sigwait() breaks when straced
From: Sanjoy Mahajan (sanjoy_at_mrao.cam.ac.uk)
Date: 07/31/05
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To: Pavel Machek <pavel@ucw.cz> Date: Sat, 30 Jul 2005 19:50:10 -0400
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 2.6.11.4 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
2.3.2.
$ 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();
sigemptyset(&mask);
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;
}
-Sanjoy
`A society of sheep must in time beget a government of wolves.'
- Bertrand de Jouvenal
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