Debugging with GDB (February 2008)

38 Debugging with GDB
• 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 5.1 [Breakpoints; watchpoints; and excep-
tions], page 33).
With a conditional breakpoint (see Section 5.1.5 [Break conditions], page 40) that de-
pends 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.
5.1.3 Deleting breakpoints
It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has
done its job and you no longer want your program to stop there. This is called deleting the
breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear command you can delete breakpoints according to where they are in your
program. With the delete command you can delete individual breakpoints, watchpoints,
or catchpoints by specifying their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores
breakpoints on the first instruction to be executed when you continue execution without
changing the execution address.
clear Delete any breakpoints at the next instruction to be executed in the selected
stack frame (see Section 6.5 [Selecting a frame], page 53). When the innermost
frame is selected, this is a good way to delete a breakpoint where your program
just stopped.
clear function
clear filename :function
Delete any breakpoints set at entry to the function function.
clear linenum
clear filename :linenum
Delete any breakpoints set at or within the code of the specified line.