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<title>Debugging with GDB: Set Catchpoints</title>
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<a name="Set-Catchpoints"></a>
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<div class="header">
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<p>
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Next: <a href="Delete-Breaks.html#Delete-Breaks" accesskey="n" rel="next">Delete Breaks</a>, Previous: <a href="Set-Watchpoints.html#Set-Watchpoints" accesskey="p" rel="prev">Set Watchpoints</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Setting-Catchpoints"></a>
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<h4 class="subsection">5.1.3 Setting Catchpoints</h4>
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<a name="index-catchpoints_002c-setting"></a>
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<a name="index-exception-handlers"></a>
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<a name="index-event-handling"></a>
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<p>You can use <em>catchpoints</em> to cause the debugger to stop for certain
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kinds of program events, such as C<tt>++</tt> exceptions or the loading of a
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shared library. Use the <code>catch</code> command to set a catchpoint.
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</p>
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<dl compact="compact">
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<dd><a name="index-catch"></a>
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</dd>
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<dt><code>catch <var>event</var></code></dt>
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<dd><p>Stop when <var>event</var> occurs. The <var>event</var> can be any of the following:
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</p>
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<dl compact="compact">
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<dt><code>throw <span class="roman">[</span><var>regexp</var><span class="roman">]</span></code></dt>
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<dt><code>rethrow <span class="roman">[</span><var>regexp</var><span class="roman">]</span></code></dt>
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<dt><code>catch <span class="roman">[</span><var>regexp</var><span class="roman">]</span></code></dt>
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<dd><a name="index-catch-throw"></a>
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<a name="index-catch-rethrow"></a>
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<a name="index-catch-catch"></a>
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<a name="index-stop-on-C_002b_002b-exceptions"></a>
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<p>The throwing, re-throwing, or catching of a C<tt>++</tt> exception.
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</p>
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<p>If <var>regexp</var> is given, then only exceptions whose type matches the
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regular expression will be caught.
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</p>
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<a name="index-_0024_005fexception_002c-convenience-variable"></a>
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<p>The convenience variable <code>$_exception</code> is available at an
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exception-related catchpoint, on some systems. This holds the
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exception being thrown.
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</p>
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<p>There are currently some limitations to C<tt>++</tt> exception handling in
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<small>GDB</small>:
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</p>
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<ul>
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<li> The support for these commands is system-dependent. Currently, only
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systems using the ‘<samp>gnu-v3</samp>’ C<tt>++</tt> ABI (see <a href="ABI.html#ABI">ABI</a>) are
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supported.
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</li><li> The regular expression feature and the <code>$_exception</code> convenience
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variable rely on the presence of some SDT probes in <code>libstdc++</code>.
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If these probes are not present, then these features cannot be used.
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These probes were first available in the GCC 4.8 release, but whether
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or not they are available in your GCC also depends on how it was
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built.
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</li><li> The <code>$_exception</code> convenience variable is only valid at the
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instruction at which an exception-related catchpoint is set.
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</li><li> When an exception-related catchpoint is hit, <small>GDB</small> stops at a
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location in the system library which implements runtime exception
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support for C<tt>++</tt>, usually <code>libstdc++</code>. You can use <code>up</code>
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(see <a href="Selection.html#Selection">Selection</a>) to get to your code.
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</li><li> If you call a function interactively, <small>GDB</small> normally returns
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control to you when the function has finished executing. If the call
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raises an exception, however, the call may bypass the mechanism that
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returns control to you and cause your program either to abort or to
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simply continue running until it hits a breakpoint, catches a signal
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that <small>GDB</small> is listening for, or exits. This is the case even if
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you set a catchpoint for the exception; catchpoints on exceptions are
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disabled within interactive calls. See <a href="Calling.html#Calling">Calling</a>, for information on
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controlling this with <code>set unwind-on-terminating-exception</code>.
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</li><li> You cannot raise an exception interactively.
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</li><li> You cannot install an exception handler interactively.
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</li></ul>
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</dd>
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<dt><code>exception</code></dt>
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<dd><a name="index-catch-exception"></a>
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<a name="index-Ada-exception-catching"></a>
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<a name="index-catch-Ada-exceptions"></a>
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<p>An Ada exception being raised. If an exception name is specified
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at the end of the command (eg <code>catch exception Program_Error</code>),
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the debugger will stop only when this specific exception is raised.
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Otherwise, the debugger stops execution when any Ada exception is raised.
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</p>
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<p>When inserting an exception catchpoint on a user-defined exception whose
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name is identical to one of the exceptions defined by the language, the
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fully qualified name must be used as the exception name. Otherwise,
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<small>GDB</small> will assume that it should stop on the pre-defined exception
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rather than the user-defined one. For instance, assuming an exception
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called <code>Constraint_Error</code> is defined in package <code>Pck</code>, then
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the command to use to catch such exceptions is <kbd>catch exception
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Pck.Constraint_Error</kbd>.
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</p>
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</dd>
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<dt><code>exception unhandled</code></dt>
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<dd><a name="index-catch-exception-unhandled"></a>
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<p>An exception that was raised but is not handled by the program.
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</p>
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</dd>
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<dt><code>assert</code></dt>
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<dd><a name="index-catch-assert"></a>
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<p>A failed Ada assertion.
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</p>
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</dd>
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<dt><code>exec</code></dt>
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<dd><a name="index-catch-exec"></a>
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<a name="index-break-on-fork_002fexec"></a>
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<p>A call to <code>exec</code>.
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</p>
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</dd>
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<dt><code>syscall</code></dt>
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<dt><code>syscall <span class="roman">[</span><var>name</var> <span class="roman">|</span> <var>number</var> <span class="roman">|</span> <span class="roman">group:</span><var>groupname</var> <span class="roman">|</span> <span class="roman">g:</span><var>groupname</var><span class="roman">]</span> …</code></dt>
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<dd><a name="index-catch-syscall"></a>
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<a name="index-break-on-a-system-call_002e"></a>
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<p>A call to or return from a system call, a.k.a. <em>syscall</em>. A
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syscall is a mechanism for application programs to request a service
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from the operating system (OS) or one of the OS system services.
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<small>GDB</small> can catch some or all of the syscalls issued by the
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debuggee, and show the related information for each syscall. If no
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argument is specified, calls to and returns from all system calls
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will be caught.
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</p>
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<p><var>name</var> can be any system call name that is valid for the
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underlying OS. Just what syscalls are valid depends on the OS. On
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GNU and Unix systems, you can find the full list of valid syscall
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names on <samp>/usr/include/asm/unistd.h</samp>.
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</p>
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<p>Normally, <small>GDB</small> knows in advance which syscalls are valid for
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each OS, so you can use the <small>GDB</small> command-line completion
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facilities (see <a href="Completion.html#Completion">command completion</a>) to list the
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available choices.
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</p>
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<p>You may also specify the system call numerically. A syscall’s
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number is the value passed to the OS’s syscall dispatcher to
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identify the requested service. When you specify the syscall by its
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name, <small>GDB</small> uses its database of syscalls to convert the name
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into the corresponding numeric code, but using the number directly
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may be useful if <small>GDB</small>’s database does not have the complete
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list of syscalls on your system (e.g., because <small>GDB</small> lags
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behind the OS upgrades).
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</p>
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<p>You may specify a group of related syscalls to be caught at once using
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the <code>group:</code> syntax (<code>g:</code> is a shorter equivalent). For
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instance, on some platforms <small>GDB</small> allows you to catch all
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network related syscalls, by passing the argument <code>group:network</code>
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to <code>catch syscall</code>. Note that not all syscall groups are
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available in every system. You can use the command completion
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facilities (see <a href="Completion.html#Completion">command completion</a>) to list the
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syscall groups available on your environment.
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</p>
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<p>The example below illustrates how this command works if you don’t provide
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arguments to it:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall
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Catchpoint 1 (syscall)
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(gdb) r
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Starting program: /tmp/catch-syscall
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Catchpoint 1 (call to syscall 'close'), \
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0xffffe424 in __kernel_vsyscall ()
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(gdb) c
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Continuing.
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Catchpoint 1 (returned from syscall 'close'), \
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0xffffe424 in __kernel_vsyscall ()
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(gdb)
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</pre></div>
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<p>Here is an example of catching a system call by name:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall chroot
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Catchpoint 1 (syscall 'chroot' [61])
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(gdb) r
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Starting program: /tmp/catch-syscall
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Catchpoint 1 (call to syscall 'chroot'), \
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0xffffe424 in __kernel_vsyscall ()
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(gdb) c
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Continuing.
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Catchpoint 1 (returned from syscall 'chroot'), \
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0xffffe424 in __kernel_vsyscall ()
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(gdb)
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</pre></div>
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<p>An example of specifying a system call numerically. In the case
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below, the syscall number has a corresponding entry in the XML
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file, so <small>GDB</small> finds its name and prints it:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall 252
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Catchpoint 1 (syscall(s) 'exit_group')
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(gdb) r
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Starting program: /tmp/catch-syscall
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Catchpoint 1 (call to syscall 'exit_group'), \
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0xffffe424 in __kernel_vsyscall ()
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(gdb) c
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Continuing.
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Program exited normally.
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(gdb)
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</pre></div>
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<p>Here is an example of catching a syscall group:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall group:process
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Catchpoint 1 (syscalls 'exit' [1] 'fork' [2] 'waitpid' [7]
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'execve' [11] 'wait4' [114] 'clone' [120] 'vfork' [190]
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'exit_group' [252] 'waitid' [284] 'unshare' [310])
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(gdb) r
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Starting program: /tmp/catch-syscall
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Catchpoint 1 (call to syscall fork), 0x00007ffff7df4e27 in open64 ()
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from /lib64/ld-linux-x86-64.so.2
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(gdb) c
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Continuing.
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</pre></div>
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<p>However, there can be situations when there is no corresponding name
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in XML file for that syscall number. In this case, <small>GDB</small> prints
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a warning message saying that it was not able to find the syscall name,
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but the catchpoint will be set anyway. See the example below:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall 764
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warning: The number '764' does not represent a known syscall.
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Catchpoint 2 (syscall 764)
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(gdb)
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</pre></div>
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<p>If you configure <small>GDB</small> using the ‘<samp>--without-expat</samp>’ option,
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it will not be able to display syscall names. Also, if your
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architecture does not have an XML file describing its system calls,
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you will not be able to see the syscall names. It is important to
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notice that these two features are used for accessing the syscall
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name database. In either case, you will see a warning like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall
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warning: Could not open "syscalls/i386-linux.xml"
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warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'.
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GDB will not be able to display syscall names.
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Catchpoint 1 (syscall)
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(gdb)
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</pre></div>
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<p>Of course, the file name will change depending on your architecture and system.
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</p>
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<p>Still using the example above, you can also try to catch a syscall by its
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number. In this case, you would see something like:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) catch syscall 252
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Catchpoint 1 (syscall(s) 252)
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</pre></div>
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<p>Again, in this case <small>GDB</small> would not be able to display syscall’s names.
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</p>
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</dd>
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<dt><code>fork</code></dt>
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<dd><a name="index-catch-fork"></a>
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<p>A call to <code>fork</code>.
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</p>
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</dd>
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<dt><code>vfork</code></dt>
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<dd><a name="index-catch-vfork"></a>
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<p>A call to <code>vfork</code>.
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</p>
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</dd>
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<dt><code>load <span class="roman">[</span>regexp<span class="roman">]</span></code></dt>
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<dt><code>unload <span class="roman">[</span>regexp<span class="roman">]</span></code></dt>
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<dd><a name="index-catch-load"></a>
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<a name="index-catch-unload"></a>
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<p>The loading or unloading of a shared library. If <var>regexp</var> is
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given, then the catchpoint will stop only if the regular expression
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matches one of the affected libraries.
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</p>
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</dd>
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<dt><code>signal <span class="roman">[</span><var>signal</var>… <span class="roman">|</span> ‘<samp>all</samp>’<span class="roman">]</span></code></dt>
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<dd><a name="index-catch-signal"></a>
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<p>The delivery of a signal.
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</p>
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<p>With no arguments, this catchpoint will catch any signal that is not
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used internally by <small>GDB</small>, specifically, all signals except
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‘<samp>SIGTRAP</samp>’ and ‘<samp>SIGINT</samp>’.
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</p>
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<p>With the argument ‘<samp>all</samp>’, all signals, including those used by
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<small>GDB</small>, will be caught. This argument cannot be used with other
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signal names.
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</p>
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<p>Otherwise, the arguments are a list of signal names as given to
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<code>handle</code> (see <a href="Signals.html#Signals">Signals</a>). Only signals specified in this list
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will be caught.
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</p>
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<p>One reason that <code>catch signal</code> can be more useful than
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<code>handle</code> is that you can attach commands and conditions to the
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catchpoint.
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</p>
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<p>When a signal is caught by a catchpoint, the signal’s <code>stop</code> and
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<code>print</code> settings, as specified by <code>handle</code>, are ignored.
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However, whether the signal is still delivered to the inferior depends
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on the <code>pass</code> setting; this can be changed in the catchpoint’s
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commands.
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</p>
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</dd>
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</dl>
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</dd>
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<dt><code>tcatch <var>event</var></code></dt>
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<dd><a name="index-tcatch"></a>
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<p>Set a catchpoint that is enabled only for one stop. The catchpoint is
|
automatically deleted after the first time the event is caught.
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</p>
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</dd>
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</dl>
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<p>Use the <code>info break</code> command to list the current catchpoints.
|
</p>
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<hr>
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