Here is a simple example of a System V semaphore. The code is very basic and does not do to much error checking. If you decide to play with this example I recommend you check out the man pages for 'ipcs' and 'ipcrm' (Respectively they list and remove semaphores).

Anyways ... as usual enjoy:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semTake(int semid);
int semGive(int semid);
int semDestroy(int semid);
void printErrno(int myerrno);

int main(void) {

	key_t semkey;
	int semid;
	pid_t pid;
	FILE * fp;
	const char * keyfilename = ".";

	printf("Start\n");

	semkey = ftok(keyfilename, 'z');
	if (semkey == -1) {
		printf("ERROR:could get key\n");
		return EXIT_FAILURE;
	}

	semid = semget(semkey, 1, (IPC_CREAT| IPC_EXCL | 0666));
	if (semid == -1) {
		printf("ERROR:could not create semaphore\n");
		printErrno(errno);
		return EXIT_FAILURE;
	}

	if ((pid = fork()) == 0) {
		int i = 0;
		int sleeptime = 1;
		for (i = 0; i < 10; i += 1) {
			if (semTake(semid) == EXIT_FAILURE) {
				printf("ERROR: semTake failed\n");
				if (semDestroy(semid) == EXIT_FAILURE) {
					return EXIT_FAILURE;
				}
				return EXIT_FAILURE;
			}
			printf("\tCHILD OWNS MUTEX FOR %d SLEEP(S) ... %d\n", sleeptime, i);
			sleep(1);
			if (semGive(semid) == EXIT_FAILURE) {
				printf("ERROR: semTake failed\n");
				if (semDestroy(semid) == EXIT_FAILURE) {
					return EXIT_FAILURE;
				}
				return EXIT_FAILURE;
			}
		}
		return EXIT_SUCCESS;
	} else if (pid > 0) {
		int i = 0;
		int status;
		int sleeptime = 1;
		for (i = 0; i < 10; i += 1) {
			if (semGive(semid) == EXIT_FAILURE) {
				printf("ERROR: semTake failed\n");
				if (semDestroy(semid) == EXIT_FAILURE) {
					return EXIT_FAILURE;
				}
				return EXIT_FAILURE;
			}
			if (semTake(semid) == EXIT_FAILURE) {
				printf("ERROR: semTake failed\n");
				if (semDestroy(semid) == EXIT_FAILURE) {
					return EXIT_FAILURE;
				}
				return EXIT_FAILURE;
			}
			printf("\tPARENT OWNS MUTEX FOR %d SLEEP(S) ... %d\n", sleeptime, i);
			sleep(2);
		}

		/*Do one last give to prevent below wait from hanging. */
		/*Without this the child will wait forever*/
		if (semGive(semid) == EXIT_FAILURE) {
			printf("ERROR: semTake failed\n");
			if (semDestroy(semid) == EXIT_FAILURE) {
				return EXIT_FAILURE;
			}
			return EXIT_FAILURE;
		}

		if (pid != waitpid(pid, &status, 0)) {
			printf("ERROR: could not join with child process\n");
			if (semDestroy(semid) == EXIT_FAILURE) {
				return EXIT_FAILURE;
			}
			return EXIT_FAILURE;
		}

		if (semDestroy(semid) == EXIT_FAILURE) {
			return EXIT_FAILURE;
		}
	} else {
		printf("ERROR:could not fork process (pid=%d)\n", pid);
		return EXIT_FAILURE;
	}

	printf("Done\n");
	fflush(stdout);
	return EXIT_SUCCESS;
}

int semTake(int semid) {
	struct sembuf sops;
	sops.sem_num = 0;
	sops.sem_op = -1; /* semaphore operation */
	sops.sem_flg = SEM_UNDO;
	if (semop(semid, &sops, 1) == -1) {
		printErrno(errno);
		return EXIT_FAILURE;
	}
	return EXIT_SUCCESS;
}

int semGive(int semid) {
	struct sembuf sops;
	sops.sem_num = 0;
	sops.sem_op = 1; /* semaphore operation */
	sops.sem_flg = SEM_UNDO;
	if (semop(semid, &sops, 1) == -1) {
		printErrno(errno);
		return EXIT_FAILURE;
	}
	return EXIT_SUCCESS;
}

int semDestroy(int semid) {
	if (semctl(semid, 0, IPC_RMID) == -1) {
		printf("ERROR: could not clean up semaphore\n");
		printErrno(errno);
		return EXIT_FAILURE;
	}
	return EXIT_SUCCESS;
}

void printErrno(int myerrno) {
	switch (myerrno) {
	case EACCES:
		printf("EACCES\n");
		break;
	case EEXIST:
		printf("EEXIST\n");
		break;
	case EINVAL:
		printf("EINVAL\n");
		break;
	case ENOENT:
		printf("ENOENT\n");
		break;
	case ENOMEM:
		printf("ENOMEM\n");
		break;
	case ENOSPC:
		printf("ENOSPC\n");
		break;
	case E2BIG:
		printf("E2BIG\n");
		break;
	case EAGAIN:
		printf("EAGAIN\n");
		break;
	case EFAULT:
		printf("EFAULT\n");
		break;
	case EFBIG:
		printf("EFBIG\n");
		break;
	case EIDRM:
		printf("EIDRM\n");
		break;
	case EINTR:
		printf("EINTR\n");
		break;
	case ERANGE:
		printf("ERANGE\n");
		break;
	default:
		printf("default\n");
		break;
	}
	fflush(stdout);
}