sigwait() breaks when straced

From: Sanjoy Mahajan
Date: Sat Jul 30 2005 - 18:52:04 EST

Pavel Machek wrote:
> If you think it is a linux bug, can you produce small test case doing
> just the sigwait, and post it on l-k with big title "sigwait() breaks
> when straced, and on suspend"?

Here it is. I haven't tested the sigwait()+suspend lately, since
suspend isn't working with any kernel except and I'm chasing
down other acpi errors in 2.6.13-*. But here's a test case for how
sigwait() breaks when straced (see C file below). It is with 2.6.13-rc4
on a Thinkpad 600X (Pentium III), Debian 'testing', libc

$ gcc waiting.c -o waiting -lpthread
$ ./waiting
Sigwaiting...[strace my pid, which is 3359]
[in another shell I run 'strace -p 3359', and get:]
sigwait() returned 4, errno=0, sig=77

In the strace window, I get

$ strace -p 3359
Process 3359 attached - interrupt to quit
write(2, "sigwait() returned 4, errno=0, s"..., 39) = 39
exit_group(4) = ?
Process 3359 detached

According to the man entry for sigwait:

The !sigwait! function never returns an error.

so the return value should not be 4 (or the docs are not right).

Here's waiting.c:

#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <errno.h>

sigset_t mask;

int main () {
int ret, id;
int sig = 77; /* easy to see if it gets changed */

id = getpid();
fprintf (stderr, "Sigwaiting...[strace my pid, which is %d]\n", id);
ret = sigwait(&mask, &sig);
fprintf (stderr,
"sigwait() returned %d, errno=%d, sig=%d\n",
ret, errno, sig);
return ret;


`A society of sheep must in time beget a government of wolves.'
- Bertrand de Jouvenal
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at