SIG_ERR

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
 
void signal_handler(int signal)
{
    printf("Received signal %d\n", signal);
}
 
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()