c - gdb freezes in malloc

Question

Suppose I have some C program like this:

#include <stdlib.h>
#include <stdbool.h>

int main()
{
    while (true) {
        void *p = malloc(1000);
        free(p);
    }
    return 0;
}

and I attach to it with gdb like this gdb a.out PID. gdb successfully attaches to it but that I try to do something like call printf("bla bla bla") gdb freezes and if I press Ctrl^C I get this:

(gdb) call printf("bla bla bla")
^C
Program received signal SIGINT, Interrupt.
__lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95  ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file or directory.
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(malloc) will be abandoned.
When the function is done executing, GDB will silently stop.

I suppose that this happens because my a.out was creating an object and acquired a lock inside malloc.c and in this moment I connected with gdb and tried to create string "bla bla bla" using malloc.

My question is how can I detect that I'm inside malloc.c and let my program finish this execution? I need to do it not inside command line but using some sort of gdb scripting (I only can execute commands inside gdb with -ex option).

Answer 1

The reason you're froze is probably a lock that's being held by your program, and is also required by printf. When you try to aquire it twice - you fail.

A possible WA is when breaking your program to call printf, just before you make the call, type finish - it will cause the current function to complete and return to the main frame. This will ensure the lock is free before you call printf.