This command attaches to a running process--one that was started outside gdb. (info files shows your active targets.) The command takes as argument a process ID. The usual way to find out the process-id of a Unix process is with the ps utility, or with the jobs -l shell command.
attach does not repeat if you press
To use attach, your program must be running in an environment which supports processes; for example, attach does not work for programs on bare-board targets that lack an operating system. You must also have permission to send the process a signal.
When you use attach, the debugger finds the program running in the process first by looking in the current working directory, then (if the program is not found) by using the source file search path (refer to Section 9.4 Specifying source directories). You can also use the file command to load the program. Refer to Section 17.1 Commands to specify files.
The first thing gdb does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the gdb commands that are ordinarily available when you start processes with run. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the continue command after attaching gdb to the process.
When you have finished debugging the attached process, you can use the
detach command to release it from gdb control. Detaching
the process continues its execution. After the detach command,
that process and gdb become completely independent once more, and you
are ready to attach another process or start one with run.
detach does not repeat if you press
If you exit gdb or use the run command while you have an attached process, you kill that process. By default, gdb asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (refer to Section 21.7 Optional warnings and messages).