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<title>Using the GNU Compiler Collection (GCC): Warning Options</title>
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<a name="Warning-Options"></a>
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<div class="header">
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<p>
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Next: <a href="Debugging-Options.html#Debugging-Options" accesskey="n" rel="next">Debugging Options</a>, Previous: <a href="Diagnostic-Message-Formatting-Options.html#Diagnostic-Message-Formatting-Options" accesskey="p" rel="prev">Diagnostic Message Formatting Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
|
</div>
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<hr>
|
<a name="Options-to-Request-or-Suppress-Warnings"></a>
|
<h3 class="section">3.8 Options to Request or Suppress Warnings</h3>
|
<a name="index-options-to-control-warnings"></a>
|
<a name="index-warning-messages"></a>
|
<a name="index-messages_002c-warning"></a>
|
<a name="index-suppressing-warnings"></a>
|
|
<p>Warnings are diagnostic messages that report constructions that
|
are not inherently erroneous but that are risky or suggest there
|
may have been an error.
|
</p>
|
<p>The following language-independent options do not enable specific
|
warnings but control the kinds of diagnostics produced by GCC.
|
</p>
|
<dl compact="compact">
|
<dd><a name="index-syntax-checking"></a>
|
</dd>
|
<dt><code>-fsyntax-only</code></dt>
|
<dd><a name="index-fsyntax_002donly"></a>
|
<p>Check the code for syntax errors, but don’t do anything beyond that.
|
</p>
|
</dd>
|
<dt><code>-fmax-errors=<var>n</var></code></dt>
|
<dd><a name="index-fmax_002derrors"></a>
|
<p>Limits the maximum number of error messages to <var>n</var>, at which point
|
GCC bails out rather than attempting to continue processing the source
|
code. If <var>n</var> is 0 (the default), there is no limit on the number
|
of error messages produced. If <samp>-Wfatal-errors</samp> is also
|
specified, then <samp>-Wfatal-errors</samp> takes precedence over this
|
option.
|
</p>
|
</dd>
|
<dt><code>-w</code></dt>
|
<dd><a name="index-w"></a>
|
<p>Inhibit all warning messages.
|
</p>
|
</dd>
|
<dt><code>-Werror</code></dt>
|
<dd><a name="index-Werror"></a>
|
<a name="index-Wno_002derror"></a>
|
<p>Make all warnings into errors.
|
</p>
|
</dd>
|
<dt><code>-Werror=</code></dt>
|
<dd><a name="index-Werror_003d"></a>
|
<a name="index-Wno_002derror_003d"></a>
|
<p>Make the specified warning into an error. The specifier for a warning
|
is appended; for example <samp>-Werror=switch</samp> turns the warnings
|
controlled by <samp>-Wswitch</samp> into errors. This switch takes a
|
negative form, to be used to negate <samp>-Werror</samp> for specific
|
warnings; for example <samp>-Wno-error=switch</samp> makes
|
<samp>-Wswitch</samp> warnings not be errors, even when <samp>-Werror</samp>
|
is in effect.
|
</p>
|
<p>The warning message for each controllable warning includes the
|
option that controls the warning. That option can then be used with
|
<samp>-Werror=</samp> and <samp>-Wno-error=</samp> as described above.
|
(Printing of the option in the warning message can be disabled using the
|
<samp>-fno-diagnostics-show-option</samp> flag.)
|
</p>
|
<p>Note that specifying <samp>-Werror=</samp><var>foo</var> automatically implies
|
<samp>-W</samp><var>foo</var>. However, <samp>-Wno-error=</samp><var>foo</var> does not
|
imply anything.
|
</p>
|
</dd>
|
<dt><code>-Wfatal-errors</code></dt>
|
<dd><a name="index-Wfatal_002derrors"></a>
|
<a name="index-Wno_002dfatal_002derrors"></a>
|
<p>This option causes the compiler to abort compilation on the first error
|
occurred rather than trying to keep going and printing further error
|
messages.
|
</p>
|
</dd>
|
</dl>
|
|
<p>You can request many specific warnings with options beginning with
|
‘<samp>-W</samp>’, for example <samp>-Wimplicit</samp> to request warnings on
|
implicit declarations. Each of these specific warning options also
|
has a negative form beginning ‘<samp>-Wno-</samp>’ to turn off warnings; for
|
example, <samp>-Wno-implicit</samp>. This manual lists only one of the
|
two forms, whichever is not the default. For further
|
language-specific options also refer to <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and
|
<a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
|
</p>
|
<p>Some options, such as <samp>-Wall</samp> and <samp>-Wextra</samp>, turn on other
|
options, such as <samp>-Wunused</samp>, which may turn on further options,
|
such as <samp>-Wunused-value</samp>. The combined effect of positive and
|
negative forms is that more specific options have priority over less
|
specific ones, independently of their position in the command-line. For
|
options of the same specificity, the last one takes effect. Options
|
enabled or disabled via pragmas (see <a href="Diagnostic-Pragmas.html#Diagnostic-Pragmas">Diagnostic Pragmas</a>) take effect
|
as if they appeared at the end of the command-line.
|
</p>
|
<p>When an unrecognized warning option is requested (e.g.,
|
<samp>-Wunknown-warning</samp>), GCC emits a diagnostic stating
|
that the option is not recognized. However, if the <samp>-Wno-</samp> form
|
is used, the behavior is slightly different: no diagnostic is
|
produced for <samp>-Wno-unknown-warning</samp> unless other diagnostics
|
are being produced. This allows the use of new <samp>-Wno-</samp> options
|
with old compilers, but if something goes wrong, the compiler
|
warns that an unrecognized option is present.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wpedantic</code></dt>
|
<dt><code>-pedantic</code></dt>
|
<dd><a name="index-pedantic-1"></a>
|
<a name="index-Wpedantic"></a>
|
<p>Issue all the warnings demanded by strict ISO C and ISO C++;
|
reject all programs that use forbidden extensions, and some other
|
programs that do not follow ISO C and ISO C++. For ISO C, follows the
|
version of the ISO C standard specified by any <samp>-std</samp> option used.
|
</p>
|
<p>Valid ISO C and ISO C++ programs should compile properly with or without
|
this option (though a rare few require <samp>-ansi</samp> or a
|
<samp>-std</samp> option specifying the required version of ISO C). However,
|
without this option, certain GNU extensions and traditional C and C++
|
features are supported as well. With this option, they are rejected.
|
</p>
|
<p><samp>-Wpedantic</samp> does not cause warning messages for use of the
|
alternate keywords whose names begin and end with ‘<samp>__</samp>’. Pedantic
|
warnings are also disabled in the expression that follows
|
<code>__extension__</code>. However, only system header files should use
|
these escape routes; application programs should avoid them.
|
See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>.
|
</p>
|
<p>Some users try to use <samp>-Wpedantic</samp> to check programs for strict ISO
|
C conformance. They soon find that it does not do quite what they want:
|
it finds some non-ISO practices, but not all—only those for which
|
ISO C <em>requires</em> a diagnostic, and some others for which
|
diagnostics have been added.
|
</p>
|
<p>A feature to report any failure to conform to ISO C might be useful in
|
some instances, but would require considerable additional work and would
|
be quite different from <samp>-Wpedantic</samp>. We don’t have plans to
|
support such a feature in the near future.
|
</p>
|
<p>Where the standard specified with <samp>-std</samp> represents a GNU
|
extended dialect of C, such as ‘<samp>gnu90</samp>’ or ‘<samp>gnu99</samp>’, there is a
|
corresponding <em>base standard</em>, the version of ISO C on which the GNU
|
extended dialect is based. Warnings from <samp>-Wpedantic</samp> are given
|
where they are required by the base standard. (It does not make sense
|
for such warnings to be given only for features not in the specified GNU
|
C dialect, since by definition the GNU dialects of C include all
|
features the compiler supports with the given option, and there would be
|
nothing to warn about.)
|
</p>
|
</dd>
|
<dt><code>-pedantic-errors</code></dt>
|
<dd><a name="index-pedantic_002derrors-1"></a>
|
<p>Give an error whenever the <em>base standard</em> (see <samp>-Wpedantic</samp>)
|
requires a diagnostic, in some cases where there is undefined behavior
|
at compile-time and in some other cases that do not prevent compilation
|
of programs that are valid according to the standard. This is not
|
equivalent to <samp>-Werror=pedantic</samp>, since there are errors enabled
|
by this option and not enabled by the latter and vice versa.
|
</p>
|
</dd>
|
<dt><code>-Wall</code></dt>
|
<dd><a name="index-Wall"></a>
|
<a name="index-Wno_002dall"></a>
|
<p>This enables all the warnings about constructions that some users
|
consider questionable, and that are easy to avoid (or modify to
|
prevent the warning), even in conjunction with macros. This also
|
enables some language-specific warnings described in <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
|
</p>
|
<p><samp>-Wall</samp> turns on the following warning flags:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">-Waddress
|
-Warray-bounds=1 <span class="roman">(only with</span> <samp>-O2</samp><span class="roman">)</span>
|
-Wbool-compare
|
-Wc++11-compat -Wc++14-compat
|
-Wchar-subscripts
|
-Wcomment
|
-Wenum-compare <span class="roman">(in C/ObjC; this is on by default in C++)</span>
|
-Wformat
|
-Wimplicit <span class="roman">(C and Objective-C only)</span>
|
-Wimplicit-int <span class="roman">(C and Objective-C only)</span>
|
-Wimplicit-function-declaration <span class="roman">(C and Objective-C only)</span>
|
-Winit-self <span class="roman">(only for C++)</span>
|
-Wlogical-not-parentheses
|
-Wmain <span class="roman">(only for C/ObjC and unless</span> <samp>-ffreestanding</samp><span class="roman">)</span>
|
-Wmaybe-uninitialized
|
-Wmemset-transposed-args
|
-Wmisleading-indentation <span class="roman">(only for C/C++)</span>
|
-Wmissing-braces <span class="roman">(only for C/ObjC)</span>
|
-Wnarrowing <span class="roman">(only for C++)</span>
|
-Wnonnull
|
-Wnonnull-compare
|
-Wopenmp-simd
|
-Wparentheses
|
-Wpointer-sign
|
-Wreorder
|
-Wreturn-type
|
-Wsequence-point
|
-Wsign-compare <span class="roman">(only in C++)</span>
|
-Wsizeof-pointer-memaccess
|
-Wstrict-aliasing
|
-Wstrict-overflow=1
|
-Wswitch
|
-Wtautological-compare
|
-Wtrigraphs
|
-Wuninitialized
|
-Wunknown-pragmas
|
-Wunused-function
|
-Wunused-label
|
-Wunused-value
|
-Wunused-variable
|
-Wvolatile-register-var
|
|
</pre></div>
|
|
<p>Note that some warning flags are not implied by <samp>-Wall</samp>. Some of
|
them warn about constructions that users generally do not consider
|
questionable, but which occasionally you might wish to check for;
|
others warn about constructions that are necessary or hard to avoid in
|
some cases, and there is no simple way to modify the code to suppress
|
the warning. Some of them are enabled by <samp>-Wextra</samp> but many of
|
them must be enabled individually.
|
</p>
|
</dd>
|
<dt><code>-Wextra</code></dt>
|
<dd><a name="index-W"></a>
|
<a name="index-Wextra"></a>
|
<a name="index-Wno_002dextra"></a>
|
<p>This enables some extra warning flags that are not enabled by
|
<samp>-Wall</samp>. (This option used to be called <samp>-W</samp>. The older
|
name is still supported, but the newer name is more descriptive.)
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">-Wclobbered
|
-Wempty-body
|
-Wignored-qualifiers
|
-Wmissing-field-initializers
|
-Wmissing-parameter-type <span class="roman">(C only)</span>
|
-Wold-style-declaration <span class="roman">(C only)</span>
|
-Woverride-init
|
-Wsign-compare <span class="roman">(C only)</span>
|
-Wtype-limits
|
-Wuninitialized
|
-Wshift-negative-value <span class="roman">(in C++03 and in C99 and newer)</span>
|
-Wunused-parameter <span class="roman">(only with</span> <samp>-Wunused</samp> <span class="roman">or</span> <samp>-Wall</samp><span class="roman">)</span>
|
-Wunused-but-set-parameter <span class="roman">(only with</span> <samp>-Wunused</samp> <span class="roman">or</span> <samp>-Wall</samp><span class="roman">)</span>
|
|
</pre></div>
|
|
<p>The option <samp>-Wextra</samp> also prints warning messages for the
|
following cases:
|
</p>
|
<ul>
|
<li> A pointer is compared against integer zero with <code><</code>, <code><=</code>,
|
<code>></code>, or <code>>=</code>.
|
|
</li><li> (C++ only) An enumerator and a non-enumerator both appear in a
|
conditional expression.
|
|
</li><li> (C++ only) Ambiguous virtual bases.
|
|
</li><li> (C++ only) Subscripting an array that has been declared <code>register</code>.
|
|
</li><li> (C++ only) Taking the address of a variable that has been declared
|
<code>register</code>.
|
|
</li><li> (C++ only) A base class is not initialized in a derived class’s copy
|
constructor.
|
|
</li></ul>
|
|
</dd>
|
<dt><code>-Wchar-subscripts</code></dt>
|
<dd><a name="index-Wchar_002dsubscripts"></a>
|
<a name="index-Wno_002dchar_002dsubscripts"></a>
|
<p>Warn if an array subscript has type <code>char</code>. This is a common cause
|
of error, as programmers often forget that this type is signed on some
|
machines.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wcomment</code></dt>
|
<dd><a name="index-Wcomment"></a>
|
<a name="index-Wno_002dcomment"></a>
|
<p>Warn whenever a comment-start sequence ‘<samp>/*</samp>’ appears in a ‘<samp>/*</samp>’
|
comment, or whenever a Backslash-Newline appears in a ‘<samp>//</samp>’ comment.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wchkp</code></dt>
|
<dd><a name="index-Wchkp"></a>
|
<p>Warn about an invalid memory access that is found by Pointer Bounds Checker
|
(<samp>-fcheck-pointer-bounds</samp>).
|
</p>
|
</dd>
|
<dt><code>-Wno-coverage-mismatch</code></dt>
|
<dd><a name="index-Wno_002dcoverage_002dmismatch"></a>
|
<p>Warn if feedback profiles do not match when using the
|
<samp>-fprofile-use</samp> option.
|
If a source file is changed between compiling with <samp>-fprofile-gen</samp> and
|
with <samp>-fprofile-use</samp>, the files with the profile feedback can fail
|
to match the source file and GCC cannot use the profile feedback
|
information. By default, this warning is enabled and is treated as an
|
error. <samp>-Wno-coverage-mismatch</samp> can be used to disable the
|
warning or <samp>-Wno-error=coverage-mismatch</samp> can be used to
|
disable the error. Disabling the error for this warning can result in
|
poorly optimized code and is useful only in the
|
case of very minor changes such as bug fixes to an existing code-base.
|
Completely disabling the warning is not recommended.
|
</p>
|
</dd>
|
<dt><code>-Wno-cpp</code></dt>
|
<dd><p><span class="roman">(C, Objective-C, C++, Objective-C++ and Fortran only)</span>
|
</p>
|
<p>Suppress warning messages emitted by <code>#warning</code> directives.
|
</p>
|
</dd>
|
<dt><code>-Wdouble-promotion <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wdouble_002dpromotion"></a>
|
<a name="index-Wno_002ddouble_002dpromotion"></a>
|
<p>Give a warning when a value of type <code>float</code> is implicitly
|
promoted to <code>double</code>. CPUs with a 32-bit “single-precision”
|
floating-point unit implement <code>float</code> in hardware, but emulate
|
<code>double</code> in software. On such a machine, doing computations
|
using <code>double</code> values is much more expensive because of the
|
overhead required for software emulation.
|
</p>
|
<p>It is easy to accidentally do computations with <code>double</code> because
|
floating-point literals are implicitly of type <code>double</code>. For
|
example, in:
|
</p><div class="smallexample">
|
<pre class="smallexample">float area(float radius)
|
{
|
return 3.14159 * radius * radius;
|
}
|
</pre></div>
|
<p>the compiler performs the entire computation with <code>double</code>
|
because the floating-point literal is a <code>double</code>.
|
</p>
|
</dd>
|
<dt><code>-Wformat</code></dt>
|
<dt><code>-Wformat=<var>n</var></code></dt>
|
<dd><a name="index-Wformat"></a>
|
<a name="index-Wno_002dformat"></a>
|
<a name="index-ffreestanding-2"></a>
|
<a name="index-fno_002dbuiltin-1"></a>
|
<a name="index-Wformat_003d"></a>
|
<p>Check calls to <code>printf</code> and <code>scanf</code>, etc., to make sure that
|
the arguments supplied have types appropriate to the format string
|
specified, and that the conversions specified in the format string make
|
sense. This includes standard functions, and others specified by format
|
attributes (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), in the <code>printf</code>,
|
<code>scanf</code>, <code>strftime</code> and <code>strfmon</code> (an X/Open extension,
|
not in the C standard) families (or other target-specific families).
|
Which functions are checked without format attributes having been
|
specified depends on the standard version selected, and such checks of
|
functions without the attribute specified are disabled by
|
<samp>-ffreestanding</samp> or <samp>-fno-builtin</samp>.
|
</p>
|
<p>The formats are checked against the format features supported by GNU
|
libc version 2.2. These include all ISO C90 and C99 features, as well
|
as features from the Single Unix Specification and some BSD and GNU
|
extensions. Other library implementations may not support all these
|
features; GCC does not support warning about features that go beyond a
|
particular library’s limitations. However, if <samp>-Wpedantic</samp> is used
|
with <samp>-Wformat</samp>, warnings are given about format features not
|
in the selected standard version (but not for <code>strfmon</code> formats,
|
since those are not in any version of the C standard). See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wformat=1</code></dt>
|
<dt><code>-Wformat</code></dt>
|
<dd><a name="index-Wformat-1"></a>
|
<a name="index-Wformat_003d1"></a>
|
<p>Option <samp>-Wformat</samp> is equivalent to <samp>-Wformat=1</samp>, and
|
<samp>-Wno-format</samp> is equivalent to <samp>-Wformat=0</samp>. Since
|
<samp>-Wformat</samp> also checks for null format arguments for several
|
functions, <samp>-Wformat</samp> also implies <samp>-Wnonnull</samp>. Some
|
aspects of this level of format checking can be disabled by the
|
options: <samp>-Wno-format-contains-nul</samp>,
|
<samp>-Wno-format-extra-args</samp>, and <samp>-Wno-format-zero-length</samp>.
|
<samp>-Wformat</samp> is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wno-format-contains-nul</code></dt>
|
<dd><a name="index-Wno_002dformat_002dcontains_002dnul"></a>
|
<a name="index-Wformat_002dcontains_002dnul"></a>
|
<p>If <samp>-Wformat</samp> is specified, do not warn about format strings that
|
contain NUL bytes.
|
</p>
|
</dd>
|
<dt><code>-Wno-format-extra-args</code></dt>
|
<dd><a name="index-Wno_002dformat_002dextra_002dargs"></a>
|
<a name="index-Wformat_002dextra_002dargs"></a>
|
<p>If <samp>-Wformat</samp> is specified, do not warn about excess arguments to a
|
<code>printf</code> or <code>scanf</code> format function. The C standard specifies
|
that such arguments are ignored.
|
</p>
|
<p>Where the unused arguments lie between used arguments that are
|
specified with ‘<samp>$</samp>’ operand number specifications, normally
|
warnings are still given, since the implementation could not know what
|
type to pass to <code>va_arg</code> to skip the unused arguments. However,
|
in the case of <code>scanf</code> formats, this option suppresses the
|
warning if the unused arguments are all pointers, since the Single
|
Unix Specification says that such unused arguments are allowed.
|
</p>
|
</dd>
|
<dt><code>-Wno-format-zero-length</code></dt>
|
<dd><a name="index-Wno_002dformat_002dzero_002dlength"></a>
|
<a name="index-Wformat_002dzero_002dlength"></a>
|
<p>If <samp>-Wformat</samp> is specified, do not warn about zero-length formats.
|
The C standard specifies that zero-length formats are allowed.
|
</p>
|
|
</dd>
|
<dt><code>-Wformat=2</code></dt>
|
<dd><a name="index-Wformat_003d2"></a>
|
<p>Enable <samp>-Wformat</samp> plus additional format checks. Currently
|
equivalent to <samp>-Wformat -Wformat-nonliteral -Wformat-security
|
-Wformat-y2k</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wformat-nonliteral</code></dt>
|
<dd><a name="index-Wformat_002dnonliteral"></a>
|
<a name="index-Wno_002dformat_002dnonliteral"></a>
|
<p>If <samp>-Wformat</samp> is specified, also warn if the format string is not a
|
string literal and so cannot be checked, unless the format function
|
takes its format arguments as a <code>va_list</code>.
|
</p>
|
</dd>
|
<dt><code>-Wformat-security</code></dt>
|
<dd><a name="index-Wformat_002dsecurity"></a>
|
<a name="index-Wno_002dformat_002dsecurity"></a>
|
<p>If <samp>-Wformat</samp> is specified, also warn about uses of format
|
functions that represent possible security problems. At present, this
|
warns about calls to <code>printf</code> and <code>scanf</code> functions where the
|
format string is not a string literal and there are no format arguments,
|
as in <code>printf (foo);</code>. This may be a security hole if the format
|
string came from untrusted input and contains ‘<samp>%n</samp>’. (This is
|
currently a subset of what <samp>-Wformat-nonliteral</samp> warns about, but
|
in future warnings may be added to <samp>-Wformat-security</samp> that are not
|
included in <samp>-Wformat-nonliteral</samp>.)
|
</p>
|
</dd>
|
<dt><code>-Wformat-signedness</code></dt>
|
<dd><a name="index-Wformat_002dsignedness"></a>
|
<a name="index-Wno_002dformat_002dsignedness"></a>
|
<p>If <samp>-Wformat</samp> is specified, also warn if the format string
|
requires an unsigned argument and the argument is signed and vice versa.
|
</p>
|
</dd>
|
<dt><code>-Wformat-y2k</code></dt>
|
<dd><a name="index-Wformat_002dy2k"></a>
|
<a name="index-Wno_002dformat_002dy2k"></a>
|
<p>If <samp>-Wformat</samp> is specified, also warn about <code>strftime</code>
|
formats that may yield only a two-digit year.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wnonnull</code></dt>
|
<dd><a name="index-Wnonnull"></a>
|
<a name="index-Wno_002dnonnull"></a>
|
<p>Warn about passing a null pointer for arguments marked as
|
requiring a non-null value by the <code>nonnull</code> function attribute.
|
</p>
|
<p><samp>-Wnonnull</samp> is included in <samp>-Wall</samp> and <samp>-Wformat</samp>. It
|
can be disabled with the <samp>-Wno-nonnull</samp> option.
|
</p>
|
</dd>
|
<dt><code>-Wnonnull-compare</code></dt>
|
<dd><a name="index-Wnonnull_002dcompare"></a>
|
<a name="index-Wno_002dnonnull_002dcompare"></a>
|
<p>Warn when comparing an argument marked with the <code>nonnull</code>
|
function attribute against null inside the function.
|
</p>
|
<p><samp>-Wnonnull-compare</samp> is included in <samp>-Wall</samp>. It
|
can be disabled with the <samp>-Wno-nonnull-compare</samp> option.
|
</p>
|
</dd>
|
<dt><code>-Wnull-dereference</code></dt>
|
<dd><a name="index-Wnull_002ddereference"></a>
|
<a name="index-Wno_002dnull_002ddereference"></a>
|
<p>Warn if the compiler detects paths that trigger erroneous or
|
undefined behavior due to dereferencing a null pointer. This option
|
is only active when <samp>-fdelete-null-pointer-checks</samp> is active,
|
which is enabled by optimizations in most targets. The precision of
|
the warnings depends on the optimization options used.
|
</p>
|
</dd>
|
<dt><code>-Winit-self <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Winit_002dself"></a>
|
<a name="index-Wno_002dinit_002dself"></a>
|
<p>Warn about uninitialized variables that are initialized with themselves.
|
Note this option can only be used with the <samp>-Wuninitialized</samp> option.
|
</p>
|
<p>For example, GCC warns about <code>i</code> being uninitialized in the
|
following snippet only when <samp>-Winit-self</samp> has been specified:
|
</p><div class="smallexample">
|
<pre class="smallexample">int f()
|
{
|
int i = i;
|
return i;
|
}
|
</pre></div>
|
|
<p>This warning is enabled by <samp>-Wall</samp> in C++.
|
</p>
|
</dd>
|
<dt><code>-Wimplicit-int <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wimplicit_002dint"></a>
|
<a name="index-Wno_002dimplicit_002dint"></a>
|
<p>Warn when a declaration does not specify a type.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wimplicit-function-declaration <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wimplicit_002dfunction_002ddeclaration"></a>
|
<a name="index-Wno_002dimplicit_002dfunction_002ddeclaration"></a>
|
<p>Give a warning whenever a function is used before being declared. In
|
C99 mode (<samp>-std=c99</samp> or <samp>-std=gnu99</samp>), this warning is
|
enabled by default and it is made into an error by
|
<samp>-pedantic-errors</samp>. This warning is also enabled by
|
<samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wimplicit <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wimplicit"></a>
|
<a name="index-Wno_002dimplicit"></a>
|
<p>Same as <samp>-Wimplicit-int</samp> and <samp>-Wimplicit-function-declaration</samp>.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wignored-qualifiers <span class="roman">(C and C++ only)</span></code></dt>
|
<dd><a name="index-Wignored_002dqualifiers"></a>
|
<a name="index-Wno_002dignored_002dqualifiers"></a>
|
<p>Warn if the return type of a function has a type qualifier
|
such as <code>const</code>. For ISO C such a type qualifier has no effect,
|
since the value returned by a function is not an lvalue.
|
For C++, the warning is only emitted for scalar types or <code>void</code>.
|
ISO C prohibits qualified <code>void</code> return types on function
|
definitions, so such return types always receive a warning
|
even without this option.
|
</p>
|
<p>This warning is also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wignored-attributes <span class="roman">(C and C++ only)</span></code></dt>
|
<dd><a name="index-Wignored_002dattributes"></a>
|
<a name="index-Wno_002dignored_002dattributes"></a>
|
<p>Warn when an attribute is ignored. This is different from the
|
<samp>-Wattributes</samp> option in that it warns whenever the compiler decides
|
to drop an attribute, not that the attribute is either unknown, used in a
|
wrong place, etc. This warning is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wmain</code></dt>
|
<dd><a name="index-Wmain"></a>
|
<a name="index-Wno_002dmain"></a>
|
<p>Warn if the type of <code>main</code> is suspicious. <code>main</code> should be
|
a function with external linkage, returning int, taking either zero
|
arguments, two, or three arguments of appropriate types. This warning
|
is enabled by default in C++ and is enabled by either <samp>-Wall</samp>
|
or <samp>-Wpedantic</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wmisleading-indentation <span class="roman">(C and C++ only)</span></code></dt>
|
<dd><a name="index-Wmisleading_002dindentation"></a>
|
<a name="index-Wno_002dmisleading_002dindentation"></a>
|
<p>Warn when the indentation of the code does not reflect the block structure.
|
Specifically, a warning is issued for <code>if</code>, <code>else</code>, <code>while</code>, and
|
<code>for</code> clauses with a guarded statement that does not use braces,
|
followed by an unguarded statement with the same indentation.
|
</p>
|
<p>In the following example, the call to “bar” is misleadingly indented as
|
if it were guarded by the “if” conditional.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample"> if (some_condition ())
|
foo ();
|
bar (); /* Gotcha: this is not guarded by the "if". */
|
</pre></div>
|
|
<p>In the case of mixed tabs and spaces, the warning uses the
|
<samp>-ftabstop=</samp> option to determine if the statements line up
|
(defaulting to 8).
|
</p>
|
<p>The warning is not issued for code involving multiline preprocessor logic
|
such as the following example.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample"> if (flagA)
|
foo (0);
|
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
|
if (flagB)
|
#endif
|
foo (1);
|
</pre></div>
|
|
<p>The warning is not issued after a <code>#line</code> directive, since this
|
typically indicates autogenerated code, and no assumptions can be made
|
about the layout of the file that the directive references.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp> in C and C++.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-braces</code></dt>
|
<dd><a name="index-Wmissing_002dbraces"></a>
|
<a name="index-Wno_002dmissing_002dbraces"></a>
|
<p>Warn if an aggregate or union initializer is not fully bracketed. In
|
the following example, the initializer for <code>a</code> is not fully
|
bracketed, but that for <code>b</code> is fully bracketed. This warning is
|
enabled by <samp>-Wall</samp> in C.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">int a[2][2] = { 0, 1, 2, 3 };
|
int b[2][2] = { { 0, 1 }, { 2, 3 } };
|
</pre></div>
|
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-include-dirs <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wmissing_002dinclude_002ddirs"></a>
|
<a name="index-Wno_002dmissing_002dinclude_002ddirs"></a>
|
<p>Warn if a user-supplied include directory does not exist.
|
</p>
|
</dd>
|
<dt><code>-Wparentheses</code></dt>
|
<dd><a name="index-Wparentheses"></a>
|
<a name="index-Wno_002dparentheses"></a>
|
<p>Warn if parentheses are omitted in certain contexts, such
|
as when there is an assignment in a context where a truth value
|
is expected, or when operators are nested whose precedence people
|
often get confused about.
|
</p>
|
<p>Also warn if a comparison like <code>x<=y<=z</code> appears; this is
|
equivalent to <code>(x<=y ? 1 : 0) <= z</code>, which is a different
|
interpretation from that of ordinary mathematical notation.
|
</p>
|
<p>Also warn about constructions where there may be confusion to which
|
<code>if</code> statement an <code>else</code> branch belongs. Here is an example of
|
such a case:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">{
|
if (a)
|
if (b)
|
foo ();
|
else
|
bar ();
|
}
|
</pre></div>
|
|
<p>In C/C++, every <code>else</code> branch belongs to the innermost possible
|
<code>if</code> statement, which in this example is <code>if (b)</code>. This is
|
often not what the programmer expected, as illustrated in the above
|
example by indentation the programmer chose. When there is the
|
potential for this confusion, GCC issues a warning when this flag
|
is specified. To eliminate the warning, add explicit braces around
|
the innermost <code>if</code> statement so there is no way the <code>else</code>
|
can belong to the enclosing <code>if</code>. The resulting code
|
looks like this:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">{
|
if (a)
|
{
|
if (b)
|
foo ();
|
else
|
bar ();
|
}
|
}
|
</pre></div>
|
|
<p>Also warn for dangerous uses of the GNU extension to
|
<code>?:</code> with omitted middle operand. When the condition
|
in the <code>?</code>: operator is a boolean expression, the omitted value is
|
always 1. Often programmers expect it to be a value computed
|
inside the conditional expression instead.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wsequence-point</code></dt>
|
<dd><a name="index-Wsequence_002dpoint"></a>
|
<a name="index-Wno_002dsequence_002dpoint"></a>
|
<p>Warn about code that may have undefined semantics because of violations
|
of sequence point rules in the C and C++ standards.
|
</p>
|
<p>The C and C++ standards define the order in which expressions in a C/C++
|
program are evaluated in terms of <em>sequence points</em>, which represent
|
a partial ordering between the execution of parts of the program: those
|
executed before the sequence point, and those executed after it. These
|
occur after the evaluation of a full expression (one which is not part
|
of a larger expression), after the evaluation of the first operand of a
|
<code>&&</code>, <code>||</code>, <code>? :</code> or <code>,</code> (comma) operator, before a
|
function is called (but after the evaluation of its arguments and the
|
expression denoting the called function), and in certain other places.
|
Other than as expressed by the sequence point rules, the order of
|
evaluation of subexpressions of an expression is not specified. All
|
these rules describe only a partial order rather than a total order,
|
since, for example, if two functions are called within one expression
|
with no sequence point between them, the order in which the functions
|
are called is not specified. However, the standards committee have
|
ruled that function calls do not overlap.
|
</p>
|
<p>It is not specified when between sequence points modifications to the
|
values of objects take effect. Programs whose behavior depends on this
|
have undefined behavior; the C and C++ standards specify that “Between
|
the previous and next sequence point an object shall have its stored
|
value modified at most once by the evaluation of an expression.
|
Furthermore, the prior value shall be read only to determine the value
|
to be stored.”. If a program breaks these rules, the results on any
|
particular implementation are entirely unpredictable.
|
</p>
|
<p>Examples of code with undefined behavior are <code>a = a++;</code>, <code>a[n]
|
= b[n++]</code> and <code>a[i++] = i;</code>. Some more complicated cases are not
|
diagnosed by this option, and it may give an occasional false positive
|
result, but in general it has been found fairly effective at detecting
|
this sort of problem in programs.
|
</p>
|
<p>The standard is worded confusingly, therefore there is some debate
|
over the precise meaning of the sequence point rules in subtle cases.
|
Links to discussions of the problem, including proposed formal
|
definitions, may be found on the GCC readings page, at
|
<a href="http://gcc.gnu.org/readings.html">http://gcc.gnu.org/readings.html</a>.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp> for C and C++.
|
</p>
|
</dd>
|
<dt><code>-Wno-return-local-addr</code></dt>
|
<dd><a name="index-Wno_002dreturn_002dlocal_002daddr"></a>
|
<a name="index-Wreturn_002dlocal_002daddr"></a>
|
<p>Do not warn about returning a pointer (or in C++, a reference) to a
|
variable that goes out of scope after the function returns.
|
</p>
|
</dd>
|
<dt><code>-Wreturn-type</code></dt>
|
<dd><a name="index-Wreturn_002dtype"></a>
|
<a name="index-Wno_002dreturn_002dtype"></a>
|
<p>Warn whenever a function is defined with a return type that defaults
|
to <code>int</code>. Also warn about any <code>return</code> statement with no
|
return value in a function whose return type is not <code>void</code>
|
(falling off the end of the function body is considered returning
|
without a value), and about a <code>return</code> statement with an
|
expression in a function whose return type is <code>void</code>.
|
</p>
|
<p>For C++, a function without return type always produces a diagnostic
|
message, even when <samp>-Wno-return-type</samp> is specified. The only
|
exceptions are <code>main</code> and functions defined in system headers.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wshift-count-negative</code></dt>
|
<dd><a name="index-Wshift_002dcount_002dnegative"></a>
|
<a name="index-Wno_002dshift_002dcount_002dnegative"></a>
|
<p>Warn if shift count is negative. This warning is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wshift-count-overflow</code></dt>
|
<dd><a name="index-Wshift_002dcount_002doverflow"></a>
|
<a name="index-Wno_002dshift_002dcount_002doverflow"></a>
|
<p>Warn if shift count >= width of type. This warning is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wshift-negative-value</code></dt>
|
<dd><a name="index-Wshift_002dnegative_002dvalue"></a>
|
<a name="index-Wno_002dshift_002dnegative_002dvalue"></a>
|
<p>Warn if left shifting a negative value. This warning is enabled by
|
<samp>-Wextra</samp> in C99 and C++11 modes (and newer).
|
</p>
|
</dd>
|
<dt><code>-Wshift-overflow</code></dt>
|
<dt><code>-Wshift-overflow=<var>n</var></code></dt>
|
<dd><a name="index-Wshift_002doverflow"></a>
|
<a name="index-Wno_002dshift_002doverflow"></a>
|
<p>Warn about left shift overflows. This warning is enabled by
|
default in C99 and C++11 modes (and newer).
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wshift-overflow=1</code></dt>
|
<dd><p>This is the warning level of <samp>-Wshift-overflow</samp> and is enabled
|
by default in C99 and C++11 modes (and newer). This warning level does
|
not warn about left-shifting 1 into the sign bit. (However, in C, such
|
an overflow is still rejected in contexts where an integer constant expression
|
is required.)
|
</p>
|
</dd>
|
<dt><code>-Wshift-overflow=2</code></dt>
|
<dd><p>This warning level also warns about left-shifting 1 into the sign bit,
|
unless C++14 mode is active.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wswitch</code></dt>
|
<dd><a name="index-Wswitch"></a>
|
<a name="index-Wno_002dswitch"></a>
|
<p>Warn whenever a <code>switch</code> statement has an index of enumerated type
|
and lacks a <code>case</code> for one or more of the named codes of that
|
enumeration. (The presence of a <code>default</code> label prevents this
|
warning.) <code>case</code> labels outside the enumeration range also
|
provoke warnings when this option is used (even if there is a
|
<code>default</code> label).
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wswitch-default</code></dt>
|
<dd><a name="index-Wswitch_002ddefault"></a>
|
<a name="index-Wno_002dswitch_002ddefault"></a>
|
<p>Warn whenever a <code>switch</code> statement does not have a <code>default</code>
|
case.
|
</p>
|
</dd>
|
<dt><code>-Wswitch-enum</code></dt>
|
<dd><a name="index-Wswitch_002denum"></a>
|
<a name="index-Wno_002dswitch_002denum"></a>
|
<p>Warn whenever a <code>switch</code> statement has an index of enumerated type
|
and lacks a <code>case</code> for one or more of the named codes of that
|
enumeration. <code>case</code> labels outside the enumeration range also
|
provoke warnings when this option is used. The only difference
|
between <samp>-Wswitch</samp> and this option is that this option gives a
|
warning about an omitted enumeration code even if there is a
|
<code>default</code> label.
|
</p>
|
</dd>
|
<dt><code>-Wswitch-bool</code></dt>
|
<dd><a name="index-Wswitch_002dbool"></a>
|
<a name="index-Wno_002dswitch_002dbool"></a>
|
<p>Warn whenever a <code>switch</code> statement has an index of boolean type
|
and the case values are outside the range of a boolean type.
|
It is possible to suppress this warning by casting the controlling
|
expression to a type other than <code>bool</code>. For example:
|
</p><div class="smallexample">
|
<pre class="smallexample">switch ((int) (a == 4))
|
{
|
…
|
}
|
</pre></div>
|
<p>This warning is enabled by default for C and C++ programs.
|
</p>
|
</dd>
|
<dt><code>-Wsync-nand <span class="roman">(C and C++ only)</span></code></dt>
|
<dd><a name="index-Wsync_002dnand"></a>
|
<a name="index-Wno_002dsync_002dnand"></a>
|
<p>Warn when <code>__sync_fetch_and_nand</code> and <code>__sync_nand_and_fetch</code>
|
built-in functions are used. These functions changed semantics in GCC 4.4.
|
</p>
|
</dd>
|
<dt><code>-Wtrigraphs</code></dt>
|
<dd><a name="index-Wtrigraphs"></a>
|
<a name="index-Wno_002dtrigraphs"></a>
|
<p>Warn if any trigraphs are encountered that might change the meaning of
|
the program (trigraphs within comments are not warned about).
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-but-set-parameter</code></dt>
|
<dd><a name="index-Wunused_002dbut_002dset_002dparameter"></a>
|
<a name="index-Wno_002dunused_002dbut_002dset_002dparameter"></a>
|
<p>Warn whenever a function parameter is assigned to, but otherwise unused
|
(aside from its declaration).
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
<p>This warning is also enabled by <samp>-Wunused</samp> together with
|
<samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-but-set-variable</code></dt>
|
<dd><a name="index-Wunused_002dbut_002dset_002dvariable"></a>
|
<a name="index-Wno_002dunused_002dbut_002dset_002dvariable"></a>
|
<p>Warn whenever a local variable is assigned to, but otherwise unused
|
(aside from its declaration).
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
<p>This warning is also enabled by <samp>-Wunused</samp>, which is enabled
|
by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-function</code></dt>
|
<dd><a name="index-Wunused_002dfunction"></a>
|
<a name="index-Wno_002dunused_002dfunction"></a>
|
<p>Warn whenever a static function is declared but not defined or a
|
non-inline static function is unused.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-label</code></dt>
|
<dd><a name="index-Wunused_002dlabel"></a>
|
<a name="index-Wno_002dunused_002dlabel"></a>
|
<p>Warn whenever a label is declared but not used.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
</dd>
|
<dt><code>-Wunused-local-typedefs <span class="roman">(C, Objective-C, C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wunused_002dlocal_002dtypedefs"></a>
|
<p>Warn when a typedef locally defined in a function is not used.
|
This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-parameter</code></dt>
|
<dd><a name="index-Wunused_002dparameter"></a>
|
<a name="index-Wno_002dunused_002dparameter"></a>
|
<p>Warn whenever a function parameter is unused aside from its declaration.
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
</dd>
|
<dt><code>-Wno-unused-result</code></dt>
|
<dd><a name="index-Wunused_002dresult"></a>
|
<a name="index-Wno_002dunused_002dresult"></a>
|
<p>Do not warn if a caller of a function marked with attribute
|
<code>warn_unused_result</code> (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>) does not use
|
its return value. The default is <samp>-Wunused-result</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused-variable</code></dt>
|
<dd><a name="index-Wunused_002dvariable"></a>
|
<a name="index-Wno_002dunused_002dvariable"></a>
|
<p>Warn whenever a local or static variable is unused aside from its
|
declaration. This option implies <samp>-Wunused-const-variable=1</samp> for C,
|
but not for C++. This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
</dd>
|
<dt><code>-Wunused-const-variable</code></dt>
|
<dt><code>-Wunused-const-variable=<var>n</var></code></dt>
|
<dd><a name="index-Wunused_002dconst_002dvariable"></a>
|
<a name="index-Wno_002dunused_002dconst_002dvariable"></a>
|
<p>Warn whenever a constant static variable is unused aside from its declaration.
|
<samp>-Wunused-const-variable=1</samp> is enabled by <samp>-Wunused-variable</samp>
|
for C, but not for C++. In C this declares variable storage, but in C++ this
|
is not an error since const variables take the place of <code>#define</code>s.
|
</p>
|
<p>To suppress this warning use the <code>unused</code> attribute
|
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wunused-const-variable=1</code></dt>
|
<dd><p>This is the warning level that is enabled by <samp>-Wunused-variable</samp> for
|
C. It warns only about unused static const variables defined in the main
|
compilation unit, but not about static const variables declared in any
|
header included.
|
</p>
|
</dd>
|
<dt><code>-Wunused-const-variable=2</code></dt>
|
<dd><p>This warning level also warns for unused constant static variables in
|
headers (excluding system headers). This is the warning level of
|
<samp>-Wunused-const-variable</samp> and must be explicitly requested since
|
in C++ this isn’t an error and in C it might be harder to clean up all
|
headers included.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wunused-value</code></dt>
|
<dd><a name="index-Wunused_002dvalue"></a>
|
<a name="index-Wno_002dunused_002dvalue"></a>
|
<p>Warn whenever a statement computes a result that is explicitly not
|
used. To suppress this warning cast the unused expression to
|
<code>void</code>. This includes an expression-statement or the left-hand
|
side of a comma expression that contains no side effects. For example,
|
an expression such as <code>x[i,j]</code> causes a warning, while
|
<code>x[(void)i,j]</code> does not.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunused</code></dt>
|
<dd><a name="index-Wunused"></a>
|
<a name="index-Wno_002dunused"></a>
|
<p>All the above <samp>-Wunused</samp> options combined.
|
</p>
|
<p>In order to get a warning about an unused function parameter, you must
|
either specify <samp>-Wextra -Wunused</samp> (note that <samp>-Wall</samp> implies
|
<samp>-Wunused</samp>), or separately specify <samp>-Wunused-parameter</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wuninitialized</code></dt>
|
<dd><a name="index-Wuninitialized"></a>
|
<a name="index-Wno_002duninitialized"></a>
|
<p>Warn if an automatic variable is used without first being initialized
|
or if a variable may be clobbered by a <code>setjmp</code> call. In C++,
|
warn if a non-static reference or non-static <code>const</code> member
|
appears in a class without constructors.
|
</p>
|
<p>If you want to warn about code that uses the uninitialized value of the
|
variable in its own initializer, use the <samp>-Winit-self</samp> option.
|
</p>
|
<p>These warnings occur for individual uninitialized or clobbered
|
elements of structure, union or array variables as well as for
|
variables that are uninitialized or clobbered as a whole. They do
|
not occur for variables or elements declared <code>volatile</code>. Because
|
these warnings depend on optimization, the exact variables or elements
|
for which there are warnings depends on the precise optimization
|
options and version of GCC used.
|
</p>
|
<p>Note that there may be no warning about a variable that is used only
|
to compute a value that itself is never used, because such
|
computations may be deleted by data flow analysis before the warnings
|
are printed.
|
</p>
|
</dd>
|
<dt><code>-Winvalid-memory-model</code></dt>
|
<dd><a name="index-Winvalid_002dmemory_002dmodel"></a>
|
<a name="index-Wno_002dinvalid_002dmemory_002dmodel"></a>
|
<p>Warn for invocations of <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins">__atomic Builtins</a>, <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>,
|
and the C11 atomic generic functions with a memory consistency argument
|
that is either invalid for the operation or outside the range of values
|
of the <code>memory_order</code> enumeration. For example, since the
|
<code>__atomic_store</code> and <code>__atomic_store_n</code> built-ins are only
|
defined for the relaxed, release, and sequentially consistent memory
|
orders the following code is diagnosed:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">void store (int *i)
|
{
|
__atomic_store_n (i, 0, memory_order_consume);
|
}
|
</pre></div>
|
|
<p><samp>-Winvalid-memory-model</samp> is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wmaybe-uninitialized</code></dt>
|
<dd><a name="index-Wmaybe_002duninitialized"></a>
|
<a name="index-Wno_002dmaybe_002duninitialized"></a>
|
<p>For an automatic variable, if there exists a path from the function
|
entry to a use of the variable that is initialized, but there exist
|
some other paths for which the variable is not initialized, the compiler
|
emits a warning if it cannot prove the uninitialized paths are not
|
executed at run time. These warnings are made optional because GCC is
|
not smart enough to see all the reasons why the code might be correct
|
in spite of appearing to have an error. Here is one example of how
|
this can happen:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">{
|
int x;
|
switch (y)
|
{
|
case 1: x = 1;
|
break;
|
case 2: x = 4;
|
break;
|
case 3: x = 5;
|
}
|
foo (x);
|
}
|
</pre></div>
|
|
<p>If the value of <code>y</code> is always 1, 2 or 3, then <code>x</code> is
|
always initialized, but GCC doesn’t know this. To suppress the
|
warning, you need to provide a default case with assert(0) or
|
similar code.
|
</p>
|
<a name="index-longjmp-warnings"></a>
|
<p>This option also warns when a non-volatile automatic variable might be
|
changed by a call to <code>longjmp</code>. These warnings as well are possible
|
only in optimizing compilation.
|
</p>
|
<p>The compiler sees only the calls to <code>setjmp</code>. It cannot know
|
where <code>longjmp</code> will be called; in fact, a signal handler could
|
call it at any point in the code. As a result, you may get a warning
|
even when there is in fact no problem because <code>longjmp</code> cannot
|
in fact be called at the place that would cause a problem.
|
</p>
|
<p>Some spurious warnings can be avoided if you declare all the functions
|
you use that never return as <code>noreturn</code>. See <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>.
|
</p>
|
<p>This warning is enabled by <samp>-Wall</samp> or <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunknown-pragmas</code></dt>
|
<dd><a name="index-Wunknown_002dpragmas"></a>
|
<a name="index-Wno_002dunknown_002dpragmas"></a>
|
<a name="index-warning-for-unknown-pragmas"></a>
|
<a name="index-unknown-pragmas_002c-warning"></a>
|
<a name="index-pragmas_002c-warning-of-unknown"></a>
|
<p>Warn when a <code>#pragma</code> directive is encountered that is not understood by
|
GCC. If this command-line option is used, warnings are even issued
|
for unknown pragmas in system header files. This is not the case if
|
the warnings are only enabled by the <samp>-Wall</samp> command-line option.
|
</p>
|
</dd>
|
<dt><code>-Wno-pragmas</code></dt>
|
<dd><a name="index-Wno_002dpragmas"></a>
|
<a name="index-Wpragmas"></a>
|
<p>Do not warn about misuses of pragmas, such as incorrect parameters,
|
invalid syntax, or conflicts between pragmas. See also
|
<samp>-Wunknown-pragmas</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-aliasing</code></dt>
|
<dd><a name="index-Wstrict_002daliasing"></a>
|
<a name="index-Wno_002dstrict_002daliasing"></a>
|
<p>This option is only active when <samp>-fstrict-aliasing</samp> is active.
|
It warns about code that might break the strict aliasing rules that the
|
compiler is using for optimization. The warning does not catch all
|
cases, but does attempt to catch the more common pitfalls. It is
|
included in <samp>-Wall</samp>.
|
It is equivalent to <samp>-Wstrict-aliasing=3</samp>
|
</p>
|
</dd>
|
<dt><code>-Wstrict-aliasing=n</code></dt>
|
<dd><a name="index-Wstrict_002daliasing_003dn"></a>
|
<p>This option is only active when <samp>-fstrict-aliasing</samp> is active.
|
It warns about code that might break the strict aliasing rules that the
|
compiler is using for optimization.
|
Higher levels correspond to higher accuracy (fewer false positives).
|
Higher levels also correspond to more effort, similar to the way <samp>-O</samp>
|
works.
|
<samp>-Wstrict-aliasing</samp> is equivalent to <samp>-Wstrict-aliasing=3</samp>.
|
</p>
|
<p>Level 1: Most aggressive, quick, least accurate.
|
Possibly useful when higher levels
|
do not warn but <samp>-fstrict-aliasing</samp> still breaks the code, as it has very few
|
false negatives. However, it has many false positives.
|
Warns for all pointer conversions between possibly incompatible types,
|
even if never dereferenced. Runs in the front end only.
|
</p>
|
<p>Level 2: Aggressive, quick, not too precise.
|
May still have many false positives (not as many as level 1 though),
|
and few false negatives (but possibly more than level 1).
|
Unlike level 1, it only warns when an address is taken. Warns about
|
incomplete types. Runs in the front end only.
|
</p>
|
<p>Level 3 (default for <samp>-Wstrict-aliasing</samp>):
|
Should have very few false positives and few false
|
negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
|
Takes care of the common pun+dereference pattern in the front end:
|
<code>*(int*)&some_float</code>.
|
If optimization is enabled, it also runs in the back end, where it deals
|
with multiple statement cases using flow-sensitive points-to information.
|
Only warns when the converted pointer is dereferenced.
|
Does not warn about incomplete types.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-overflow</code></dt>
|
<dt><code>-Wstrict-overflow=<var>n</var></code></dt>
|
<dd><a name="index-Wstrict_002doverflow"></a>
|
<a name="index-Wno_002dstrict_002doverflow"></a>
|
<p>This option is only active when <samp>-fstrict-overflow</samp> is active.
|
It warns about cases where the compiler optimizes based on the
|
assumption that signed overflow does not occur. Note that it does not
|
warn about all cases where the code might overflow: it only warns
|
about cases where the compiler implements some optimization. Thus
|
this warning depends on the optimization level.
|
</p>
|
<p>An optimization that assumes that signed overflow does not occur is
|
perfectly safe if the values of the variables involved are such that
|
overflow never does, in fact, occur. Therefore this warning can
|
easily give a false positive: a warning about code that is not
|
actually a problem. To help focus on important issues, several
|
warning levels are defined. No warnings are issued for the use of
|
undefined signed overflow when estimating how many iterations a loop
|
requires, in particular when determining whether a loop will be
|
executed at all.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wstrict-overflow=1</code></dt>
|
<dd><p>Warn about cases that are both questionable and easy to avoid. For
|
example, with <samp>-fstrict-overflow</samp>, the compiler simplifies
|
<code>x + 1 > x</code> to <code>1</code>. This level of
|
<samp>-Wstrict-overflow</samp> is enabled by <samp>-Wall</samp>; higher levels
|
are not, and must be explicitly requested.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-overflow=2</code></dt>
|
<dd><p>Also warn about other cases where a comparison is simplified to a
|
constant. For example: <code>abs (x) >= 0</code>. This can only be
|
simplified when <samp>-fstrict-overflow</samp> is in effect, because
|
<code>abs (INT_MIN)</code> overflows to <code>INT_MIN</code>, which is less than
|
zero. <samp>-Wstrict-overflow</samp> (with no level) is the same as
|
<samp>-Wstrict-overflow=2</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-overflow=3</code></dt>
|
<dd><p>Also warn about other cases where a comparison is simplified. For
|
example: <code>x + 1 > 1</code> is simplified to <code>x > 0</code>.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-overflow=4</code></dt>
|
<dd><p>Also warn about other simplifications not covered by the above cases.
|
For example: <code>(x * 10) / 5</code> is simplified to <code>x * 2</code>.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-overflow=5</code></dt>
|
<dd><p>Also warn about cases where the compiler reduces the magnitude of a
|
constant involved in a comparison. For example: <code>x + 2 > y</code> is
|
simplified to <code>x + 1 >= y</code>. This is reported only at the
|
highest warning level because this simplification applies to many
|
comparisons, so this warning level gives a very large number of
|
false positives.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wsuggest-attribute=<span class="roman">[</span>pure<span class="roman">|</span>const<span class="roman">|</span>noreturn<span class="roman">|</span>format<span class="roman">]</span></code></dt>
|
<dd><a name="index-Wsuggest_002dattribute_003d"></a>
|
<a name="index-Wno_002dsuggest_002dattribute_003d"></a>
|
<p>Warn for cases where adding an attribute may be beneficial. The
|
attributes currently supported are listed below.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wsuggest-attribute=pure</code></dt>
|
<dt><code>-Wsuggest-attribute=const</code></dt>
|
<dt><code>-Wsuggest-attribute=noreturn</code></dt>
|
<dd><a name="index-Wsuggest_002dattribute_003dpure"></a>
|
<a name="index-Wno_002dsuggest_002dattribute_003dpure"></a>
|
<a name="index-Wsuggest_002dattribute_003dconst"></a>
|
<a name="index-Wno_002dsuggest_002dattribute_003dconst"></a>
|
<a name="index-Wsuggest_002dattribute_003dnoreturn"></a>
|
<a name="index-Wno_002dsuggest_002dattribute_003dnoreturn"></a>
|
|
<p>Warn about functions that might be candidates for attributes
|
<code>pure</code>, <code>const</code> or <code>noreturn</code>. The compiler only warns for
|
functions visible in other compilation units or (in the case of <code>pure</code> and
|
<code>const</code>) if it cannot prove that the function returns normally. A function
|
returns normally if it doesn’t contain an infinite loop or return abnormally
|
by throwing, calling <code>abort</code> or trapping. This analysis requires option
|
<samp>-fipa-pure-const</samp>, which is enabled by default at <samp>-O</samp> and
|
higher. Higher optimization levels improve the accuracy of the analysis.
|
</p>
|
</dd>
|
<dt><code>-Wsuggest-attribute=format</code></dt>
|
<dt><code>-Wmissing-format-attribute</code></dt>
|
<dd><a name="index-Wsuggest_002dattribute_003dformat"></a>
|
<a name="index-Wmissing_002dformat_002dattribute"></a>
|
<a name="index-Wno_002dsuggest_002dattribute_003dformat"></a>
|
<a name="index-Wno_002dmissing_002dformat_002dattribute"></a>
|
<a name="index-Wformat-2"></a>
|
<a name="index-Wno_002dformat-1"></a>
|
|
<p>Warn about function pointers that might be candidates for <code>format</code>
|
attributes. Note these are only possible candidates, not absolute ones.
|
GCC guesses that function pointers with <code>format</code> attributes that
|
are used in assignment, initialization, parameter passing or return
|
statements should have a corresponding <code>format</code> attribute in the
|
resulting type. I.e. the left-hand side of the assignment or
|
initialization, the type of the parameter variable, or the return type
|
of the containing function respectively should also have a <code>format</code>
|
attribute to avoid the warning.
|
</p>
|
<p>GCC also warns about function definitions that might be
|
candidates for <code>format</code> attributes. Again, these are only
|
possible candidates. GCC guesses that <code>format</code> attributes
|
might be appropriate for any function that calls a function like
|
<code>vprintf</code> or <code>vscanf</code>, but this might not always be the
|
case, and some functions for which <code>format</code> attributes are
|
appropriate may not be detected.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wsuggest-final-types</code></dt>
|
<dd><a name="index-Wno_002dsuggest_002dfinal_002dtypes"></a>
|
<a name="index-Wsuggest_002dfinal_002dtypes"></a>
|
<p>Warn about types with virtual methods where code quality would be improved
|
if the type were declared with the C++11 <code>final</code> specifier,
|
or, if possible,
|
declared in an anonymous namespace. This allows GCC to more aggressively
|
devirtualize the polymorphic calls. This warning is more effective with link
|
time optimization, where the information about the class hierarchy graph is
|
more complete.
|
</p>
|
</dd>
|
<dt><code>-Wsuggest-final-methods</code></dt>
|
<dd><a name="index-Wno_002dsuggest_002dfinal_002dmethods"></a>
|
<a name="index-Wsuggest_002dfinal_002dmethods"></a>
|
<p>Warn about virtual methods where code quality would be improved if the method
|
were declared with the C++11 <code>final</code> specifier,
|
or, if possible, its type were
|
declared in an anonymous namespace or with the <code>final</code> specifier.
|
This warning is
|
more effective with link time optimization, where the information about the
|
class hierarchy graph is more complete. It is recommended to first consider
|
suggestions of <samp>-Wsuggest-final-types</samp> and then rebuild with new
|
annotations.
|
</p>
|
</dd>
|
<dt><code>-Wsuggest-override</code></dt>
|
<dd><p>Warn about overriding virtual functions that are not marked with the override
|
keyword.
|
</p>
|
</dd>
|
<dt><code>-Warray-bounds</code></dt>
|
<dt><code>-Warray-bounds=<var>n</var></code></dt>
|
<dd><a name="index-Wno_002darray_002dbounds"></a>
|
<a name="index-Warray_002dbounds"></a>
|
<p>This option is only active when <samp>-ftree-vrp</samp> is active
|
(default for <samp>-O2</samp> and above). It warns about subscripts to arrays
|
that are always out of bounds. This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Warray-bounds=1</code></dt>
|
<dd><p>This is the warning level of <samp>-Warray-bounds</samp> and is enabled
|
by <samp>-Wall</samp>; higher levels are not, and must be explicitly requested.
|
</p>
|
</dd>
|
<dt><code>-Warray-bounds=2</code></dt>
|
<dd><p>This warning level also warns about out of bounds access for
|
arrays at the end of a struct and for arrays accessed through
|
pointers. This warning level may give a larger number of
|
false positives and is deactivated by default.
|
</p></dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wbool-compare</code></dt>
|
<dd><a name="index-Wno_002dbool_002dcompare"></a>
|
<a name="index-Wbool_002dcompare"></a>
|
<p>Warn about boolean expression compared with an integer value different from
|
<code>true</code>/<code>false</code>. For instance, the following comparison is
|
always false:
|
</p><div class="smallexample">
|
<pre class="smallexample">int n = 5;
|
…
|
if ((n > 1) == 2) { … }
|
</pre></div>
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wduplicated-cond</code></dt>
|
<dd><a name="index-Wno_002dduplicated_002dcond"></a>
|
<a name="index-Wduplicated_002dcond"></a>
|
<p>Warn about duplicated conditions in an if-else-if chain. For instance,
|
warn for the following code:
|
</p><div class="smallexample">
|
<pre class="smallexample">if (p->q != NULL) { … }
|
else if (p->q != NULL) { … }
|
</pre></div>
|
|
</dd>
|
<dt><code>-Wframe-address</code></dt>
|
<dd><a name="index-Wno_002dframe_002daddress"></a>
|
<a name="index-Wframe_002daddress"></a>
|
<p>Warn when the ‘<samp>__builtin_frame_address</samp>’ or ‘<samp>__builtin_return_address</samp>’
|
is called with an argument greater than 0. Such calls may return indeterminate
|
values or crash the program. The warning is included in <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wno-discarded-qualifiers <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002ddiscarded_002dqualifiers"></a>
|
<a name="index-Wdiscarded_002dqualifiers"></a>
|
<p>Do not warn if type qualifiers on pointers are being discarded.
|
Typically, the compiler warns if a <code>const char *</code> variable is
|
passed to a function that takes a <code>char *</code> parameter. This option
|
can be used to suppress such a warning.
|
</p>
|
</dd>
|
<dt><code>-Wno-discarded-array-qualifiers <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002ddiscarded_002darray_002dqualifiers"></a>
|
<a name="index-Wdiscarded_002darray_002dqualifiers"></a>
|
<p>Do not warn if type qualifiers on arrays which are pointer targets
|
are being discarded. Typically, the compiler warns if a
|
<code>const int (*)[]</code> variable is passed to a function that
|
takes a <code>int (*)[]</code> parameter. This option can be used to
|
suppress such a warning.
|
</p>
|
</dd>
|
<dt><code>-Wno-incompatible-pointer-types <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002dincompatible_002dpointer_002dtypes"></a>
|
<a name="index-Wincompatible_002dpointer_002dtypes"></a>
|
<p>Do not warn when there is a conversion between pointers that have incompatible
|
types. This warning is for cases not covered by <samp>-Wno-pointer-sign</samp>,
|
which warns for pointer argument passing or assignment with different
|
signedness.
|
</p>
|
</dd>
|
<dt><code>-Wno-int-conversion <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002dint_002dconversion"></a>
|
<a name="index-Wint_002dconversion"></a>
|
<p>Do not warn about incompatible integer to pointer and pointer to integer
|
conversions. This warning is about implicit conversions; for explicit
|
conversions the warnings <samp>-Wno-int-to-pointer-cast</samp> and
|
<samp>-Wno-pointer-to-int-cast</samp> may be used.
|
</p>
|
</dd>
|
<dt><code>-Wno-div-by-zero</code></dt>
|
<dd><a name="index-Wno_002ddiv_002dby_002dzero"></a>
|
<a name="index-Wdiv_002dby_002dzero"></a>
|
<p>Do not warn about compile-time integer division by zero. Floating-point
|
division by zero is not warned about, as it can be a legitimate way of
|
obtaining infinities and NaNs.
|
</p>
|
</dd>
|
<dt><code>-Wsystem-headers</code></dt>
|
<dd><a name="index-Wsystem_002dheaders"></a>
|
<a name="index-Wno_002dsystem_002dheaders"></a>
|
<a name="index-warnings-from-system-headers"></a>
|
<a name="index-system-headers_002c-warnings-from"></a>
|
<p>Print warning messages for constructs found in system header files.
|
Warnings from system headers are normally suppressed, on the assumption
|
that they usually do not indicate real problems and would only make the
|
compiler output harder to read. Using this command-line option tells
|
GCC to emit warnings from system headers as if they occurred in user
|
code. However, note that using <samp>-Wall</samp> in conjunction with this
|
option does <em>not</em> warn about unknown pragmas in system
|
headers—for that, <samp>-Wunknown-pragmas</samp> must also be used.
|
</p>
|
</dd>
|
<dt><code>-Wtautological-compare</code></dt>
|
<dd><a name="index-Wtautological_002dcompare"></a>
|
<a name="index-Wno_002dtautological_002dcompare"></a>
|
<p>Warn if a self-comparison always evaluates to true or false. This
|
warning detects various mistakes such as:
|
</p><div class="smallexample">
|
<pre class="smallexample">int i = 1;
|
…
|
if (i > i) { … }
|
</pre></div>
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wtrampolines</code></dt>
|
<dd><a name="index-Wtrampolines"></a>
|
<a name="index-Wno_002dtrampolines"></a>
|
<p>Warn about trampolines generated for pointers to nested functions.
|
A trampoline is a small piece of data or code that is created at run
|
time on the stack when the address of a nested function is taken, and is
|
used to call the nested function indirectly. For some targets, it is
|
made up of data only and thus requires no special treatment. But, for
|
most targets, it is made up of code and thus requires the stack to be
|
made executable in order for the program to work properly.
|
</p>
|
</dd>
|
<dt><code>-Wfloat-equal</code></dt>
|
<dd><a name="index-Wfloat_002dequal"></a>
|
<a name="index-Wno_002dfloat_002dequal"></a>
|
<p>Warn if floating-point values are used in equality comparisons.
|
</p>
|
<p>The idea behind this is that sometimes it is convenient (for the
|
programmer) to consider floating-point values as approximations to
|
infinitely precise real numbers. If you are doing this, then you need
|
to compute (by analyzing the code, or in some other way) the maximum or
|
likely maximum error that the computation introduces, and allow for it
|
when performing comparisons (and when producing output, but that’s a
|
different problem). In particular, instead of testing for equality, you
|
should check to see whether the two values have ranges that overlap; and
|
this is done with the relational operators, so equality comparisons are
|
probably mistaken.
|
</p>
|
</dd>
|
<dt><code>-Wtraditional <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wtraditional"></a>
|
<a name="index-Wno_002dtraditional"></a>
|
<p>Warn about certain constructs that behave differently in traditional and
|
ISO C. Also warn about ISO C constructs that have no traditional C
|
equivalent, and/or problematic constructs that should be avoided.
|
</p>
|
<ul>
|
<li> Macro parameters that appear within string literals in the macro body.
|
In traditional C macro replacement takes place within string literals,
|
but in ISO C it does not.
|
|
</li><li> In traditional C, some preprocessor directives did not exist.
|
Traditional preprocessors only considered a line to be a directive
|
if the ‘<samp>#</samp>’ appeared in column 1 on the line. Therefore
|
<samp>-Wtraditional</samp> warns about directives that traditional C
|
understands but ignores because the ‘<samp>#</samp>’ does not appear as the
|
first character on the line. It also suggests you hide directives like
|
<code>#pragma</code> not understood by traditional C by indenting them. Some
|
traditional implementations do not recognize <code>#elif</code>, so this option
|
suggests avoiding it altogether.
|
|
</li><li> A function-like macro that appears without arguments.
|
|
</li><li> The unary plus operator.
|
|
</li><li> The ‘<samp>U</samp>’ integer constant suffix, or the ‘<samp>F</samp>’ or ‘<samp>L</samp>’ floating-point
|
constant suffixes. (Traditional C does support the ‘<samp>L</samp>’ suffix on integer
|
constants.) Note, these suffixes appear in macros defined in the system
|
headers of most modern systems, e.g. the ‘<samp>_MIN</samp>’/‘<samp>_MAX</samp>’ macros in <code><limits.h></code>.
|
Use of these macros in user code might normally lead to spurious
|
warnings, however GCC’s integrated preprocessor has enough context to
|
avoid warning in these cases.
|
|
</li><li> A function declared external in one block and then used after the end of
|
the block.
|
|
</li><li> A <code>switch</code> statement has an operand of type <code>long</code>.
|
|
</li><li> A non-<code>static</code> function declaration follows a <code>static</code> one.
|
This construct is not accepted by some traditional C compilers.
|
|
</li><li> The ISO type of an integer constant has a different width or
|
signedness from its traditional type. This warning is only issued if
|
the base of the constant is ten. I.e. hexadecimal or octal values, which
|
typically represent bit patterns, are not warned about.
|
|
</li><li> Usage of ISO string concatenation is detected.
|
|
</li><li> Initialization of automatic aggregates.
|
|
</li><li> Identifier conflicts with labels. Traditional C lacks a separate
|
namespace for labels.
|
|
</li><li> Initialization of unions. If the initializer is zero, the warning is
|
omitted. This is done under the assumption that the zero initializer in
|
user code appears conditioned on e.g. <code>__STDC__</code> to avoid missing
|
initializer warnings and relies on default initialization to zero in the
|
traditional C case.
|
|
</li><li> Conversions by prototypes between fixed/floating-point values and vice
|
versa. The absence of these prototypes when compiling with traditional
|
C causes serious problems. This is a subset of the possible
|
conversion warnings; for the full set use <samp>-Wtraditional-conversion</samp>.
|
|
</li><li> Use of ISO C style function definitions. This warning intentionally is
|
<em>not</em> issued for prototype declarations or variadic functions
|
because these ISO C features appear in your code when using
|
libiberty’s traditional C compatibility macros, <code>PARAMS</code> and
|
<code>VPARAMS</code>. This warning is also bypassed for nested functions
|
because that feature is already a GCC extension and thus not relevant to
|
traditional C compatibility.
|
</li></ul>
|
|
</dd>
|
<dt><code>-Wtraditional-conversion <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wtraditional_002dconversion"></a>
|
<a name="index-Wno_002dtraditional_002dconversion"></a>
|
<p>Warn if a prototype causes a type conversion that is different from what
|
would happen to the same argument in the absence of a prototype. This
|
includes conversions of fixed point to floating and vice versa, and
|
conversions changing the width or signedness of a fixed-point argument
|
except when the same as the default promotion.
|
</p>
|
</dd>
|
<dt><code>-Wdeclaration-after-statement <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wdeclaration_002dafter_002dstatement"></a>
|
<a name="index-Wno_002ddeclaration_002dafter_002dstatement"></a>
|
<p>Warn when a declaration is found after a statement in a block. This
|
construct, known from C++, was introduced with ISO C99 and is by default
|
allowed in GCC. It is not supported by ISO C90. See <a href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>.
|
</p>
|
</dd>
|
<dt><code>-Wundef</code></dt>
|
<dd><a name="index-Wundef"></a>
|
<a name="index-Wno_002dundef"></a>
|
<p>Warn if an undefined identifier is evaluated in an <code>#if</code> directive.
|
</p>
|
</dd>
|
<dt><code>-Wno-endif-labels</code></dt>
|
<dd><a name="index-Wno_002dendif_002dlabels"></a>
|
<a name="index-Wendif_002dlabels"></a>
|
<p>Do not warn whenever an <code>#else</code> or an <code>#endif</code> are followed by text.
|
</p>
|
</dd>
|
<dt><code>-Wshadow</code></dt>
|
<dd><a name="index-Wshadow"></a>
|
<a name="index-Wno_002dshadow"></a>
|
<p>Warn whenever a local variable or type declaration shadows another
|
variable, parameter, type, class member (in C++), or instance variable
|
(in Objective-C) or whenever a built-in function is shadowed. Note
|
that in C++, the compiler warns if a local variable shadows an
|
explicit typedef, but not if it shadows a struct/class/enum.
|
</p>
|
</dd>
|
<dt><code>-Wno-shadow-ivar <span class="roman">(Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002dshadow_002divar"></a>
|
<a name="index-Wshadow_002divar"></a>
|
<p>Do not warn whenever a local variable shadows an instance variable in an
|
Objective-C method.
|
</p>
|
</dd>
|
<dt><code>-Wlarger-than=<var>len</var></code></dt>
|
<dd><a name="index-Wlarger_002dthan_003dlen"></a>
|
<a name="index-Wlarger_002dthan_002dlen"></a>
|
<p>Warn whenever an object of larger than <var>len</var> bytes is defined.
|
</p>
|
</dd>
|
<dt><code>-Wframe-larger-than=<var>len</var></code></dt>
|
<dd><a name="index-Wframe_002dlarger_002dthan"></a>
|
<p>Warn if the size of a function frame is larger than <var>len</var> bytes.
|
The computation done to determine the stack frame size is approximate
|
and not conservative.
|
The actual requirements may be somewhat greater than <var>len</var>
|
even if you do not get a warning. In addition, any space allocated
|
via <code>alloca</code>, variable-length arrays, or related constructs
|
is not included by the compiler when determining
|
whether or not to issue a warning.
|
</p>
|
</dd>
|
<dt><code>-Wno-free-nonheap-object</code></dt>
|
<dd><a name="index-Wno_002dfree_002dnonheap_002dobject"></a>
|
<a name="index-Wfree_002dnonheap_002dobject"></a>
|
<p>Do not warn when attempting to free an object that was not allocated
|
on the heap.
|
</p>
|
</dd>
|
<dt><code>-Wstack-usage=<var>len</var></code></dt>
|
<dd><a name="index-Wstack_002dusage"></a>
|
<p>Warn if the stack usage of a function might be larger than <var>len</var> bytes.
|
The computation done to determine the stack usage is conservative.
|
Any space allocated via <code>alloca</code>, variable-length arrays, or related
|
constructs is included by the compiler when determining whether or not to
|
issue a warning.
|
</p>
|
<p>The message is in keeping with the output of <samp>-fstack-usage</samp>.
|
</p>
|
<ul>
|
<li> If the stack usage is fully static but exceeds the specified amount, it’s:
|
|
<div class="smallexample">
|
<pre class="smallexample"> warning: stack usage is 1120 bytes
|
</pre></div>
|
</li><li> If the stack usage is (partly) dynamic but bounded, it’s:
|
|
<div class="smallexample">
|
<pre class="smallexample"> warning: stack usage might be 1648 bytes
|
</pre></div>
|
</li><li> If the stack usage is (partly) dynamic and not bounded, it’s:
|
|
<div class="smallexample">
|
<pre class="smallexample"> warning: stack usage might be unbounded
|
</pre></div>
|
</li></ul>
|
|
</dd>
|
<dt><code>-Wunsafe-loop-optimizations</code></dt>
|
<dd><a name="index-Wunsafe_002dloop_002doptimizations"></a>
|
<a name="index-Wno_002dunsafe_002dloop_002doptimizations"></a>
|
<p>Warn if the loop cannot be optimized because the compiler cannot
|
assume anything on the bounds of the loop indices. With
|
<samp>-funsafe-loop-optimizations</samp> warn if the compiler makes
|
such assumptions.
|
</p>
|
</dd>
|
<dt><code>-Wno-pedantic-ms-format <span class="roman">(MinGW targets only)</span></code></dt>
|
<dd><a name="index-Wno_002dpedantic_002dms_002dformat"></a>
|
<a name="index-Wpedantic_002dms_002dformat"></a>
|
<p>When used in combination with <samp>-Wformat</samp>
|
and <samp>-pedantic</samp> without GNU extensions, this option
|
disables the warnings about non-ISO <code>printf</code> / <code>scanf</code> format
|
width specifiers <code>I32</code>, <code>I64</code>, and <code>I</code> used on Windows targets,
|
which depend on the MS runtime.
|
</p>
|
</dd>
|
<dt><code>-Wplacement-new</code></dt>
|
<dt><code>-Wplacement-new=<var>n</var></code></dt>
|
<dd><a name="index-Wplacement_002dnew"></a>
|
<a name="index-Wno_002dplacement_002dnew"></a>
|
<p>Warn about placement new expressions with undefined behavior, such as
|
constructing an object in a buffer that is smaller than the type of
|
the object. For example, the placement new expression below is diagnosed
|
because it attempts to construct an array of 64 integers in a buffer only
|
64 bytes large.
|
</p><div class="smallexample">
|
<pre class="smallexample">char buf [64];
|
new (buf) int[64];
|
</pre></div>
|
<p>This warning is enabled by default.
|
</p>
|
<dl compact="compact">
|
<dt><code>-Wplacement-new=1</code></dt>
|
<dd><p>This is the default warning level of <samp>-Wplacement-new</samp>. At this
|
level the warning is not issued for some strictly undefined constructs that
|
GCC allows as extensions for compatibility with legacy code. For example,
|
the following <code>new</code> expression is not diagnosed at this level even
|
though it has undefined behavior according to the C++ standard because
|
it writes past the end of the one-element array.
|
</p><div class="smallexample">
|
<pre class="smallexample">struct S { int n, a[1]; };
|
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
|
new (s->a)int [32]();
|
</pre></div>
|
|
</dd>
|
<dt><code>-Wplacement-new=2</code></dt>
|
<dd><p>At this level, in addition to diagnosing all the same constructs as at level
|
1, a diagnostic is also issued for placement new expressions that construct
|
an object in the last member of structure whose type is an array of a single
|
element and whose size is less than the size of the object being constructed.
|
While the previous example would be diagnosed, the following construct makes
|
use of the flexible member array extension to avoid the warning at level 2.
|
</p><div class="smallexample">
|
<pre class="smallexample">struct S { int n, a[]; };
|
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
|
new (s->a)int [32]();
|
</pre></div>
|
|
</dd>
|
</dl>
|
|
</dd>
|
<dt><code>-Wpointer-arith</code></dt>
|
<dd><a name="index-Wpointer_002darith"></a>
|
<a name="index-Wno_002dpointer_002darith"></a>
|
<p>Warn about anything that depends on the “size of” a function type or
|
of <code>void</code>. GNU C assigns these types a size of 1, for
|
convenience in calculations with <code>void *</code> pointers and pointers
|
to functions. In C++, warn also when an arithmetic operation involves
|
<code>NULL</code>. This warning is also enabled by <samp>-Wpedantic</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wtype-limits</code></dt>
|
<dd><a name="index-Wtype_002dlimits"></a>
|
<a name="index-Wno_002dtype_002dlimits"></a>
|
<p>Warn if a comparison is always true or always false due to the limited
|
range of the data type, but do not warn for constant expressions. For
|
example, warn if an unsigned variable is compared against zero with
|
<code><</code> or <code>>=</code>. This warning is also enabled by
|
<samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wbad-function-cast <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wbad_002dfunction_002dcast"></a>
|
<a name="index-Wno_002dbad_002dfunction_002dcast"></a>
|
<p>Warn when a function call is cast to a non-matching type.
|
For example, warn if a call to a function returning an integer type
|
is cast to a pointer type.
|
</p>
|
</dd>
|
<dt><code>-Wc90-c99-compat <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wc90_002dc99_002dcompat"></a>
|
<a name="index-Wno_002dc90_002dc99_002dcompat"></a>
|
<p>Warn about features not present in ISO C90, but present in ISO C99.
|
For instance, warn about use of variable length arrays, <code>long long</code>
|
type, <code>bool</code> type, compound literals, designated initializers, and so
|
on. This option is independent of the standards mode. Warnings are disabled
|
in the expression that follows <code>__extension__</code>.
|
</p>
|
</dd>
|
<dt><code>-Wc99-c11-compat <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wc99_002dc11_002dcompat"></a>
|
<a name="index-Wno_002dc99_002dc11_002dcompat"></a>
|
<p>Warn about features not present in ISO C99, but present in ISO C11.
|
For instance, warn about use of anonymous structures and unions,
|
<code>_Atomic</code> type qualifier, <code>_Thread_local</code> storage-class specifier,
|
<code>_Alignas</code> specifier, <code>Alignof</code> operator, <code>_Generic</code> keyword,
|
and so on. This option is independent of the standards mode. Warnings are
|
disabled in the expression that follows <code>__extension__</code>.
|
</p>
|
</dd>
|
<dt><code>-Wc++-compat <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wc_002b_002b_002dcompat"></a>
|
<p>Warn about ISO C constructs that are outside of the common subset of
|
ISO C and ISO C++, e.g. request for implicit conversion from
|
<code>void *</code> to a pointer to non-<code>void</code> type.
|
</p>
|
</dd>
|
<dt><code>-Wc++11-compat <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wc_002b_002b11_002dcompat"></a>
|
<p>Warn about C++ constructs whose meaning differs between ISO C++ 1998
|
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
|
in ISO C++ 2011. This warning turns on <samp>-Wnarrowing</samp> and is
|
enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wc++14-compat <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wc_002b_002b14_002dcompat"></a>
|
<p>Warn about C++ constructs whose meaning differs between ISO C++ 2011
|
and ISO C++ 2014. This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wcast-qual</code></dt>
|
<dd><a name="index-Wcast_002dqual"></a>
|
<a name="index-Wno_002dcast_002dqual"></a>
|
<p>Warn whenever a pointer is cast so as to remove a type qualifier from
|
the target type. For example, warn if a <code>const char *</code> is cast
|
to an ordinary <code>char *</code>.
|
</p>
|
<p>Also warn when making a cast that introduces a type qualifier in an
|
unsafe way. For example, casting <code>char **</code> to <code>const char **</code>
|
is unsafe, as in this example:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample"> /* p is char ** value. */
|
const char **q = (const char **) p;
|
/* Assignment of readonly string to const char * is OK. */
|
*q = "string";
|
/* Now char** pointer points to read-only memory. */
|
**p = 'b';
|
</pre></div>
|
|
</dd>
|
<dt><code>-Wcast-align</code></dt>
|
<dd><a name="index-Wcast_002dalign"></a>
|
<a name="index-Wno_002dcast_002dalign"></a>
|
<p>Warn whenever a pointer is cast such that the required alignment of the
|
target is increased. For example, warn if a <code>char *</code> is cast to
|
an <code>int *</code> on machines where integers can only be accessed at
|
two- or four-byte boundaries.
|
</p>
|
</dd>
|
<dt><code>-Wwrite-strings</code></dt>
|
<dd><a name="index-Wwrite_002dstrings"></a>
|
<a name="index-Wno_002dwrite_002dstrings"></a>
|
<p>When compiling C, give string constants the type <code>const
|
char[<var>length</var>]</code> so that copying the address of one into a
|
non-<code>const</code> <code>char *</code> pointer produces a warning. These
|
warnings help you find at compile time code that can try to write
|
into a string constant, but only if you have been very careful about
|
using <code>const</code> in declarations and prototypes. Otherwise, it is
|
just a nuisance. This is why we did not make <samp>-Wall</samp> request
|
these warnings.
|
</p>
|
<p>When compiling C++, warn about the deprecated conversion from string
|
literals to <code>char *</code>. This warning is enabled by default for C++
|
programs.
|
</p>
|
</dd>
|
<dt><code>-Wclobbered</code></dt>
|
<dd><a name="index-Wclobbered"></a>
|
<a name="index-Wno_002dclobbered"></a>
|
<p>Warn for variables that might be changed by <code>longjmp</code> or
|
<code>vfork</code>. This warning is also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wconditionally-supported <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wconditionally_002dsupported"></a>
|
<a name="index-Wno_002dconditionally_002dsupported"></a>
|
<p>Warn for conditionally-supported (C++11 [intro.defs]) constructs.
|
</p>
|
</dd>
|
<dt><code>-Wconversion</code></dt>
|
<dd><a name="index-Wconversion"></a>
|
<a name="index-Wno_002dconversion"></a>
|
<p>Warn for implicit conversions that may alter a value. This includes
|
conversions between real and integer, like <code>abs (x)</code> when
|
<code>x</code> is <code>double</code>; conversions between signed and unsigned,
|
like <code>unsigned ui = -1</code>; and conversions to smaller types, like
|
<code>sqrtf (M_PI)</code>. Do not warn for explicit casts like <code>abs
|
((int) x)</code> and <code>ui = (unsigned) -1</code>, or if the value is not
|
changed by the conversion like in <code>abs (2.0)</code>. Warnings about
|
conversions between signed and unsigned integers can be disabled by
|
using <samp>-Wno-sign-conversion</samp>.
|
</p>
|
<p>For C++, also warn for confusing overload resolution for user-defined
|
conversions; and conversions that never use a type conversion
|
operator: conversions to <code>void</code>, the same type, a base class or a
|
reference to them. Warnings about conversions between signed and
|
unsigned integers are disabled by default in C++ unless
|
<samp>-Wsign-conversion</samp> is explicitly enabled.
|
</p>
|
</dd>
|
<dt><code>-Wno-conversion-null <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wconversion_002dnull"></a>
|
<a name="index-Wno_002dconversion_002dnull"></a>
|
<p>Do not warn for conversions between <code>NULL</code> and non-pointer
|
types. <samp>-Wconversion-null</samp> is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wzero-as-null-pointer-constant <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wzero_002das_002dnull_002dpointer_002dconstant"></a>
|
<a name="index-Wno_002dzero_002das_002dnull_002dpointer_002dconstant"></a>
|
<p>Warn when a literal ‘<samp>0</samp>’ is used as null pointer constant. This can
|
be useful to facilitate the conversion to <code>nullptr</code> in C++11.
|
</p>
|
</dd>
|
<dt><code>-Wsubobject-linkage <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wsubobject_002dlinkage"></a>
|
<a name="index-Wno_002dsubobject_002dlinkage"></a>
|
<p>Warn if a class type has a base or a field whose type uses the anonymous
|
namespace or depends on a type with no linkage. If a type A depends on
|
a type B with no or internal linkage, defining it in multiple
|
translation units would be an ODR violation because the meaning of B
|
is different in each translation unit. If A only appears in a single
|
translation unit, the best way to silence the warning is to give it
|
internal linkage by putting it in an anonymous namespace as well. The
|
compiler doesn’t give this warning for types defined in the main .C
|
file, as those are unlikely to have multiple definitions.
|
<samp>-Wsubobject-linkage</samp> is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wdate-time</code></dt>
|
<dd><a name="index-Wdate_002dtime"></a>
|
<a name="index-Wno_002ddate_002dtime"></a>
|
<p>Warn when macros <code>__TIME__</code>, <code>__DATE__</code> or <code>__TIMESTAMP__</code>
|
are encountered as they might prevent bit-wise-identical reproducible
|
compilations.
|
</p>
|
</dd>
|
<dt><code>-Wdelete-incomplete <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wdelete_002dincomplete"></a>
|
<a name="index-Wno_002ddelete_002dincomplete"></a>
|
<p>Warn when deleting a pointer to incomplete type, which may cause
|
undefined behavior at runtime. This warning is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wuseless-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wuseless_002dcast"></a>
|
<a name="index-Wno_002duseless_002dcast"></a>
|
<p>Warn when an expression is casted to its own type.
|
</p>
|
</dd>
|
<dt><code>-Wempty-body</code></dt>
|
<dd><a name="index-Wempty_002dbody"></a>
|
<a name="index-Wno_002dempty_002dbody"></a>
|
<p>Warn if an empty body occurs in an <code>if</code>, <code>else</code> or <code>do
|
while</code> statement. This warning is also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wenum-compare</code></dt>
|
<dd><a name="index-Wenum_002dcompare"></a>
|
<a name="index-Wno_002denum_002dcompare"></a>
|
<p>Warn about a comparison between values of different enumerated types.
|
In C++ enumeral mismatches in conditional expressions are also
|
diagnosed and the warning is enabled by default. In C this warning is
|
enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wjump-misses-init <span class="roman">(C, Objective-C only)</span></code></dt>
|
<dd><a name="index-Wjump_002dmisses_002dinit"></a>
|
<a name="index-Wno_002djump_002dmisses_002dinit"></a>
|
<p>Warn if a <code>goto</code> statement or a <code>switch</code> statement jumps
|
forward across the initialization of a variable, or jumps backward to a
|
label after the variable has been initialized. This only warns about
|
variables that are initialized when they are declared. This warning is
|
only supported for C and Objective-C; in C++ this sort of branch is an
|
error in any case.
|
</p>
|
<p><samp>-Wjump-misses-init</samp> is included in <samp>-Wc++-compat</samp>. It
|
can be disabled with the <samp>-Wno-jump-misses-init</samp> option.
|
</p>
|
</dd>
|
<dt><code>-Wsign-compare</code></dt>
|
<dd><a name="index-Wsign_002dcompare"></a>
|
<a name="index-Wno_002dsign_002dcompare"></a>
|
<a name="index-warning-for-comparison-of-signed-and-unsigned-values"></a>
|
<a name="index-comparison-of-signed-and-unsigned-values_002c-warning"></a>
|
<a name="index-signed-and-unsigned-values_002c-comparison-warning"></a>
|
<p>Warn when a comparison between signed and unsigned values could produce
|
an incorrect result when the signed value is converted to unsigned.
|
In C++, this warning is also enabled by <samp>-Wall</samp>. In C, it is
|
also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wsign-conversion</code></dt>
|
<dd><a name="index-Wsign_002dconversion"></a>
|
<a name="index-Wno_002dsign_002dconversion"></a>
|
<p>Warn for implicit conversions that may change the sign of an integer
|
value, like assigning a signed integer expression to an unsigned
|
integer variable. An explicit cast silences the warning. In C, this
|
option is enabled also by <samp>-Wconversion</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wfloat-conversion</code></dt>
|
<dd><a name="index-Wfloat_002dconversion"></a>
|
<a name="index-Wno_002dfloat_002dconversion"></a>
|
<p>Warn for implicit conversions that reduce the precision of a real value.
|
This includes conversions from real to integer, and from higher precision
|
real to lower precision real values. This option is also enabled by
|
<samp>-Wconversion</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wno-scalar-storage-order</code></dt>
|
<dd><a name="index-_002dWno_002dscalar_002dstorage_002dorder"></a>
|
<a name="index-_002dWscalar_002dstorage_002dorder"></a>
|
<p>Do not warn on suspicious constructs involving reverse scalar storage order.
|
</p>
|
</dd>
|
<dt><code>-Wsized-deallocation <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wsized_002ddeallocation"></a>
|
<a name="index-Wno_002dsized_002ddeallocation"></a>
|
<p>Warn about a definition of an unsized deallocation function
|
</p><div class="smallexample">
|
<pre class="smallexample">void operator delete (void *) noexcept;
|
void operator delete[] (void *) noexcept;
|
</pre></div>
|
<p>without a definition of the corresponding sized deallocation function
|
</p><div class="smallexample">
|
<pre class="smallexample">void operator delete (void *, std::size_t) noexcept;
|
void operator delete[] (void *, std::size_t) noexcept;
|
</pre></div>
|
<p>or vice versa. Enabled by <samp>-Wextra</samp> along with
|
<samp>-fsized-deallocation</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wsizeof-pointer-memaccess</code></dt>
|
<dd><a name="index-Wsizeof_002dpointer_002dmemaccess"></a>
|
<a name="index-Wno_002dsizeof_002dpointer_002dmemaccess"></a>
|
<p>Warn for suspicious length parameters to certain string and memory built-in
|
functions if the argument uses <code>sizeof</code>. This warning warns e.g.
|
about <code>memset (ptr, 0, sizeof (ptr));</code> if <code>ptr</code> is not an array,
|
but a pointer, and suggests a possible fix, or about
|
<code>memcpy (&foo, ptr, sizeof (&foo));</code>. This warning is enabled by
|
<samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wsizeof-array-argument</code></dt>
|
<dd><a name="index-Wsizeof_002darray_002dargument"></a>
|
<a name="index-Wno_002dsizeof_002darray_002dargument"></a>
|
<p>Warn when the <code>sizeof</code> operator is applied to a parameter that is
|
declared as an array in a function definition. This warning is enabled by
|
default for C and C++ programs.
|
</p>
|
</dd>
|
<dt><code>-Wmemset-transposed-args</code></dt>
|
<dd><a name="index-Wmemset_002dtransposed_002dargs"></a>
|
<a name="index-Wno_002dmemset_002dtransposed_002dargs"></a>
|
<p>Warn for suspicious calls to the <code>memset</code> built-in function, if the
|
second argument is not zero and the third argument is zero. This warns e.g. about <code>memset (buf, sizeof buf, 0)</code> where most probably
|
<code>memset (buf, 0, sizeof buf)</code> was meant instead. The diagnostics
|
is only emitted if the third argument is literal zero. If it is some
|
expression that is folded to zero, a cast of zero to some type, etc.,
|
it is far less likely that the user has mistakenly exchanged the arguments
|
and no warning is emitted. This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Waddress</code></dt>
|
<dd><a name="index-Waddress"></a>
|
<a name="index-Wno_002daddress"></a>
|
<p>Warn about suspicious uses of memory addresses. These include using
|
the address of a function in a conditional expression, such as
|
<code>void func(void); if (func)</code>, and comparisons against the memory
|
address of a string literal, such as <code>if (x == "abc")</code>. Such
|
uses typically indicate a programmer error: the address of a function
|
always evaluates to true, so their use in a conditional usually
|
indicate that the programmer forgot the parentheses in a function
|
call; and comparisons against string literals result in unspecified
|
behavior and are not portable in C, so they usually indicate that the
|
programmer intended to use <code>strcmp</code>. This warning is enabled by
|
<samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wlogical-op</code></dt>
|
<dd><a name="index-Wlogical_002dop"></a>
|
<a name="index-Wno_002dlogical_002dop"></a>
|
<p>Warn about suspicious uses of logical operators in expressions.
|
This includes using logical operators in contexts where a
|
bit-wise operator is likely to be expected. Also warns when
|
the operands of a logical operator are the same:
|
</p><div class="smallexample">
|
<pre class="smallexample">extern int a;
|
if (a < 0 && a < 0) { … }
|
</pre></div>
|
|
</dd>
|
<dt><code>-Wlogical-not-parentheses</code></dt>
|
<dd><a name="index-Wlogical_002dnot_002dparentheses"></a>
|
<a name="index-Wno_002dlogical_002dnot_002dparentheses"></a>
|
<p>Warn about logical not used on the left hand side operand of a comparison.
|
This option does not warn if the RHS operand is of a boolean type. Its
|
purpose is to detect suspicious code like the following:
|
</p><div class="smallexample">
|
<pre class="smallexample">int a;
|
…
|
if (!a > 1) { … }
|
</pre></div>
|
|
<p>It is possible to suppress the warning by wrapping the LHS into
|
parentheses:
|
</p><div class="smallexample">
|
<pre class="smallexample">if ((!a) > 1) { … }
|
</pre></div>
|
|
<p>This warning is enabled by <samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Waggregate-return</code></dt>
|
<dd><a name="index-Waggregate_002dreturn"></a>
|
<a name="index-Wno_002daggregate_002dreturn"></a>
|
<p>Warn if any functions that return structures or unions are defined or
|
called. (In languages where you can return an array, this also elicits
|
a warning.)
|
</p>
|
</dd>
|
<dt><code>-Wno-aggressive-loop-optimizations</code></dt>
|
<dd><a name="index-Wno_002daggressive_002dloop_002doptimizations"></a>
|
<a name="index-Waggressive_002dloop_002doptimizations"></a>
|
<p>Warn if in a loop with constant number of iterations the compiler detects
|
undefined behavior in some statement during one or more of the iterations.
|
</p>
|
</dd>
|
<dt><code>-Wno-attributes</code></dt>
|
<dd><a name="index-Wno_002dattributes"></a>
|
<a name="index-Wattributes"></a>
|
<p>Do not warn if an unexpected <code>__attribute__</code> is used, such as
|
unrecognized attributes, function attributes applied to variables,
|
etc. This does not stop errors for incorrect use of supported
|
attributes.
|
</p>
|
</dd>
|
<dt><code>-Wno-builtin-macro-redefined</code></dt>
|
<dd><a name="index-Wno_002dbuiltin_002dmacro_002dredefined"></a>
|
<a name="index-Wbuiltin_002dmacro_002dredefined"></a>
|
<p>Do not warn if certain built-in macros are redefined. This suppresses
|
warnings for redefinition of <code>__TIMESTAMP__</code>, <code>__TIME__</code>,
|
<code>__DATE__</code>, <code>__FILE__</code>, and <code>__BASE_FILE__</code>.
|
</p>
|
</dd>
|
<dt><code>-Wstrict-prototypes <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wstrict_002dprototypes"></a>
|
<a name="index-Wno_002dstrict_002dprototypes"></a>
|
<p>Warn if a function is declared or defined without specifying the
|
argument types. (An old-style function definition is permitted without
|
a warning if preceded by a declaration that specifies the argument
|
types.)
|
</p>
|
</dd>
|
<dt><code>-Wold-style-declaration <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wold_002dstyle_002ddeclaration"></a>
|
<a name="index-Wno_002dold_002dstyle_002ddeclaration"></a>
|
<p>Warn for obsolescent usages, according to the C Standard, in a
|
declaration. For example, warn if storage-class specifiers like
|
<code>static</code> are not the first things in a declaration. This warning
|
is also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wold-style-definition <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wold_002dstyle_002ddefinition"></a>
|
<a name="index-Wno_002dold_002dstyle_002ddefinition"></a>
|
<p>Warn if an old-style function definition is used. A warning is given
|
even if there is a previous prototype.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-parameter-type <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wmissing_002dparameter_002dtype"></a>
|
<a name="index-Wno_002dmissing_002dparameter_002dtype"></a>
|
<p>A function parameter is declared without a type specifier in K&R-style
|
functions:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">void foo(bar) { }
|
</pre></div>
|
|
<p>This warning is also enabled by <samp>-Wextra</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-prototypes <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wmissing_002dprototypes"></a>
|
<a name="index-Wno_002dmissing_002dprototypes"></a>
|
<p>Warn if a global function is defined without a previous prototype
|
declaration. This warning is issued even if the definition itself
|
provides a prototype. Use this option to detect global functions
|
that do not have a matching prototype declaration in a header file.
|
This option is not valid for C++ because all function declarations
|
provide prototypes and a non-matching declaration declares an
|
overload rather than conflict with an earlier declaration.
|
Use <samp>-Wmissing-declarations</samp> to detect missing declarations in C++.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-declarations</code></dt>
|
<dd><a name="index-Wmissing_002ddeclarations"></a>
|
<a name="index-Wno_002dmissing_002ddeclarations"></a>
|
<p>Warn if a global function is defined without a previous declaration.
|
Do so even if the definition itself provides a prototype.
|
Use this option to detect global functions that are not declared in
|
header files. In C, no warnings are issued for functions with previous
|
non-prototype declarations; use <samp>-Wmissing-prototypes</samp> to detect
|
missing prototypes. In C++, no warnings are issued for function templates,
|
or for inline functions, or for functions in anonymous namespaces.
|
</p>
|
</dd>
|
<dt><code>-Wmissing-field-initializers</code></dt>
|
<dd><a name="index-Wmissing_002dfield_002dinitializers"></a>
|
<a name="index-Wno_002dmissing_002dfield_002dinitializers"></a>
|
<a name="index-W-1"></a>
|
<a name="index-Wextra-1"></a>
|
<a name="index-Wno_002dextra-1"></a>
|
<p>Warn if a structure’s initializer has some fields missing. For
|
example, the following code causes such a warning, because
|
<code>x.h</code> is implicitly zero:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct s { int f, g, h; };
|
struct s x = { 3, 4 };
|
</pre></div>
|
|
<p>This option does not warn about designated initializers, so the following
|
modification does not trigger a warning:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct s { int f, g, h; };
|
struct s x = { .f = 3, .g = 4 };
|
</pre></div>
|
|
<p>In C++ this option does not warn either about the empty { }
|
initializer, for example:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct s { int f, g, h; };
|
s x = { };
|
</pre></div>
|
|
<p>This warning is included in <samp>-Wextra</samp>. To get other <samp>-Wextra</samp>
|
warnings without this one, use <samp>-Wextra -Wno-missing-field-initializers</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wno-multichar</code></dt>
|
<dd><a name="index-Wno_002dmultichar"></a>
|
<a name="index-Wmultichar"></a>
|
<p>Do not warn if a multicharacter constant (‘<samp>'FOOF'</samp>’) is used.
|
Usually they indicate a typo in the user’s code, as they have
|
implementation-defined values, and should not be used in portable code.
|
</p>
|
</dd>
|
<dt><code>-Wnormalized<span class="roman">[</span>=<span class="roman"><</span>none<span class="roman">|</span>id<span class="roman">|</span>nfc<span class="roman">|</span>nfkc<span class="roman">>]</span></code></dt>
|
<dd><a name="index-Wnormalized_003d"></a>
|
<a name="index-Wnormalized"></a>
|
<a name="index-Wno_002dnormalized"></a>
|
<a name="index-NFC"></a>
|
<a name="index-NFKC"></a>
|
<a name="index-character-set_002c-input-normalization"></a>
|
<p>In ISO C and ISO C++, two identifiers are different if they are
|
different sequences of characters. However, sometimes when characters
|
outside the basic ASCII character set are used, you can have two
|
different character sequences that look the same. To avoid confusion,
|
the ISO 10646 standard sets out some <em>normalization rules</em> which
|
when applied ensure that two sequences that look the same are turned into
|
the same sequence. GCC can warn you if you are using identifiers that
|
have not been normalized; this option controls that warning.
|
</p>
|
<p>There are four levels of warning supported by GCC. The default is
|
<samp>-Wnormalized=nfc</samp>, which warns about any identifier that is
|
not in the ISO 10646 “C” normalized form, <em>NFC</em>. NFC is the
|
recommended form for most uses. It is equivalent to
|
<samp>-Wnormalized</samp>.
|
</p>
|
<p>Unfortunately, there are some characters allowed in identifiers by
|
ISO C and ISO C++ that, when turned into NFC, are not allowed in
|
identifiers. That is, there’s no way to use these symbols in portable
|
ISO C or C++ and have all your identifiers in NFC.
|
<samp>-Wnormalized=id</samp> suppresses the warning for these characters.
|
It is hoped that future versions of the standards involved will correct
|
this, which is why this option is not the default.
|
</p>
|
<p>You can switch the warning off for all characters by writing
|
<samp>-Wnormalized=none</samp> or <samp>-Wno-normalized</samp>. You should
|
only do this if you are using some other normalization scheme (like
|
“D”), because otherwise you can easily create bugs that are
|
literally impossible to see.
|
</p>
|
<p>Some characters in ISO 10646 have distinct meanings but look identical
|
in some fonts or display methodologies, especially once formatting has
|
been applied. For instance <code>\u207F</code>, “SUPERSCRIPT LATIN SMALL
|
LETTER N”, displays just like a regular <code>n</code> that has been
|
placed in a superscript. ISO 10646 defines the <em>NFKC</em>
|
normalization scheme to convert all these into a standard form as
|
well, and GCC warns if your code is not in NFKC if you use
|
<samp>-Wnormalized=nfkc</samp>. This warning is comparable to warning
|
about every identifier that contains the letter O because it might be
|
confused with the digit 0, and so is not the default, but may be
|
useful as a local coding convention if the programming environment
|
cannot be fixed to display these characters distinctly.
|
</p>
|
</dd>
|
<dt><code>-Wno-deprecated</code></dt>
|
<dd><a name="index-Wno_002ddeprecated"></a>
|
<a name="index-Wdeprecated"></a>
|
<p>Do not warn about usage of deprecated features. See <a href="Deprecated-Features.html#Deprecated-Features">Deprecated Features</a>.
|
</p>
|
</dd>
|
<dt><code>-Wno-deprecated-declarations</code></dt>
|
<dd><a name="index-Wno_002ddeprecated_002ddeclarations"></a>
|
<a name="index-Wdeprecated_002ddeclarations"></a>
|
<p>Do not warn about uses of functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>),
|
variables (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>), and types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>) marked as deprecated by using the <code>deprecated</code>
|
attribute.
|
</p>
|
</dd>
|
<dt><code>-Wno-overflow</code></dt>
|
<dd><a name="index-Wno_002doverflow"></a>
|
<a name="index-Woverflow"></a>
|
<p>Do not warn about compile-time overflow in constant expressions.
|
</p>
|
</dd>
|
<dt><code>-Wno-odr</code></dt>
|
<dd><a name="index-Wno_002dodr"></a>
|
<a name="index-Wodr"></a>
|
<p>Warn about One Definition Rule violations during link-time optimization.
|
Requires <samp>-flto-odr-type-merging</samp> to be enabled. Enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wopenmp-simd</code></dt>
|
<dd><a name="index-Wopenm_002dsimd"></a>
|
<p>Warn if the vectorizer cost model overrides the OpenMP or the Cilk Plus
|
simd directive set by user. The <samp>-fsimd-cost-model=unlimited</samp>
|
option can be used to relax the cost model.
|
</p>
|
</dd>
|
<dt><code>-Woverride-init <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Woverride_002dinit"></a>
|
<a name="index-Wno_002doverride_002dinit"></a>
|
<a name="index-W-2"></a>
|
<a name="index-Wextra-2"></a>
|
<a name="index-Wno_002dextra-2"></a>
|
<p>Warn if an initialized field without side effects is overridden when
|
using designated initializers (see <a href="Designated-Inits.html#Designated-Inits">Designated
|
Initializers</a>).
|
</p>
|
<p>This warning is included in <samp>-Wextra</samp>. To get other
|
<samp>-Wextra</samp> warnings without this one, use <samp>-Wextra
|
-Wno-override-init</samp>.
|
</p>
|
</dd>
|
<dt><code>-Woverride-init-side-effects <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Woverride_002dinit_002dside_002deffects"></a>
|
<a name="index-Wno_002doverride_002dinit_002dside_002deffects"></a>
|
<p>Warn if an initialized field with side effects is overridden when
|
using designated initializers (see <a href="Designated-Inits.html#Designated-Inits">Designated
|
Initializers</a>). This warning is enabled by default.
|
</p>
|
</dd>
|
<dt><code>-Wpacked</code></dt>
|
<dd><a name="index-Wpacked"></a>
|
<a name="index-Wno_002dpacked"></a>
|
<p>Warn if a structure is given the packed attribute, but the packed
|
attribute has no effect on the layout or size of the structure.
|
Such structures may be mis-aligned for little benefit. For
|
instance, in this code, the variable <code>f.x</code> in <code>struct bar</code>
|
is misaligned even though <code>struct bar</code> does not itself
|
have the packed attribute:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct foo {
|
int x;
|
char a, b, c, d;
|
} __attribute__((packed));
|
struct bar {
|
char z;
|
struct foo f;
|
};
|
</pre></div>
|
|
</dd>
|
<dt><code>-Wpacked-bitfield-compat</code></dt>
|
<dd><a name="index-Wpacked_002dbitfield_002dcompat"></a>
|
<a name="index-Wno_002dpacked_002dbitfield_002dcompat"></a>
|
<p>The 4.1, 4.2 and 4.3 series of GCC ignore the <code>packed</code> attribute
|
on bit-fields of type <code>char</code>. This has been fixed in GCC 4.4 but
|
the change can lead to differences in the structure layout. GCC
|
informs you when the offset of such a field has changed in GCC 4.4.
|
For example there is no longer a 4-bit padding between field <code>a</code>
|
and <code>b</code> in this structure:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">struct foo
|
{
|
char a:4;
|
char b:8;
|
} __attribute__ ((packed));
|
</pre></div>
|
|
<p>This warning is enabled by default. Use
|
<samp>-Wno-packed-bitfield-compat</samp> to disable this warning.
|
</p>
|
</dd>
|
<dt><code>-Wpadded</code></dt>
|
<dd><a name="index-Wpadded"></a>
|
<a name="index-Wno_002dpadded"></a>
|
<p>Warn if padding is included in a structure, either to align an element
|
of the structure or to align the whole structure. Sometimes when this
|
happens it is possible to rearrange the fields of the structure to
|
reduce the padding and so make the structure smaller.
|
</p>
|
</dd>
|
<dt><code>-Wredundant-decls</code></dt>
|
<dd><a name="index-Wredundant_002ddecls"></a>
|
<a name="index-Wno_002dredundant_002ddecls"></a>
|
<p>Warn if anything is declared more than once in the same scope, even in
|
cases where multiple declaration is valid and changes nothing.
|
</p>
|
</dd>
|
<dt><code>-Wnested-externs <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wnested_002dexterns"></a>
|
<a name="index-Wno_002dnested_002dexterns"></a>
|
<p>Warn if an <code>extern</code> declaration is encountered within a function.
|
</p>
|
</dd>
|
<dt><code>-Wno-inherited-variadic-ctor</code></dt>
|
<dd><a name="index-Winherited_002dvariadic_002dctor"></a>
|
<a name="index-Wno_002dinherited_002dvariadic_002dctor"></a>
|
<p>Suppress warnings about use of C++11 inheriting constructors when the
|
base class inherited from has a C variadic constructor; the warning is
|
on by default because the ellipsis is not inherited.
|
</p>
|
</dd>
|
<dt><code>-Winline</code></dt>
|
<dd><a name="index-Winline"></a>
|
<a name="index-Wno_002dinline"></a>
|
<p>Warn if a function that is declared as inline cannot be inlined.
|
Even with this option, the compiler does not warn about failures to
|
inline functions declared in system headers.
|
</p>
|
<p>The compiler uses a variety of heuristics to determine whether or not
|
to inline a function. For example, the compiler takes into account
|
the size of the function being inlined and the amount of inlining
|
that has already been done in the current function. Therefore,
|
seemingly insignificant changes in the source program can cause the
|
warnings produced by <samp>-Winline</samp> to appear or disappear.
|
</p>
|
</dd>
|
<dt><code>-Wno-invalid-offsetof <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
|
<dd><a name="index-Wno_002dinvalid_002doffsetof"></a>
|
<a name="index-Winvalid_002doffsetof"></a>
|
<p>Suppress warnings from applying the <code>offsetof</code> macro to a non-POD
|
type. According to the 2014 ISO C++ standard, applying <code>offsetof</code>
|
to a non-standard-layout type is undefined. In existing C++ implementations,
|
however, <code>offsetof</code> typically gives meaningful results.
|
This flag is for users who are aware that they are
|
writing nonportable code and who have deliberately chosen to ignore the
|
warning about it.
|
</p>
|
<p>The restrictions on <code>offsetof</code> may be relaxed in a future version
|
of the C++ standard.
|
</p>
|
</dd>
|
<dt><code>-Wno-int-to-pointer-cast</code></dt>
|
<dd><a name="index-Wno_002dint_002dto_002dpointer_002dcast"></a>
|
<a name="index-Wint_002dto_002dpointer_002dcast"></a>
|
<p>Suppress warnings from casts to pointer type of an integer of a
|
different size. In C++, casting to a pointer type of smaller size is
|
an error. <samp>Wint-to-pointer-cast</samp> is enabled by default.
|
</p>
|
|
</dd>
|
<dt><code>-Wno-pointer-to-int-cast <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wno_002dpointer_002dto_002dint_002dcast"></a>
|
<a name="index-Wpointer_002dto_002dint_002dcast"></a>
|
<p>Suppress warnings from casts from a pointer to an integer type of a
|
different size.
|
</p>
|
</dd>
|
<dt><code>-Winvalid-pch</code></dt>
|
<dd><a name="index-Winvalid_002dpch"></a>
|
<a name="index-Wno_002dinvalid_002dpch"></a>
|
<p>Warn if a precompiled header (see <a href="Precompiled-Headers.html#Precompiled-Headers">Precompiled Headers</a>) is found in
|
the search path but can’t be used.
|
</p>
|
</dd>
|
<dt><code>-Wlong-long</code></dt>
|
<dd><a name="index-Wlong_002dlong"></a>
|
<a name="index-Wno_002dlong_002dlong"></a>
|
<p>Warn if <code>long long</code> type is used. This is enabled by either
|
<samp>-Wpedantic</samp> or <samp>-Wtraditional</samp> in ISO C90 and C++98
|
modes. To inhibit the warning messages, use <samp>-Wno-long-long</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wvariadic-macros</code></dt>
|
<dd><a name="index-Wvariadic_002dmacros"></a>
|
<a name="index-Wno_002dvariadic_002dmacros"></a>
|
<p>Warn if variadic macros are used in ISO C90 mode, or if the GNU
|
alternate syntax is used in ISO C99 mode. This is enabled by either
|
<samp>-Wpedantic</samp> or <samp>-Wtraditional</samp>. To inhibit the warning
|
messages, use <samp>-Wno-variadic-macros</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wvarargs</code></dt>
|
<dd><a name="index-Wvarargs"></a>
|
<a name="index-Wno_002dvarargs"></a>
|
<p>Warn upon questionable usage of the macros used to handle variable
|
arguments like <code>va_start</code>. This is default. To inhibit the
|
warning messages, use <samp>-Wno-varargs</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wvector-operation-performance</code></dt>
|
<dd><a name="index-Wvector_002doperation_002dperformance"></a>
|
<a name="index-Wno_002dvector_002doperation_002dperformance"></a>
|
<p>Warn if vector operation is not implemented via SIMD capabilities of the
|
architecture. Mainly useful for the performance tuning.
|
Vector operation can be implemented <code>piecewise</code>, which means that the
|
scalar operation is performed on every vector element;
|
<code>in parallel</code>, which means that the vector operation is implemented
|
using scalars of wider type, which normally is more performance efficient;
|
and <code>as a single scalar</code>, which means that vector fits into a
|
scalar type.
|
</p>
|
</dd>
|
<dt><code>-Wno-virtual-move-assign</code></dt>
|
<dd><a name="index-Wvirtual_002dmove_002dassign"></a>
|
<a name="index-Wno_002dvirtual_002dmove_002dassign"></a>
|
<p>Suppress warnings about inheriting from a virtual base with a
|
non-trivial C++11 move assignment operator. This is dangerous because
|
if the virtual base is reachable along more than one path, it is
|
moved multiple times, which can mean both objects end up in the
|
moved-from state. If the move assignment operator is written to avoid
|
moving from a moved-from object, this warning can be disabled.
|
</p>
|
</dd>
|
<dt><code>-Wvla</code></dt>
|
<dd><a name="index-Wvla"></a>
|
<a name="index-Wno_002dvla"></a>
|
<p>Warn if variable length array is used in the code.
|
<samp>-Wno-vla</samp> prevents the <samp>-Wpedantic</samp> warning of
|
the variable length array.
|
</p>
|
</dd>
|
<dt><code>-Wvolatile-register-var</code></dt>
|
<dd><a name="index-Wvolatile_002dregister_002dvar"></a>
|
<a name="index-Wno_002dvolatile_002dregister_002dvar"></a>
|
<p>Warn if a register variable is declared volatile. The volatile
|
modifier does not inhibit all optimizations that may eliminate reads
|
and/or writes to register variables. This warning is enabled by
|
<samp>-Wall</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wdisabled-optimization</code></dt>
|
<dd><a name="index-Wdisabled_002doptimization"></a>
|
<a name="index-Wno_002ddisabled_002doptimization"></a>
|
<p>Warn if a requested optimization pass is disabled. This warning does
|
not generally indicate that there is anything wrong with your code; it
|
merely indicates that GCC’s optimizers are unable to handle the code
|
effectively. Often, the problem is that your code is too big or too
|
complex; GCC refuses to optimize programs when the optimization
|
itself is likely to take inordinate amounts of time.
|
</p>
|
</dd>
|
<dt><code>-Wpointer-sign <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wpointer_002dsign"></a>
|
<a name="index-Wno_002dpointer_002dsign"></a>
|
<p>Warn for pointer argument passing or assignment with different signedness.
|
This option is only supported for C and Objective-C. It is implied by
|
<samp>-Wall</samp> and by <samp>-Wpedantic</samp>, which can be disabled with
|
<samp>-Wno-pointer-sign</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wstack-protector</code></dt>
|
<dd><a name="index-Wstack_002dprotector"></a>
|
<a name="index-Wno_002dstack_002dprotector"></a>
|
<p>This option is only active when <samp>-fstack-protector</samp> is active. It
|
warns about functions that are not protected against stack smashing.
|
</p>
|
</dd>
|
<dt><code>-Woverlength-strings</code></dt>
|
<dd><a name="index-Woverlength_002dstrings"></a>
|
<a name="index-Wno_002doverlength_002dstrings"></a>
|
<p>Warn about string constants that are longer than the “minimum
|
maximum” length specified in the C standard. Modern compilers
|
generally allow string constants that are much longer than the
|
standard’s minimum limit, but very portable programs should avoid
|
using longer strings.
|
</p>
|
<p>The limit applies <em>after</em> string constant concatenation, and does
|
not count the trailing NUL. In C90, the limit was 509 characters; in
|
C99, it was raised to 4095. C++98 does not specify a normative
|
minimum maximum, so we do not diagnose overlength strings in C++.
|
</p>
|
<p>This option is implied by <samp>-Wpedantic</samp>, and can be disabled with
|
<samp>-Wno-overlength-strings</samp>.
|
</p>
|
</dd>
|
<dt><code>-Wunsuffixed-float-constants <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><a name="index-Wunsuffixed_002dfloat_002dconstants"></a>
|
|
<p>Issue a warning for any floating constant that does not have
|
a suffix. When used together with <samp>-Wsystem-headers</samp> it
|
warns about such constants in system header files. This can be useful
|
when preparing code to use with the <code>FLOAT_CONST_DECIMAL64</code> pragma
|
from the decimal floating-point extension to C99.
|
</p>
|
</dd>
|
<dt><code>-Wno-designated-init <span class="roman">(C and Objective-C only)</span></code></dt>
|
<dd><p>Suppress warnings when a positional initializer is used to initialize
|
a structure that has been marked with the <code>designated_init</code>
|
attribute.
|
</p>
|
</dd>
|
<dt><code>-Whsa</code></dt>
|
<dd><p>Issue a warning when HSAIL cannot be emitted for the compiled function or
|
OpenMP construct.
|
</p>
|
</dd>
|
</dl>
|
|
<hr>
|
<div class="header">
|
<p>
|
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