Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.16 sigsuspendFunction 359

10.16 sigsuspendFunction

We have seen how we can change the signal mask for a process to block and unblock

selected signals. We can use this technique to protect critical regions of code that we

don’t want interrupted by a signal. But what if we want to unblock a signal and then

pause,waiting for the previously blocked signal to occur? Assuming that the signal is

SIGINT,the incorrect way to do this is

sigset_t newmask, oldmask;

sigemptyset(&newmask);

sigaddset(&newmask, SIGINT);

/* block SIGINT and save current signal mask */

if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)

err_sys("SIG_BLOCK error");

/* critical region of code */

/* restore signal mask, which unblocks SIGINT */

if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)

err_sys("SIG_SETMASK error");

/* window is open */

pause(); /* wait for signal to occur */

/* continue processing */

If the signal is sent to the process while it is blocked, the signal delivery will be deferred

until the signal is unblocked. To the application, this can look as if the signal occurs

between the unblocking and thepause(depending on how the kernel implements

signals). If this happens, or if the signal does occur between the unblocking and the

pause, we have a problem. Any occurrence of the signal in this window of time is lost,

in the sense that we might not see the signal again, in which case thepausewill block

indefinitely.This is another problem with the earlier unreliable signals.

To correct this problem, we need a way to both restorethe signal mask and put the

process to sleep in a single atomic operation. This feature is provided by the

sigsuspendfunction.

#include <signal.h>

int sigsuspend(const sigset_t *sigmask);

Returns:−1witherrnoset toEINTR

The signal mask of the process is set to the value pointed to bysigmask.Then the

process is suspended until a signal is caught or until a signal occurs that terminates the

process. Ifasignal is caught and if the signal handler returns, then sigsuspend

returns, and the signal mask of the process is set to its value beforethe call to

sigsuspend.

Note that there is no successful return from this function. If it returns to the caller,it

always returns−1witherrnoset toEINTR(indicating an interrupted system call).