Red Hat ENTERPRISE LINUX 3 - SECURITY GUIDE, Manual

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Chapter 7. Stopping and Continuing 41
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
(refer to Section 7.1 Breakpoints, watchpoints, and catchpoints ).
With a conditional breakpoint (refer to Section 7.1.6 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.
7.1.4. 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.