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<title>Using the GNU Compiler Collection (GCC): Basic Asm</title>
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<a name="Basic-Asm"></a>
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<p>
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Next: <a href="Extended-Asm.html#Extended-Asm" accesskey="n" rel="next">Extended Asm</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</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>
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<hr>
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<a name="Basic-Asm-_002d_002d_002d-Assembler-Instructions-Without-Operands"></a>
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<h4 class="subsection">6.44.1 Basic Asm — Assembler Instructions Without Operands</h4>
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<a name="index-basic-asm"></a>
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<a name="index-assembly-language-in-C_002c-basic"></a>
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<p>A basic <code>asm</code> statement has the following syntax:
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</p>
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<div class="example">
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<pre class="example">asm <span class="roman">[</span> volatile <span class="roman">]</span> ( <var>AssemblerInstructions</var> )
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</pre></div>
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<p>The <code>asm</code> keyword is a GNU extension.
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When writing code that can be compiled with <samp>-ansi</samp> and the
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various <samp>-std</samp> options, use <code>__asm__</code> instead of
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<code>asm</code> (see <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>).
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</p>
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<a name="Qualifiers-1"></a>
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<h4 class="subsubheading">Qualifiers</h4>
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<dl compact="compact">
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<dt><code>volatile</code></dt>
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<dd><p>The optional <code>volatile</code> qualifier has no effect.
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All basic <code>asm</code> blocks are implicitly volatile.
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</p></dd>
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</dl>
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<a name="Parameters"></a>
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<h4 class="subsubheading">Parameters</h4>
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<dl compact="compact">
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<dt><var>AssemblerInstructions</var></dt>
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<dd><p>This is a literal string that specifies the assembler code. The string can
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contain any instructions recognized by the assembler, including directives.
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GCC does not parse the assembler instructions themselves and
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does not know what they mean or even whether they are valid assembler input.
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</p>
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<p>You may place multiple assembler instructions together in a single <code>asm</code>
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string, separated by the characters normally used in assembly code for the
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system. A combination that works in most places is a newline to break the
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line, plus a tab character (written as ‘<samp>\n\t</samp>’).
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Some assemblers allow semicolons as a line separator. However,
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note that some assembler dialects use semicolons to start a comment.
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</p></dd>
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</dl>
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<a name="Remarks"></a>
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<h4 class="subsubheading">Remarks</h4>
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<p>Using extended <code>asm</code> (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>) typically produces
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smaller, safer, and more efficient code, and in most cases it is a
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better solution than basic <code>asm</code>. However, there are two
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situations where only basic <code>asm</code> can be used:
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</p>
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<ul>
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<li> Extended <code>asm</code> statements have to be inside a C
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function, so to write inline assembly language at file scope (“top-level”),
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outside of C functions, you must use basic <code>asm</code>.
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You can use this technique to emit assembler directives,
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define assembly language macros that can be invoked elsewhere in the file,
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or write entire functions in assembly language.
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</li><li> Functions declared
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with the <code>naked</code> attribute also require basic <code>asm</code>
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(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).
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</li></ul>
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<p>Safely accessing C data and calling functions from basic <code>asm</code> is more
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complex than it may appear. To access C data, it is better to use extended
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<code>asm</code>.
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</p>
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<p>Do not expect a sequence of <code>asm</code> statements to remain perfectly
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consecutive after compilation. If certain instructions need to remain
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consecutive in the output, put them in a single multi-instruction <code>asm</code>
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statement. Note that GCC’s optimizers can move <code>asm</code> statements
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relative to other code, including across jumps.
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</p>
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<p><code>asm</code> statements may not perform jumps into other <code>asm</code> statements.
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GCC does not know about these jumps, and therefore cannot take
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account of them when deciding how to optimize. Jumps from <code>asm</code> to C
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labels are only supported in extended <code>asm</code>.
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</p>
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<p>Under certain circumstances, GCC may duplicate (or remove duplicates of) your
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assembly code when optimizing. This can lead to unexpected duplicate
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symbol errors during compilation if your assembly code defines symbols or
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labels.
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</p>
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<p><strong>Warning:</strong> The C standards do not specify semantics for <code>asm</code>,
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making it a potential source of incompatibilities between compilers. These
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incompatibilities may not produce compiler warnings/errors.
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</p>
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<p>GCC does not parse basic <code>asm</code>’s <var>AssemblerInstructions</var>, which
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means there is no way to communicate to the compiler what is happening
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inside them. GCC has no visibility of symbols in the <code>asm</code> and may
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discard them as unreferenced. It also does not know about side effects of
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the assembler code, such as modifications to memory or registers. Unlike
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some compilers, GCC assumes that no changes to either memory or registers
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occur. This assumption may change in a future release.
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</p>
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<p>To avoid complications from future changes to the semantics and the
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compatibility issues between compilers, consider replacing basic <code>asm</code>
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with extended <code>asm</code>. See
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<a href="https://gcc.gnu.org/wiki/ConvertBasicAsmToExtended">How to convert
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from basic asm to extended asm</a> for information about how to perform this
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conversion.
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</p>
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<p>The compiler copies the assembler instructions in a basic <code>asm</code>
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verbatim to the assembly language output file, without
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processing dialects or any of the ‘<samp>%</samp>’ operators that are available with
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extended <code>asm</code>. This results in minor differences between basic
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<code>asm</code> strings and extended <code>asm</code> templates. For example, to refer to
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registers you might use ‘<samp>%eax</samp>’ in basic <code>asm</code> and
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‘<samp>%%eax</samp>’ in extended <code>asm</code>.
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</p>
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<p>On targets such as x86 that support multiple assembler dialects,
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all basic <code>asm</code> blocks use the assembler dialect specified by the
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<samp>-masm</samp> command-line option (see <a href="x86-Options.html#x86-Options">x86 Options</a>).
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Basic <code>asm</code> provides no
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mechanism to provide different assembler strings for different dialects.
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</p>
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<p>Here is an example of basic <code>asm</code> for i386:
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</p>
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<div class="example">
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<pre class="example">/* Note that this code will not compile with -masm=intel */
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#define DebugBreak() asm("int $3")
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</pre></div>
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<div class="header">
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<p>
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Next: <a href="Extended-Asm.html#Extended-Asm" accesskey="n" rel="next">Extended Asm</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</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>
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