IPC critical sections with semaphores.

Semaphores.cpp

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
#include <stdarg.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#define MAX_CHILD 4
#define SEM_MUTEX 0


int main () {
	int sem_id = getuid();
	
	int glob_sem_id = semget(sem_id, 1, 0666);

	if (glob_sem_id == -1) {
		// Create new semafor.
		glob_sem_id = semget(sem_id, 1, 0666 | IPC_CREAT);
		if (glob_sem_id == -1) {
			printf("Error while creating semaphore.\n");
			exit(1);
		}
	}

	int ret = semctl(glob_sem_id, SEM_MUTEX, SETVAL, 1);	

	if (ret == -1) {
		printf("Error while creating SEM_MUTEX.\n");
		exit(1);
	}

	int mypipe[2];

	if (pipe(mypipe) == -1) {
		printf("Error while creating pipe.\n");
		exit(1);
	}

	int pid = fork();

	if (pid == -1) {
		printf("Error while forking process.\n");
		exit(1);
	}

	if (pid != 0) {
		// Parent process.
		close(mypipe[1]);

		char buffer[100];
		int l = 0;

		while (1) {
			l = read( mypipe[0], buffer, sizeof( buffer ) );
			if (l == -1)
			{
				printf("Nelze cist z roury - handle %d !", mypipe[0]);
				exit(1);
			}
			else {
				write(1, buffer, l);
			}
		}
	}
	else {
	 	struct sembuf op[1];	

		// Child process.
		for (int children = 0; children < MAX_CHILD; children += 1) {
			
			int childpid = fork();
			
			if (childpid == 0) {
				close(mypipe[0]);

				char buffer[100];
				int l = 0;

				srand(getpid());

				while (1) {
					op[0].sem_num = SEM_MUTEX;
					op[0].sem_op = -1;
					op[0].sem_flg = 0;

					int ret = semop(glob_sem_id, op, 1);	
					if (ret == -1) {
						printf("Error while entering critical section. %s\n", strerror( errno ) );
						exit(1);
					}

					/****** Critical section. ******/
					l = sprintf(buffer, "%d: ", getpid());
					write(mypipe[1], buffer, l);
					for (int i = 0; i < 10; i += 1) {
						sleep(1);
						l = sprintf(buffer, "%d ", rand() % 100);
						write(mypipe[1], buffer, l);
					}
					l = sprintf(buffer, "\n");
					write(mypipe[1], buffer, l);
					/***** Critical section end *****/

					op[ 0 ].sem_num = SEM_MUTEX;
					op[ 0 ].sem_op = 1;
					op[ 0 ].sem_flg = 0;

					ret = semop(glob_sem_id, op, 1);	
					if (ret == -1) {
						printf("Error while leaving critical section.\n");
						exit(1);
					}
				}
			}
		}
	}

	return 0;
}