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(gdb.info)Set Catchpoints

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Setting catchpoints
-------------------

   You can use "catchpoints" to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library.  Use the `catch' command to set a catchpoint.

`catch EVENT'
     Stop when EVENT occurs.  EVENT can be any of the following:
    `throw'
          The throwing of a C++ exception.

    `catch'
          The catching of a C++ exception.

    `exec'
          A call to `exec'.  This is currently only available for HP-UX.

    `fork'
          A call to `fork'.  This is currently only available for HP-UX.

    `vfork'
          A call to `vfork'.  This is currently only available for
          HP-UX.

    `load'
    `load LIBNAME'
          The dynamic loading of any shared library, or the loading of
          the library LIBNAME.  This is currently only available for
          HP-UX.

    `unload'
    `unload LIBNAME'
          The unloading of any dynamically loaded shared library, or
          the unloading of the library LIBNAME.  This is currently only
          available for HP-UX.

`tcatch EVENT'
     Set a catchpoint that is enabled only for one stop.  The
     catchpoint is automatically deleted after the first time the event
     is caught.

   Use the `info break' command to list the current catchpoints.

   There are currently some limitations to C++ exception handling
(`catch throw' and `catch catch') in GDB:

   * If you call a function interactively, GDB 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 GDB 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.

   * You cannot raise an exception interactively.

   * You cannot install an exception handler interactively.

   Sometimes `catch' is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better
to stop _before_ the exception handler is called, since that way you
can see the stack before any unwinding takes place.  If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.

   To stop just before an exception handler is called, you need some
knowledge of the implementation.  In the case of GNU C++, exceptions are
raised by calling a library function named `__raise_exception' which
has the following ANSI C interface:

         /* ADDR is where the exception identifier is stored.
            ID is the exception identifier.  */
         void __raise_exception (void **addr, void *id);

To make the debugger catch all exceptions before any stack unwinding
takes place, set a breakpoint on `__raise_exception' (Note:
Breakpoints; watchpoints; and exceptions.).

   With a conditional breakpoint (Note: Break conditions.)
that depends on the value of ID, you can stop your program when a
specific exception is raised.  You can use multiple conditional
breakpoints to stop your program when any of a number of exceptions are
raised.