catching signals while reading from pipe with select()

Question

using select() with pipe - this is what I am doing and now I need to catch SIGTERM on that. how can I do it? Do I have to do it when select() returns error ( < 0 ) ?

Answer 1

First, SIGTERM will kill your process if not caught, and select() will not return. Thus, you must install a signal handler for SIGTERM. Do that using sigaction().

However, the SIGTERM signal can arrive at a moment where your thread is not blocked at select(). It would be a rare condition, if your process is mostly sleeping on the file descriptors, but it can otherwise happen. This means that either your signal handler must do something to inform the main routine of the interruption, namely, setting some flag variable (of type sig_atomic_t), or you must guarantee that SIGTERM is only delivered when the process is sleeping on select().

I'll go with the latter approach, since it's simpler, albeit less flexible (see end of the post).

So, you block SIGTERM just before calling select(), and reblock it right away after the function returns, so that your process only receives the signal while sleeping inside select(). But note that this actually creates a race condition. If the signal arrives just after the unblock, but just before select() is called, the system call will not have been called yet and thus it will not return -1. If the signal arrives just after select() returns successfully, but just before the re-block, you have also lost the signal.

Thus, you must use pselect() for that. It does the blocking/unblocking around select() atomically.

First, block SIGTERM using sigprocmask() before entering the pselect() loop. After that, just call pselect() with the original mask returned by sigprocmask(). This way you guarantee your process will only be interrupted while sleeping on select().

In summary:

1. Install a handler for SIGTERM (that does nothing);
2. Before entering the pselect() loop, block SIGTERM using sigprocmask();
3. Call pselect() with the old signal mask returned by sigprocmask();
4. Inside the pselect() loop, now you can check safely whether pselect() returned -1 and errno is EINTR.

Please note that if, after pselect() returns successfully, you do a lot of work, you may experience bigger latency when responding to SIGTERM (since the process must do all processing and return to pselect() before actually processing the signal). If this is a problem, you must use a flag variable inside the signal handler, so that you can check for this variable in a number of specific points in your code. Using a flag variable does not eliminate the race condition and does not eliminate the need for pselect(), though.

Remember: whenever you need to wait on some file descriptors or for the delivery of a signal, you must use pselect() (or ppoll(), for the systems that support it).

Edit: nothing better than a code example to illustrate the usage.

#define _POSIX_C_SOURCE 200809L
#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
#include <unistd.h>

// Signal handler to catch SIGTERM.
void sigterm(int signo) {
    (void)signo;
}

int main(void) {
    // Install the signal handler for SIGTERM.
    struct sigaction s;
    s.sa_handler = sigterm;
    sigemptyset(&s.sa_mask);
    s.sa_flags = 0;
    sigaction(SIGTERM, &s, NULL);

    // Block SIGTERM.
    sigset_t sigset, oldset;
    sigemptyset(&sigset);
    sigaddset(&sigset, SIGTERM);
    sigprocmask(SIG_BLOCK, &sigset, &oldset);

    // Enter the pselect() loop, using the original mask as argument.
    fd_set set;
    FD_ZERO(&set);
    FD_SET(0, &set);
    while (pselect(1, &set, NULL, NULL, NULL, &oldset) >= 0) {
        // Do some processing. Note that the process will not be
        // interrupted while inside this loop.
        sleep(5);
    }

    // See why pselect() has failed.
    if (errno == EINTR)
        puts("Interrupted by SIGTERM.");
    else
        perror("pselect()");
    return EXIT_SUCCESS;
}