GDB (not) handling SIGINT...?

[Simon Marchi]

On 2018-11-13 17:37, Paul Smith wrote:
> Hi all; I'm using GDB 8.1 on a modern 64bit GNU/Linux system (Ubuntu
> 18.04LTS).
> 
> Recently I added a call to sigtimedwait() for SIGINT into my
> (multithreaded) program and now I'm having an issue with GDB.
> 
> What I want is that if I attach to my program then continue, then use
> ^C at the GDB terminal, I should get a (gdb) prompt back but I do NOT
> want the SIGINT delivered to my program to wake up my sigtimedwait()
> call (because it will cause my program to do various things that I
> don't want it to do).
> 
> I see that SIGINT is set to nopass:
> 
>   (gdb) info signals SIGINT
>   Signal        Stop      Print   Pass to program Description
>   SIGINT        Yes       Yes     No              Interrupt
> 
> but yet when I use ^C at the GDB prompt my sigtimedwait() call inside
> my program does return with "2" (SIGINT), which I don't want.
> 
> I guess I don't understand what the docs mean when they say that the
> signal is not passed to the process under debug.  Note that in my case
> I'm attaching to the program from a completely different terminal so
> there's no issue with process groups etc. and as far as I can
> understand it, the signal should only be delivered to GDB not my
> process, so unless there's some weird magic at work here it must be GDB
> forwarding that signal to my process.
> 
> Anyone have any thoughts about this?
> 
> Cheers!

I was able to reproduce it with, it really sounds like a bug.

If others want to try, here's my test program:

#include <stdio.h>
#include <signal.h>
#include <stdlib.h>

int main()
{
	sigset_t set;
	sigemptyset(&set);
	sigaddset(&set, SIGINT);
	sigprocmask(SIG_BLOCK, &set, NULL);
	for (int i = 0; i < 10; i++) {
		int n = sigwaitinfo(&set, NULL);
		printf("signal %d\n", n);
	}
}


Run in a terminal, attach with GDB in another terminal, continue, then 
ctrl-C in GDB's terminal.  The first call to sigwaitinfo returns -1, I 
think it's expected as the syscall gets interrupted.  But the subsequent 
calls return 2, showing that indeed the process has received a SIGINT.

Paul, could you please file a bug?  You can re-use this test program if 
you want.

Simon