using gdb to debug a interactive program that reads input from stdin

Question

I'm writing a client/server program in C.

My client has a thread reading input from stdin, it's just a while(1) loop to read input from stdin. Whenever it reads a line, it deliver it to another thread that handles message parsing and framing.

As I enter gdb, the command line is occupied by gdb prompt and I can no longer input lines into stdin.

Is there a way to do it? (I don't want to redirect stdin to an input file because I've tried this method and it didn't work)

Gamin

Answer 1

Run your program in one terminal and attach to it from gdb in another terminal.

To attach to a running program, find the process ID (PID) of the program you want to attach to, then execute gdb <executable> <PID>.

Personally

how to attach a process to gdb? – Gamin

What if I want to debug program startup? – Albaalbacete

@Gamin that command will attach directly your process. It required root permission in my try. – Aladdin

Answer 2

As an addition to Jonathan Reinhart's answer, here is a oneliner to attach to a running program by name:

gdb -p $(pgrep <executable-name>)

Paranymph

This works if there is only one running instance of <executable-name>. – Personally

One of these does the job: gdb -p $(pgrep <executable-name> | select); gdb -p $(pgrep <executable-name> | peco); gdb -p $(pgrep <executable-name> | fzf ); – Paranymph

Answer 3

As a clarification you don't need the executable name if you do know the process id of the program. This will allow you to attach a program directly.

gdb -p PID

Goggle

This will work, but I don't think GDB will know where to find the executable in order to access the symbols and debug info. – Personally

In principle if the executable is built with debug symbols or say RelWithDebInfo modern gdb is able to attach the executable corresponding to the PID specified. – Goggle

Oh it probably uses /proc/$PID/exe. – Personally