SIG_ERR

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
 
void signal_handler(int sig)
{
 printf ("Received signal: %d\n", sig);
}
 
int main(void)
{
 /* Install a signal handler. */
 if (signal (SIGTERM, signal_handler) == SIG_ERR)
 {
 printf ("Error while installing a signal handler.\n");
 exit (EXIT_FAILURE );
 }
 
 printf ("Sending signal: %d\n", SIGTERM );
 if (raise (SIGTERM ) != 0)
 {
 printf ("Error while raising the SIGTERM signal.\n");
 exit (EXIT_FAILURE );
 }
 
 printf ("Exit main()\n");
 return EXIT_SUCCESS ;
}

Sending signal: 15
Received signal: 15
Exit main()