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| <a name="Set-Catchpoints"></a> |
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| |
| <h4 class="subsection">5.1.3 Setting Catchpoints</h4> |
| |
| <p><a name="index-catchpoints_002c-setting-284"></a><a name="index-exception-handlers-285"></a><a name="index-event-handling-286"></a> |
| You can use <dfn>catchpoints</dfn> to cause the debugger to stop for certain |
| kinds of program events, such as C<tt>++</tt> exceptions or the loading of a |
| shared library. Use the <code>catch</code> command to set a catchpoint. |
| |
| |
| <a name="index-catch-287"></a> |
| <dl><dt><code>catch </code><var>event</var><dd>Stop when <var>event</var> occurs. The <var>event</var> can be any of the following: |
| |
| <dl> |
| <dt><code>throw </code><span class="roman">[</span><var>regexp</var><span class="roman">]</span><dt><code>rethrow </code><span class="roman">[</span><var>regexp</var><span class="roman">]</span><dt><code>catch </code><span class="roman">[</span><var>regexp</var><span class="roman">]</span><dd><a name="index-catch-throw-288"></a><a name="index-catch-rethrow-289"></a><a name="index-catch-catch-290"></a><a name="index-stop-on-C_0040t_007b_002b_002b_007d-exceptions-291"></a>The throwing, re-throwing, or catching of a C<tt>++</tt> exception. |
| |
| <p>If <var>regexp</var> is given, then only exceptions whose type matches the |
| regular expression will be caught. |
| |
| <p><a name="index-g_t_0024_005fexception_0040r_007b_002c-convenience-variable_007d-292"></a>The convenience variable <code>$_exception</code> is available at an |
| exception-related catchpoint, on some systems. This holds the |
| exception being thrown. |
| |
| <p>There are currently some limitations to C<tt>++</tt> exception handling in |
| <span class="sc">gdb</span>: |
| |
| <ul> |
| <li>The support for these commands is system-dependent. Currently, only |
| systems using the ‘<samp><span class="samp">gnu-v3</span></samp>’ C<tt>++</tt> ABI (see <a href="ABI.html#ABI">ABI</a>) are |
| supported. |
| |
| <li>The regular expression feature and the <code>$_exception</code> convenience |
| variable rely on the presence of some SDT probes in <code>libstdc++</code>. |
| If these probes are not present, then these features cannot be used. |
| These probes were first available in the GCC 4.8 release, but whether |
| or not they are available in your GCC also depends on how it was |
| built. |
| |
| <li>The <code>$_exception</code> convenience variable is only valid at the |
| instruction at which an exception-related catchpoint is set. |
| |
| <li>When an exception-related catchpoint is hit, <span class="sc">gdb</span> stops at a |
| location in the system library which implements runtime exception |
| support for C<tt>++</tt>, usually <code>libstdc++</code>. You can use <code>up</code> |
| (see <a href="Selection.html#Selection">Selection</a>) to get to your code. |
| |
| <li>If you call a function interactively, <span class="sc">gdb</span> normally returns |
| control to you when the function has finished executing. If the call |
| raises an exception, however, the call may bypass the mechanism that |
| returns control to you and cause your program either to abort or to |
| simply continue running until it hits a breakpoint, catches a signal |
| that <span class="sc">gdb</span> is listening for, or exits. This is the case even if |
| you set a catchpoint for the exception; catchpoints on exceptions are |
| disabled within interactive calls. See <a href="Calling.html#Calling">Calling</a>, for information on |
| controlling this with <code>set unwind-on-terminating-exception</code>. |
| |
| <li>You cannot raise an exception interactively. |
| |
| <li>You cannot install an exception handler interactively. |
| </ul> |
| |
| <br><dt><code>exception</code><dd><a name="index-catch-exception-293"></a><a name="index-Ada-exception-catching-294"></a><a name="index-catch-Ada-exceptions-295"></a>An Ada exception being raised. If an exception name is specified |
| at the end of the command (eg <code>catch exception Program_Error</code>), |
| the debugger will stop only when this specific exception is raised. |
| Otherwise, the debugger stops execution when any Ada exception is raised. |
| |
| <p>When inserting an exception catchpoint on a user-defined exception whose |
| name is identical to one of the exceptions defined by the language, the |
| fully qualified name must be used as the exception name. Otherwise, |
| <span class="sc">gdb</span> will assume that it should stop on the pre-defined exception |
| rather than the user-defined one. For instance, assuming an exception |
| called <code>Constraint_Error</code> is defined in package <code>Pck</code>, then |
| the command to use to catch such exceptions is <kbd>catch exception |
| Pck.Constraint_Error</kbd>. |
| |
| <br><dt><code>handlers</code><dd><a name="index-catch-handlers-296"></a><a name="index-Ada-exception-handlers-catching-297"></a><a name="index-catch-Ada-exceptions-when-handled-298"></a>An Ada exception being handled. If an exception name is |
| specified at the end of the command |
| (eg <kbd>catch handlers Program_Error</kbd>), the debugger will stop |
| only when this specific exception is handled. |
| Otherwise, the debugger stops execution when any Ada exception is handled. |
| |
| <p>When inserting a handlers catchpoint on a user-defined |
| exception whose name is identical to one of the exceptions |
| defined by the language, the fully qualified name must be used |
| as the exception name. Otherwise, <span class="sc">gdb</span> will assume that it |
| should stop on the pre-defined exception rather than the |
| user-defined one. For instance, assuming an exception called |
| <code>Constraint_Error</code> is defined in package <code>Pck</code>, then the |
| command to use to catch such exceptions handling is |
| <kbd>catch handlers Pck.Constraint_Error</kbd>. |
| |
| <br><dt><code>exception unhandled</code><dd><a name="index-catch-exception-unhandled-299"></a>An exception that was raised but is not handled by the program. |
| |
| <br><dt><code>assert</code><dd><a name="index-catch-assert-300"></a>A failed Ada assertion. |
| |
| <br><dt><code>exec</code><dd><a name="index-catch-exec-301"></a><a name="index-break-on-fork_002fexec-302"></a>A call to <code>exec</code>. |
| |
| <p><a name="catch-syscall"></a><br><dt><code>syscall</code><dt><code>syscall </code><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> ...</code><dd><a name="index-catch-syscall-303"></a><a name="index-break-on-a-system-call_002e-304"></a>A call to or return from a system call, a.k.a. <dfn>syscall</dfn>. A |
| syscall is a mechanism for application programs to request a service |
| from the operating system (OS) or one of the OS system services. |
| <span class="sc">gdb</span> can catch some or all of the syscalls issued by the |
| debuggee, and show the related information for each syscall. If no |
| argument is specified, calls to and returns from all system calls |
| will be caught. |
| |
| <p><var>name</var> can be any system call name that is valid for the |
| underlying OS. Just what syscalls are valid depends on the OS. On |
| GNU and Unix systems, you can find the full list of valid syscall |
| names on <samp><span class="file">/usr/include/asm/unistd.h</span></samp>. |
| |
| <!-- For MS-Windows, the syscall names and the corresponding numbers --> |
| <!-- can be found, e.g., on this URL: --> |
| <!-- http://www.metasploit.com/users/opcode/syscalls.html --> |
| <!-- but we don't support Windows syscalls yet. --> |
| <p>Normally, <span class="sc">gdb</span> knows in advance which syscalls are valid for |
| each OS, so you can use the <span class="sc">gdb</span> command-line completion |
| facilities (see <a href="Completion.html#Completion">command completion</a>) to list the |
| available choices. |
| |
| <p>You may also specify the system call numerically. A syscall's |
| number is the value passed to the OS's syscall dispatcher to |
| identify the requested service. When you specify the syscall by its |
| name, <span class="sc">gdb</span> uses its database of syscalls to convert the name |
| into the corresponding numeric code, but using the number directly |
| may be useful if <span class="sc">gdb</span>'s database does not have the complete |
| list of syscalls on your system (e.g., because <span class="sc">gdb</span> lags |
| behind the OS upgrades). |
| |
| <p>You may specify a group of related syscalls to be caught at once using |
| the <code>group:</code> syntax (<code>g:</code> is a shorter equivalent). For |
| instance, on some platforms <span class="sc">gdb</span> allows you to catch all |
| network related syscalls, by passing the argument <code>group:network</code> |
| to <code>catch syscall</code>. Note that not all syscall groups are |
| available in every system. You can use the command completion |
| facilities (see <a href="Completion.html#Completion">command completion</a>) to list the |
| syscall groups available on your environment. |
| |
| <p>The example below illustrates how this command works if you don't provide |
| arguments to it: |
| |
| <pre class="smallexample"> (gdb) catch syscall |
| Catchpoint 1 (syscall) |
| (gdb) r |
| Starting program: /tmp/catch-syscall |
| |
| Catchpoint 1 (call to syscall 'close'), \ |
| 0xffffe424 in __kernel_vsyscall () |
| (gdb) c |
| Continuing. |
| |
| Catchpoint 1 (returned from syscall 'close'), \ |
| 0xffffe424 in __kernel_vsyscall () |
| (gdb) |
| </pre> |
| <p>Here is an example of catching a system call by name: |
| |
| <pre class="smallexample"> (gdb) catch syscall chroot |
| Catchpoint 1 (syscall 'chroot' [61]) |
| (gdb) r |
| Starting program: /tmp/catch-syscall |
| |
| Catchpoint 1 (call to syscall 'chroot'), \ |
| 0xffffe424 in __kernel_vsyscall () |
| (gdb) c |
| Continuing. |
| |
| Catchpoint 1 (returned from syscall 'chroot'), \ |
| 0xffffe424 in __kernel_vsyscall () |
| (gdb) |
| </pre> |
| <p>An example of specifying a system call numerically. In the case |
| below, the syscall number has a corresponding entry in the XML |
| file, so <span class="sc">gdb</span> finds its name and prints it: |
| |
| <pre class="smallexample"> (gdb) catch syscall 252 |
| Catchpoint 1 (syscall(s) 'exit_group') |
| (gdb) r |
| Starting program: /tmp/catch-syscall |
| |
| Catchpoint 1 (call to syscall 'exit_group'), \ |
| 0xffffe424 in __kernel_vsyscall () |
| (gdb) c |
| Continuing. |
| |
| Program exited normally. |
| (gdb) |
| </pre> |
| <p>Here is an example of catching a syscall group: |
| |
| <pre class="smallexample"> (gdb) catch syscall group:process |
| Catchpoint 1 (syscalls 'exit' [1] 'fork' [2] 'waitpid' [7] |
| 'execve' [11] 'wait4' [114] 'clone' [120] 'vfork' [190] |
| 'exit_group' [252] 'waitid' [284] 'unshare' [310]) |
| (gdb) r |
| Starting program: /tmp/catch-syscall |
| |
| Catchpoint 1 (call to syscall fork), 0x00007ffff7df4e27 in open64 () |
| from /lib64/ld-linux-x86-64.so.2 |
| |
| (gdb) c |
| Continuing. |
| </pre> |
| <p>However, there can be situations when there is no corresponding name |
| in XML file for that syscall number. In this case, <span class="sc">gdb</span> prints |
| a warning message saying that it was not able to find the syscall name, |
| but the catchpoint will be set anyway. See the example below: |
| |
| <pre class="smallexample"> (gdb) catch syscall 764 |
| warning: The number '764' does not represent a known syscall. |
| Catchpoint 2 (syscall 764) |
| (gdb) |
| </pre> |
| <p>If you configure <span class="sc">gdb</span> using the ‘<samp><span class="samp">--without-expat</span></samp>’ option, |
| it will not be able to display syscall names. Also, if your |
| architecture does not have an XML file describing its system calls, |
| you will not be able to see the syscall names. It is important to |
| notice that these two features are used for accessing the syscall |
| name database. In either case, you will see a warning like this: |
| |
| <pre class="smallexample"> (gdb) catch syscall |
| warning: Could not open "syscalls/i386-linux.xml" |
| warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'. |
| GDB will not be able to display syscall names. |
| Catchpoint 1 (syscall) |
| (gdb) |
| </pre> |
| <p>Of course, the file name will change depending on your architecture and system. |
| |
| <p>Still using the example above, you can also try to catch a syscall by its |
| number. In this case, you would see something like: |
| |
| <pre class="smallexample"> (gdb) catch syscall 252 |
| Catchpoint 1 (syscall(s) 252) |
| </pre> |
| <p>Again, in this case <span class="sc">gdb</span> would not be able to display syscall's names. |
| |
| <br><dt><code>fork</code><dd><a name="index-catch-fork-305"></a>A call to <code>fork</code>. |
| |
| <br><dt><code>vfork</code><dd><a name="index-catch-vfork-306"></a>A call to <code>vfork</code>. |
| |
| <br><dt><code>load </code><span class="roman">[</span><code>regexp</code><span class="roman">]</span><dt><code>unload </code><span class="roman">[</span><code>regexp</code><span class="roman">]</span><dd><a name="index-catch-load-307"></a><a name="index-catch-unload-308"></a>The loading or unloading of a shared library. If <var>regexp</var> is |
| given, then the catchpoint will stop only if the regular expression |
| matches one of the affected libraries. |
| |
| <br><dt><code>signal </code><span class="roman">[</span><var>signal</var><code>... </code><span class="roman">|</span><code> ‘</code><samp><span class="samp">all</span></samp><code>’</code><span class="roman">]</span><dd><a name="index-catch-signal-309"></a>The delivery of a signal. |
| |
| <p>With no arguments, this catchpoint will catch any signal that is not |
| used internally by <span class="sc">gdb</span>, specifically, all signals except |
| ‘<samp><span class="samp">SIGTRAP</span></samp>’ and ‘<samp><span class="samp">SIGINT</span></samp>’. |
| |
| <p>With the argument ‘<samp><span class="samp">all</span></samp>’, all signals, including those used by |
| <span class="sc">gdb</span>, will be caught. This argument cannot be used with other |
| signal names. |
| |
| <p>Otherwise, the arguments are a list of signal names as given to |
| <code>handle</code> (see <a href="Signals.html#Signals">Signals</a>). Only signals specified in this list |
| will be caught. |
| |
| <p>One reason that <code>catch signal</code> can be more useful than |
| <code>handle</code> is that you can attach commands and conditions to the |
| catchpoint. |
| |
| <p>When a signal is caught by a catchpoint, the signal's <code>stop</code> and |
| <code>print</code> settings, as specified by <code>handle</code>, are ignored. |
| However, whether the signal is still delivered to the inferior depends |
| on the <code>pass</code> setting; this can be changed in the catchpoint's |
| commands. |
| |
| </dl> |
| |
| <br><dt><code>tcatch </code><var>event</var><dd><a name="index-tcatch-310"></a>Set a catchpoint that is enabled only for one stop. The catchpoint is |
| automatically deleted after the first time the event is caught. |
| |
| </dl> |
| |
| <p>Use the <code>info break</code> command to list the current catchpoints. |
| |
| </body></html> |
| |
