The Linux Programming Interface

Signals: Advanced Features 473

operation (Section 5.3) to set the O_NONBLOCK flag for the file descriptor, so that reads
are nonblocking and will fail with the error EAGAIN if no signals are pending.
When a signal is read from a signalfd file descriptor, it is consumed and ceases
to be pending for the process.

Listing 22-7: Using signalfd() to read signals

––––––––––––––––––––––––––––––––––––––––––––––––––signals/signalfd_sigval.c
#include <sys/signalfd.h>
#include <signal.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
sigset_t mask;
int sfd, j;
struct signalfd_siginfo fdsi;
ssize_t s;

if (argc < 2 || strcmp(argv[1], "--help") == 0)
usageErr("%s sig-num...\n", argv[0]);

printf("%s: PID = %ld\n", argv[0], (long) getpid());

sigemptyset(&mask);
for (j = 1; j < argc; j++)
sigaddset(&mask, atoi(argv[j]));

if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
errExit("sigprocmask");

sfd = signalfd(-1, &mask, 0);
if (sfd == -1)
errExit("signalfd");

for (;;) {
s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
if (s != sizeof(struct signalfd_siginfo))
errExit("read");

printf("%s: got signal %d", argv[0], fdsi.ssi_signo);
if (fdsi.ssi_code == SI_QUEUE) {
printf("; ssi_pid = %d; ", fdsi.ssi_pid);
printf("ssi_int = %d", fdsi.ssi_int);
}
printf("\n");
}
}
––––––––––––––––––––––––––––––––––––––––––––––––––signals/signalfd_sigval.c