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sigprocmask

Syntax

 
#include <signal.h>

int sigprocmask (int how, const sigset_t *new_set, sigset_t *old_set)

Description

This function is used to examine and/or change the program's current signal mask. The current signal mask determines which signals are blocked from being delivered to the program. A signal is blocked if its bit in the mask is set. (See section sigismember, See section sigaddset, See section sigdelset, See section sigemptyset, See section sigfillset, for information about functions to manipulate the signal masks.) When a blocked signal happens, it is not delivered to the program until such time as that signal is unblocked by another call to sigprocmask. Thus blocking a signal is an alternative to ignoring it (by setting its handler to SIG_IGN, see section signal), but has an advantage of not missing the signal entirely.

The value of the argument how determines the operation: if it is SIG_BLOCK, the set pointed to by the argument new_set is added to the current signal mask. If the value is SIG_UNBLOCK, the set pointed to by new_set is removed from the current signal mask. If the value is SIG_SETMASK, the current mask is replaced by the set pointed to by new_set.

If the argument old_set is not NULL, the previous mask is stored in the space pointed to by old_set. If the value of the argument new_set is NULL, the value of how is not significant and the process signal mask is unchanged; thus, the call with a zero new_set can be used to inquire about currently blocked signals, without changing the current set.

If the new set defined by the call causes some pending signals to be unblocked, they are all delivered (by calling raise) before the call to sigprocmask returns.

The DJGPP implementation only records a single occurrence of any given signal, so when the signal is unblocked, its handler will be called at most once.

It is not possible to block CPU exceptions such as Page Fault, General Protection Fault etc. (mapped to SIGSEGV signal); for these, sigprocmask will behave as if the call succeeded, but when an exception happens, the signal handler will be called anyway (the default handler will abort the program).

Also note that there are no provisions to save and restore any additional info about the signal beyond the fact that it happened. A signal handler might need such info to handle the signal intelligently. For example, a handler for SIGFPE might need to examine the status word of the FPU to see what exactly went wrong. But if the signal was blocked and is delivered after a call to sigprocmask has unblocked it, that information is lost. Therefore, if you need access to such auxiliary information in the signal handler, don't block that signal.

Return Value

0 on success, -1 for illegal value of sig or illegal address in new_set or old_set.

Portability

ANSI/ISO C No
POSIX 1003.2-1992; 1003.1-2001

Example

 
#include <conio.h>
#include <signal.h>

static void
sig_catcher (int signo)
{
  cprintf ("\r\nGot signal %d\r\n", signo);
}

int
main (void)
{
  sigset_t sigmask, prevmask;

  signal (SIGINT, sig_catcher);

  sigemptyset (&sigmask);

  sigaddset (&sigmask, SIGINT);
  if (sigprocmask (SIG_SETMASK, &sigmask, &prevmask) == 0)
    cputs ("SIGINT blocked.  Try to interrupt me now.\r\n");
  while (!kbhit ())
    ;
  cputs ("See?  I wasn't interrupted.\r\n");
  cputs ("But now I will unblock SIGINT, and then get the signal.\r\n");
  sigprocmask (SIG_UNBLOCK, &sigmask, &prevmask);
  return 0;
}


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  Copyright 2004   by DJ Delorie     Updated Apr 2004