Cancellation points in signal handlers? - c

Question

What happens if a program calls a function which is a cancellation point from a signal handler? There are a number of functions which POSIX specifies as both async-signal-safe and cancellation points. If a signal handler calls such a function and cancellation is acted upon, the result is quite similar to what would happen if the thread had enabled asynchronous cancellation - actually much worse, because all the cancellation cleanup handlers, which are probably not async-signal-safe, would be called from a signal-handler context.
What does POSIX actually specify in this case, and what do implementations actually do? I can't find any language in POSIX that would forbid cancellation points in signal handlers from being acted upon, nor any such protection in the glibc/nptl source.

Answer 1

I'm not aware that POSIX even dares to mention this topic, but I haven't done an exhaustive search.
Some brief experimentation with a gcc/nptl system reveals that, as I suspected and I think you did too, there is no such protection in NPTL - the cancellation handlers do indeed get called, from within the signal handler context.
The program below (apologies for the hackiness etc) displays the following output:
Signal handler called
Sent cancellation
Cleanup called
In sighandler
... indicating that:
the signal handler got called
the other thread then called pthread_cancel()
the cancellation handler then got called, without the signal handler completing
Here's the program:
#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
pthread_t mainthread;
int in_sighandler = 0;
void
cleanup (void *arg)
{
write(1, "Cleanup called\n", strlen("Cleanup called\n"));
if (in_sighandler) {
write(1, "In sighandler\n", strlen("In sighandler\n"));
} else {
write(1, "Not in sighandler\n", strlen("In sighandler\n"));
}
}
void
sighandler (int sig, siginfo_t *siginfo, void *arg)
{
in_sighandler = 1;
write(1,"Signal handler called\n", strlen("Signal handler called\n")); // write() is a CP
usleep(3000000); // usleep() is a CP; not strictly async-signal-safe but happens to be so in Linux
write(1, "Signal handler exit\n", strlen("Signal handler exit\n"));
in_sighandler = 0;
}
void *
thread (void *arg)
{
sleep(1);
pthread_kill(mainthread, SIGUSR1);
usleep(500000);
pthread_cancel(mainthread);
printf("Sent cancellation\n");
return (NULL);
}
int
main (int argc, char **argv)
{
int rc;
struct sigaction sa;
pthread_t threadid;
mainthread = pthread_self();
// Set up a signal handler to test its cancellation properties
sa.sa_sigaction = &sighandler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_SIGINFO;
rc = sigaction(SIGUSR1, &sa, NULL);
assert(rc == 0);
// Set up a thread to send us signals and cancel us
rc = pthread_create(&threadid, NULL, &thread, NULL);
assert(rc == 0);
// Set up cleanup handlers and loop forever
pthread_cleanup_push(&cleanup, NULL);
while (1) {
sleep(60);
}
pthread_cleanup_pop(0);
return (0);
}