10.12. sigprocmask Function

Recall from Section 10.8 that the signal mask of a process is the set of signals currently blocked from delivery to that process. A process can examine its signal mask, change its signal mask, or perform both operations in one step by calling the following function.

#include <signal.h> int sigprocmask(int how, const sigset_t *restrict set, sigset_t *restrict oset);
Returns: 0 if OK, 1 on error

First, if oset is a non-null pointer, the current signal mask for the process is returned through oset.

Second, if set is a non-null pointer, the how argument indicates how the current signal mask is modified. Figure 10.13 describes the possible values for how. SIG_BLOCK is an inclusive-OR operation, whereas SIG_SETMASK is an assignment. Note that SIGKILL and SIGSTOP can't be blocked.

Figure 10.13. Ways to change current signal mask using sigprocmask

how	Description
SIG_BLOCK	The new signal mask for the process is the union of its current signal mask and the signal set pointed to by set. That is, set contains the additional signals that we want to block.
SIG_UNBLOCK	The new signal mask for the process is the intersection of its current signal mask and the complement of the signal set pointed to by set. That is, set contains the signals that we want to unblock.
SIG_SETMASK	The new signal mask for the process is replaced by the value of the signal set pointed to by set.

If set is a null pointer, the signal mask of the process is not changed, and how is ignored.

After calling sigprocmask, if any unblocked signals are pending, at least one of these signals is delivered to the process before sigprocmask returns.

The sigprocmask function is defined only for single-threaded processes. A separate function is provided to manipulate a thread's signal mask in a multithreaded process. We'll discuss this in Section 12.8.

Example

Figure 10.14 shows a function that prints the names of the signals in the signal mask of the calling process. We call this function from the programs shown in Figure 10.20 and Figure 10.22.

To save space, we don't test the signal mask for every signal that we listed in Figure 10.1. (See Exercise 10.9.)

Figure 10.14. Print the signal mask for the process

#include "apue.h"
#include <errno.h>

void
pr_mask(const char *str)
{
    sigset_t    sigset;
    int         errno_save;

    errno_save = errno;     /* we can be called by signal handlers */
    if (sigprocmask(0, NULL, &sigset) < 0)
        err_sys("sigprocmask error");

    printf("%s", str);
    if (sigismember(&sigset, SIGINT))   printf("SIGINT ");
    if (sigismember(&sigset, SIGQUIT))  printf("SIGQUIT ");
    if (sigismember(&sigset, SIGUSR1))  printf("SIGUSR1 ");
    if (sigismember(&sigset, SIGALRM))  printf("SIGALRM ");

    /* remaining signals can go here */

    printf("\n");
    errno = errno_save;
}