Breakpoints and exceptions

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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 calls.

- `throw`
The throwing of a C++ exception.
- `catch`
The catching of a C++ exception.
- `exec`
A call to `exec`.
- `fork`
A call to `fork`
- `vfork`
A call to `vfork`
- `load`
- `load` `libname`
The dynamic loading of any shared library, or the loading of the library, `libname`.
- `unload`
- `unload libname`
The unloading of any dynamically loaded shared library, or the unloading of the library, `libname`.

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 (see previous `event` calls for event definitions).

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, as the following discussion describes.

- 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` (see Breakpoints, watchpoints, and exceptions).

With a conditional breakpoint (see 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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