HP gnu source-level debugger 5992-4701 manual Debugging programs with multiple threads

On some operating systems, a program cannot be executed outside GDB while you have breakpoints set on it inside GDB. You can use the kill command in this situation to permit running your program outside the debugger.

The kill command is also useful if you wish to recompile and relink your program, since on many systems it is impossible to modify an executable file while it is running in a process. In this case, when you next type run, GDB notices that the file has changed, and reads the symbol table again (while trying to preserve your current breakpoint settings).

4.9 Debugging programs with multiple threads

In some operating systems, such as HP-UX and Solaris, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes―except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and private memory.

GDB provides these facilities for debugging multi-thread programs:

•automatic notification of new threads

•thread-specific breakpoints

WARNING! These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from 'info threads', and always rejects the thread command, like this:

((gdb)) info threads

((gdb)) thread 1

Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads.

Following commands are used to debug multi-threaded programs:

•'thread threadno', a command to switch among threads

•'info threads', a command to inquire about existing threads

•'thread apply [threadno] [all] args', a command to apply a command to a list of threads

The GDB thread debugging facility allows you to observe all threads while your program runs―but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.

Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the form '[New systag]'. systag is a

46 Running Programs Under GDB