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.

exception

An Ada exception being raised. If an exception name is specified at the end of the command (eg catch exception Program_Error), the debugger will stop only when this specific exception is raised. Otherwise, the debugger stops execution when any Ada exception is raised.

exception unhandled

An exception that was raised but is not handled by the program.

assert

A failed Ada assertion.

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 No value for GDBN:

• If you call a function interactively, No value for GDBN 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 No value for GDBN 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 (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.