pgsjc/handling_SIGTSTP.c

+/* 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");
     }
 }