wait() gets Interupted system call - c

Question

I thought that wait() fuction will wait until the proces has done, however it receives a signal -1. Does anyone know the reason of the problem? May be the problem is my shared memory? So I tried to make a debugging, and in the debbuging mode there is no problem like when I run my code in normal mode.
#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include <unistd.h>
#include <errno.h>
#define PROCESSES 3
struct shdata
{
int x;
};
void childf(int shared_memory, int index)
{
// connect shared memory
struct shdata* shm = (struct shdata*)shmat(shared_memory, NULL, 0);
if(shm == (void*)-1)
{
perror("shmat");
exit(0);
}
// initialize x as 0
if(index == 0)
{
shm->x = 0;
}
// increment x
shm->x++;
//show x
printf("Proces %d: x = %d\n", index, shm->x);
// disconnect shared memory
if(shmdt(shm) == -1)
{
perror("shmdt");
exit(0);
}
// end child process
exit(0);
}
int main(int argc, const char * argv[]) {
// create shared memory
int shared_memory = shmget(IPC_PRIVATE, 4096, 0600 | IPC_CREAT | IPC_EXCL);
if(shared_memory == -1)
{
perror("shmget");
return 1;
}
// create child processes
for (int i = 0; i < PROCESSES; i++)
{
int pid = fork();
if(pid == -1)
{
perror("fork");
return 5;
}
if(pid == 0)
{
childf(shared_memory, i);
}
}
// wait for child processes
for(int i = 0; i < PROCESSES; i++)
{
int wait_res = wait(NULL);
if(wait_res < 0)
{
perror("wait");
return 6;
}
}
// delete shared memory
int delete_memory = shmctl(shared_memory, IPC_RMID, NULL);
if(delete_memory == -1)
{
perror("shmctl");
return 4;
}
return 0;
}
There what I gets:
Proces 0: x = 1 Proces 1: x = 2 Proces 2: x = 3 wait: Interrupted system call Program ended with exit code: 6
But from time to time I dont receive this error. So what is the problem?
I expected:
Proces 0: x = 1 Proces 1: x = 2 Proces 2: x = 3 Program ended with exit code: 0

Answer 1

An otherwise benign signal can always interrupt wait (and other blocking system calls). If you are not interested in signals, just go back to waiting.
Instead of this
wait_res = wait(NULL);
use this:
while ((wait_res = wait(NULL)) == -1) {
if (errno != EINTR) break;
}