SIGTERM signal handler not printing to a console - c

Question

I am trying to catch the SIGTERM signal and print a message in the handler from a Linux daemon:
void SigStop_Handler(int sig)
{
D(printf("****************** HANDLED STOP SIGNAL ******************\n"));
printf("\n");
}
int main(int argc, char *argv[])
{
signal(SIGTERM, SigStop_Handler);
while(true)
{
//do something
}
return 0;
}
The program runs as a daemon, started from a command line:
systemctl start abc
The deameon will be stopped by:
systemctl stop abc
When the daemon is stopping, I expected that the message will be printed on the console. However the message is not printed and the command line doesn’t return to the command-promt. It does return after a while (timeout). The daemon will be stopped, but the message will not be printed.
What am I doing wrong?

Answer 1

Two items to consider: terminating the daemon when signal is received, and getting the message.
In the context of systemctl, you want your signal handler to exit the program. Otherwise,
it will just 'print' the message the messages, and resume processing. Try
void SigStop_Handler(int sig)
{
D(fprintf(stderr, "****************** HANDLED STOP SIGNAL ******************\n"));
printf("\n");
exit(sig+128) ;
}
Also, given that the process run as a daemon, stdout is not going to be connected to the terminal. Instead, it will get redirected into log file. Also, suggesting using stderr to send messages, instead of stdout (see code above). Also, look at StandardOutput and StandardErrro configuration entries for your service.
The delay that you currently observe occurs because 'systemctl' wait for few seconds after sending the TERM signal - giving the daemon change to terminate. If the daemon does not terminate voluntarily, a KILL signal will be sent, which will force the daemon to terminate, unconditionally, immediately.