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Debugging with GDB

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5.1.3 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:

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 (see section Breakpoints; watchpoints; and exceptions).

With a conditional breakpoint (see section 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.


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