Reputation: 58352
What's the proper way to log a shutdown message when an application (a C++ daemon, in my case) receives a SIGTERM or SIGINT?
According to CERT and the signal(7) manpage, many functions (including, presumably, those used by most logging libraries) aren't safe to call from signal handlers.
Upvotes: 2
Views: 1645
Reputation: 24174
Vlad Lazarenko wrote a great blog post earlier this year on this very topic. On Linux it boils down to creating a signal descriptor with signalfd(2)
and use an event loop such as poll(2)
or epoll_wait(2)
. Here is Vlad's example reading from the descriptor
#include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
sigset_t mask;
int sfd;
struct signalfd_siginfo fdsi;
ssize_t s;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGQUIT);
/* Block signals so that they aren't handled
according to their default dispositions */
if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
handle_error("sigprocmask");
sfd = signalfd(-1, &mask, 0);
if (sfd == -1)
handle_error("signalfd");
for (;;) {
s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
if (s != sizeof(struct signalfd_siginfo))
handle_error("read");
if (fdsi.ssi_signo == SIGINT) {
printf("Got SIGINT\n");
} else if (fdsi.ssi_signo == SIGQUIT) {
printf("Got SIGQUIT\n");
exit(EXIT_SUCCESS);
} else {
printf("Read unexpected signal\n");
}
}
}
This example can easily be extended to integrate into an event loop.
Upvotes: 1
Reputation: 652
Logging could be done not from handler, but after it:
int received_sigterm = 0;
void
sigterm_handler(int sig)
{
received_sigterm = 1;
}
void
loop(void)
{
for(;;) {
sleep(1);
if (received_sigterm)
log("finish\n");
}
}
int
main()
{
log("start\n");
signal(SIGTERM, sigterm_handler);
loop();
}
The concept is borrowed from openssh-6.1 sources.
Upvotes: 0