I'm seeing something odd, and I'm hoping someone can help. Running
linux 2.6.9 on ppc.
I was writing a small test program to verify behaviour of network code
when interrupted by a signal. To make the code bulletproof (or so I
hoped) I used setjmp/longjmp to recover from the case where the syscall
was being automatically restarted. Everything seemed to be okay.
However, I then reversed the order of the tests (doing the SA_RESTART
case first). At this point I got the strange behaviour that the third
signal was never delivered. Strace and gdb both show the recv() call
running forever, and the signal never gets delivered.
It this point I suspected my setjmp/longjmp code was bad, so to test it
I changed the recv() call to a pause() call. After that change,
everything worked fine. I changed it to a read() call, and it breaks
again with the third signal never being delivered.
Looking at the strace output, the only interesting thing I see is that
both recv() and read() return with (errno == ERESTARTSYS), while pause()
returns with (errno == ERESTARTNOHAND).
I've included the source code as well as traces of the pause() and
recv() runs. If anyone has any ideas I'd love to hear them....
Chris
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <string.h>
#include <linux/stddef.h>
#include <stdio.h>
#include <signal.h>
#include <errno.h>
#include <setjmp.h>
#include <unistd.h>
jmp_buf env;
volatile int signalled;
void handler(int sig)
{
if (signalled)
longjmp(env, 1);
signalled = 1;
struct itimerval itv = {{0,0},{0,500000}};
setitimer(ITIMER_REAL,&itv,0);
}
void dotest(int sock, int flags)
{
int rc = setjmp(env);
if (rc) {
printf("system call was not interrupted\n");
return;
}
struct sigaction action;
action.sa_handler = handler;
action.sa_flags = flags;
sigaction(SIGALRM, &action, 0);
signalled = 0;
struct itimerval itv = {{0,0},{0,500000}};
setitimer(ITIMER_REAL,&itv,0);
char buf;
rc = recv(sock, &buf, 1, 0);
if(rc < 0) {
int localerrno = errno;
printf("recv() interrupted, errno: %d\n", localerrno);
return;
}
}
int main()
{
int sock = socket (PF_INET, SOCK_DGRAM, 0);
printf("\ntesting behaviour of recv() on signal interruption with
restarted syscalls:\n");
dotest(sock, SA_RESTART);
printf("testing behaviour of recv() on signal interruption with
non-restarted syscalls:\n");
dotest(sock, 0);
return 0;
}
using pause()
write(1, "testing behaviour of recv() on s"..., 76testing behaviour of
recv() on signal interruption with restarted syscalls:
) = 76
rt_sigaction(SIGALRM, {0x10000844, [], SA_RESTART}, NULL, 8) = 0
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
pause() = ? ERESTARTNOHAND (To be restarted)
--- SIGALRM (Alarm clock) @ 0 (0) ---
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
sigreturn() = ? (mask now [])
write(1, "recv() interrupted, errno: 4\n", 29recv() interrupted, errno: 4
) = 29
write(1, "testing behaviour of recv() on s"..., 80testing behaviour of
recv() on signal interruption with non-restarted syscalls:
) = 80
rt_sigaction(SIGALRM, {0x10000844, [], 0}, NULL, 8) = 0
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
pause() = ? ERESTARTNOHAND (To be restarted)
--- SIGALRM (Alarm clock) @ 0 (0) ---
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
sigreturn() = ? (mask now [])
write(1, "recv() interrupted, errno: 4\n", 29recv() interrupted, errno: 4
) = 29
using recv()
write(1, "testing behaviour of recv() on s"..., 76testing behaviour of
recv() on signal interruption with restarted syscalls:
) = 76
rt_sigaction(SIGALRM, {0x100008a4, [], SA_RESTART}, NULL, 8) = 0
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
recv(3, 0x7ffff620, 1, 0) = ? ERESTARTSYS (To be restarted)
--- SIGALRM (Alarm clock) @ 0 (0) ---
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
sigreturn() = ? (mask now [])
recv(3, 0x7ffff620, 1, 0) = ? ERESTARTSYS (To be restarted)
--- SIGALRM (Alarm clock) @ 0 (0) ---
write(1, "system call was not interrupted\n", 32system call was not
interrupted
) = 32
write(1, "testing behaviour of recv() on s"..., 80testing behaviour of
recv() on signal interruption with non-restarted syscalls:
) = 80
rt_sigaction(SIGALRM, {0x100008a4, [], 0}, NULL, 8) = 0
setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 500000}}, NULL) = 0
recv(3,
Friesen, Christopher [CAR:VC21:EXCH] wrote:
> It this point I suspected my setjmp/longjmp code was bad, so to test it
> I changed the recv() call to a pause() call. After that change,
> everything worked fine. I changed it to a read() call, and it breaks
> again with the third signal never being delivered.
My bad, didn't notice that pause() doesn't get restarted. So the
setjmp/longjmp code isn't being called with pause.
Also, behaviour is the same with 2.6.5 on a P4.
Chris
Bah...
As all of you no doubt suspected, I should have been using
sigsetjmp()/siglongjmp().
Sorry to waste your time.
Chris