pgsjc/handling_SIGTSTP.c

This is pgsjc/handling_SIGTSTP.c (Listing 34-6, page 724), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* handling_SIGTSTP.c

   Demonstrate the correct way to catch SIGTSTP and raise it again (so that a
   parent process that is monitoring this program can see that it was stopped
   by SIGTSTP).
*/
#include <signal.h>
#include "tlpi_hdr.h"
static void                             /* Handler for SIGTSTP */
tstpHandler(int sig)
{
    sigset_t tstpMask, prevMask;
    int savedErrno;
    struct sigaction sa;

    savedErrno = errno;                 /* In case we change 'errno' here */

    printf("Caught SIGTSTP\n");         /* UNSAFE (see Section 21.1.2) */

    if (signal(SIGTSTP, SIG_DFL) == SIG_ERR)
        errExit("signal");              /* Set handling to default */

    raise(SIGTSTP);                     /* Generate a further SIGTSTP */

    /* Unblock SIGTSTP; the pending SIGTSTP immediately suspends the program */

    sigemptyset(&tstpMask);
    sigaddset(&tstpMask, SIGTSTP);
    if (sigprocmask(SIG_UNBLOCK, &tstpMask, &prevMask) == -1)
        errExit("sigprocmask");

    /* Execution resumes here after SIGCONT */

    if (sigprocmask(SIG_SETMASK, &prevMask, NULL) == -1)
        errExit("sigprocmask");         /* Reblock SIGTSTP */

    sigemptyset(&sa.sa_mask);           /* Reestablish handler */
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = tstpHandler;
    if (sigaction(SIGTSTP, &sa, NULL) == -1)
        errExit("sigaction");

    printf("Exiting SIGTSTP handler\n");
    errno = savedErrno;
}
int
main(int argc, char *argv[])
{
    struct sigaction sa;

    /* Only establish handler for SIGTSTP if it is not being ignored */

    if (sigaction(SIGTSTP, NULL, &sa) == -1)
        errExit("sigaction");

    if (sa.sa_handler != SIG_IGN) {
        sigemptyset(&sa.sa_mask);
        sa.sa_flags = SA_RESTART;
        sa.sa_handler = tstpHandler;
        if (sigaction(SIGTSTP, &sa, NULL) == -1)
            errExit("sigaction");
    }

    for (;;) {                          /* Wait for signals */
        pause();
        printf("Main\n");
    }
}

 

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