Oh, sorry. I tried on a different machine, and realized one have to use
"run&".

So, here's what I did:

$ cat main.c
    #include <stdio.h>
    #include <unistd.h>

    int main(void) {
        for (int i = 0; ; i++) {
            printf("%d\n", i);
            sleep(1);
        }
    }

$ gcc -g -o break main.c
$ emacs -Q main.c
M-x gdb RET
# in *gud-break*, NOTE it's "run&"
(gdb) run&
# in main.c
(goto-line 6)
(gud-break 1) ; or C-x C-a C-b
# nothing happens
M-: (gud-call "break 6") RET
# breakpoint is set and process execution is paused on hitting that
breakpoint

Hope that's more clear now!

On Wed, Apr 26, 2023 at 8:10 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: tatrics@gmail.com
> > Date: Tue, 25 Apr 2023 21:18:52 +0200
> >
> > When I try to set a breakpoint with
> >     "C-x C-a C-b" or (gud-break 1)
> > it doesn't seem to work.
>
> I cannot reproduce this: "C-x C-a C-b" does work for me.  Please
> describe the recipe for reproducing the problem completely, starting
> from "emacs -Q" and including all the commands you need to type/invoke
> to reproduce the issue.
>
> Thanks.
>