Lost signals from child to parent - c

Question

I'm creating several child processes which send a signal to their parent process and die. I simply count them. But I never get the right count. Some signals never get caught by the handler.
How should I code this?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
int ended = 0;
void handler(int sig){
ended++;
}
int main(int argc, char **argv){
int i;
pid_t pid, ppid;
if (signal(SIGUSR1, handler) < 0) {
fprintf(stderr, "signal failed.\n");
exit (-1);
}
ppid = getpid();
for (i = 0; i < 50; i++){
if ((pid = fork()) < 0){
fprintf(stderr, "fork failed.\n");
exit(-1);
}
if (pid == 0){
kill(ppid, SIGUSR1);
exit(0);
}
}
while (wait(NULL) > 0);
printf("ended = %d\n", ended);
return 0;
}
The output for this program is sometimes 47, others 39... but never 50

Answer 1

The problem here is that a signal acts as a hardware interruption where your handler function would be the ISR (Interrupt Service Routine). Then if multiple signals of the same value happens "at the same time" linux kernel treat them as only one signal. Signal are not designed to be used in this manner. A signal should be used to inform of the state of a process to another. To achieve communications between processes you should use IPC (InterProcess Communications) mechanisms such as queue, sockets, or pipes.

Answer 2

Thanks,
I found the problem can be solved using Real Time Signals. Just changing SIGUSR1 with SIGRTMIN. Real Time Signals are queued (http://man7.org/linux/man-pages/man7/signal.7.html).
Are there any negative side effects in this solution?

Answer 3

it was working..
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
int ended = 0;
void handler(int sig)
{
ended++;
}
int main(int argc, char **argv)
{
int i;
pid_t pid, ppid;
if (signal(SIGUSR1, handler) < 0)
{
fprintf(stderr, "signal failed.\n");
exit (-1);
}
ppid = getpid();
for (i = 0; i < 50; i++)
{
if ((pid = fork()) < 0)
{
fprintf(stderr, "fork failed.\n");
exit(-1);
}
if (pid == 0)
{
kill(ppid, SIGUSR1);
exit(0);
}
}
while (wait(NULL) > 0);
printf("ended = %d\n", ended);
return 0;
}