POSIX

Signals

Syntax#

  • unsigned alarm(unsigned seconds);
  • int kill(pid_t pid, int sig);

Parameters#

Function, Parameter(s), Return Value Description
alarm() function name
unsigned seconds Seconds to raise an alarm or 0 to cancel any pending alarm
>= 0 0 if no other alarm was pending, else the number of seconds the pending alarm still had open. This function won’t fail.
- -
kill() function name
pid_t pid .
int sig 0 or signal ID
0, -1 On success 0 is returned, -1 on failure with setting errno to EINVAL, EPERMor ESRCH.
## Raising SIGALARM with the default action
Using alarm, user can schedule SIGALARM signal to be raised after specified interval. In case user did not blocked, ignored or specified explicit signal handler for this signal, the default action for this signal will be performed on arrival. Per specification default action for SIGALARM is to terminate the process: 
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char** argv)
{
    printf("Hello!\n");

    // Set the alarm for five second
    alarm(5); // Per POSIX, this cannot fail

    // Now sleep for 15 seconds
    for (int i = 1; i <= 15; i++)
    {
        printf("%d\n", i);
        sleep(1);
    }

    // And print the message before successful exit
    printf("Goodbye!\n");

    return EXIT_SUCCESS;
}

This outputs:

Hello!
1
2
3
4
5
[2]    35086 alarm      ./a.out

Setting signal handler using sigaction and raising signals using raise

In order for a program to react to a certain signal, other than using default action, custom signal handler can be installed using sigaction. sigaction receives three arguments - signal to act on, pointer to sigaction_t structure which, if not NULL, is describing new behaviour and pointer to sigaction_t which, if not NULL will be filled with the old behaviour (so one can restore it). Raising signals in same process can be done with raise method. If more control is needed (to send the signal to some other process, kill or pthread_kill can be used, which accept the destination process id or thread id).

#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Signals are numbered from 1, signal 0 doesn't exist
volatile sig_atomic_t last_received_signal = 0;

// Signal handler, will set the global variable
// to indicate what is the last signal received.
// There should be as less work as possible inside
// signal handler routine, and one must take care only
// to call reentrant functions (in case of signal arriving
// while program is already executing same function)
void signal_catcher(int signo, siginfo_t *info, void *context)
{
    last_received_signal = info->si_signo;
}

int main (int argc, char** argv)
{
    // Setup a signal handler for SIGUSR1 and SIGUSR2
    struct sigaction act;
    memset(&act, 0, sizeof act);

    // sigact structure holding old configuration
    // (will be filled by sigaction):
    struct sigaction old_1;
    memset(&old_1, 0, sizeof old_1);
    struct sigaction old_2;
    memset(&old_2, 0, sizeof old_2);

    act.sa_sigaction = signal_catcher;
    // When passing sa_sigaction, SA_SIGINFO flag
    // must be specified. Otherwise, function pointed
    // by act.sa_handler will be invoked
    act.sa_flags = SA_SIGINFO;

    if (0 != sigaction(SIGUSR1, &act, &old_1))
    {
        perror("sigaction () failed installing SIGUSR1 handler");
        return EXIT_FAILURE;
    }

    if (0 != sigaction(SIGUSR2, &act, &old_2))
    {
        perror("sigaction() failed installing SIGUSR2 handler");
        return EXIT_FAILURE;
    }

    // Main body of "work" during which two signals
    // will be raised, after 5 and 10 seconds, and which
    // will print last received signal
    for (int i = 1; i <= 15; i++)
    {
        if (i == 5)
        {
            if (0 != raise(SIGUSR1))
            {
                perror("Can't raise SIGUSR1");
                return EXIT_FAILURE;
            }
        }

        if (i == 10)
        {
            if (0 != raise(SIGUSR2))
            {
                perror("Can't raise SIGUSR2");
                return EXIT_FAILURE;
            }
        }

        printf("Tick #%d, last caught signal: %d\n",
            i, last_received_signal);

        sleep(1);
    }

    // Restore old signal handlers
    if (0 != sigaction(SIGUSR1, &old_1, NULL))
    {
        perror("sigaction() failed restoring SIGUSR1 handler");
        return EXIT_FAILURE;
    }

    if (0 != sigaction(SIGUSR2, &old_2, NULL))
    {
        perror("sigaction() failed restoring SIGUSR2 handler");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

This outputs:

Tick #1, last caught signal: 0
Tick #2, last caught signal: 0
Tick #3, last caught signal: 0
Tick #4, last caught signal: 0
Tick #5, last caught signal: 30
Tick #6, last caught signal: 30
Tick #7, last caught signal: 30
Tick #8, last caught signal: 30
Tick #9, last caught signal: 30
Tick #10, last caught signal: 31
Tick #11, last caught signal: 31
Tick #12, last caught signal: 31
Tick #13, last caught signal: 31
Tick #14, last caught signal: 31
Tick #15, last caught signal: 31

A process committing suicide using kill()

A process can (try to) send a signal to any other process using the kill() function.

To do so, the sending process needs to known the receiving process’ PID. As, without introducing a race, a process can only be sure of its own PID (and the PIDs of its children) the most simple example to demonstrate the usage of kill() is to have a process send a signal to itself.

Below an example of a process initiating its own termination by sending itself a kill-signal (SIGKILL):

#define _POSIX_C_SOURCE 1

#include <sys/types.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>


int main (void)
{
  pid_t pid = getpid(); /* Get my iown process ID. */

  kill(pid, SIGKILL); /* Send myself a KILL signal. */

  puts("Signal delivery initiated.");  /* Although not guaranteed, 
                                  practically the program never gets here. */

  pause(); /* Wait to die. */

  puts("This never gets printed.");
}

Output:

Killed

(… or alike, depending on the implementation)

Handle SIGPIPE generated by write() in a thread-safe manner

When write() is called for a named or unnamed pipe or stream socket whose reading end is closed, two things happen:

  1. SIGPIPE signal is sent to the process that called write()
  1. SIGPIPE signal is sent to the thread that called write()
  1. EPIPE error is returned by write()

There are several ways to deal with SIGPIPE:

  • For sockets, SIGPIPE may be disabled by setting platform-specific options like MSG_NOSIGNAL in Linux and SO_NOSIGPIPE in BSD (works only for send, but not for write). This is not portable.

  • For FIFOs (named pipes), SIGPIPE will not be generated if writer uses O_RDWR instead of O_WRONLY, so that reading end is always opened. However, this disables EPIPE too.

  • We can ignore SIGPIPE or set global handler. This is a good solution, but it’s not acceptable if you don’t control the whole application (e.g. you’re writing a library).

  • With recent POSIX versions, we can use the fact that SIGPIPE is send to the thread that called write() and handle it using synchronous signal handling technique.

Code below demonstrates thread-safe SIGPIPE handling for POSIX.1-2004 and later.

It’s inspired by this post:

  • First, add SIGPIPE to signal mask of current thread using pthread_sigmask().
  • Check if there is already pending SIGPIPE using sigpending().
  • Call write(). If reading end is closed, SIGPIPE will be added to pending signals mask and EPIPE will be returned.
  • If write() returned EPIPE, and SIGPIPE was not already pending before write(), remove it from pending signals mask using sigtimedwait().
  • Restore original signal mask using pthread_sigmask().

Source code:

#include <unistd.h>
#include <time.h>
#include <errno.h>
#include <sys/signal.h>

ssize_t safe_write(int fd, const void* buf, size_t bufsz)
{
    sigset_t sig_block, sig_restore, sig_pending;

    sigemptyset(&sig_block);
    sigaddset(&sig_block, SIGPIPE);

    /* Block SIGPIPE for this thread.
     *
     * This works since kernel sends SIGPIPE to the thread that called write(),
     * not to the whole process.
     */
    if (pthread_sigmask(SIG_BLOCK, &sig_block, &sig_restore) != 0) {
        return -1;
    }

    /* Check if SIGPIPE is already pending.
     */
    int sigpipe_pending = -1;
    if (sigpending(&sig_pending) != -1) {
        sigpipe_pending = sigismember(&sig_pending, SIGPIPE);
    }

    if (sigpipe_pending == -1) {
        pthread_sigmask(SIG_SETMASK, &sig_restore, NULL);
        return -1;
    }

    ssize_t ret;
    while ((ret = write(fd, buf, bufsz)) == -1) {
        if (errno != EINTR)
            break;
    }

    /* Fetch generated SIGPIPE if write() failed with EPIPE.
     *
     * However, if SIGPIPE was already pending before calling write(), it was
     * also generated and blocked by caller, and caller may expect that it can
     * fetch it later. Since signals are not queued, we don't fetch it in this
     * case.
     */
    if (ret == -1 && errno == EPIPE && sigpipe_pending == 0) {
        struct timespec ts;
        ts.tv_sec = 0;
        ts.tv_nsec = 0;

        int sig;
        while ((sig = sigtimedwait(&sig_block, 0, &ts)) == -1) {
            if (errno != EINTR)
                break;
        }
    }

    pthread_sigmask(SIG_SETMASK, &sig_restore, NULL);
    return ret;
}
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