global semaphores for synchronisation - counter - c

Question

Below i increment a counter 1000 in two different processes and use a global semaphores for synchronisation. But iCounter doesn't give me 2000? Could someone explain to me why?
#include <stdio.h>
#include <semaphore.h>
#include <stdlib.h>
#include <fcntl.h>
#define SEM_NAME "/sync"
#define NUMBER_OF_PROCESSES 2
int main ()
{
int iCounter = 0;
pid_t pid[NUMBER_OF_PROCESSES];
sem_t *sSem = sem_open (SEM_NAME,O_CREAT | O_EXCL, S_IRWXU | S_IRWXG | S_IRWXO, 1);
int i, iStatus;
for (i = 0; i < NUMBER_OF_PROCESSES; i++)
{
pid[i] = fork ();
if (pid[i] < 0)
{
printf ("Could not create process\n");
exit (1);
}
else if (pid[i] == 0)
{
int i;
for (i = 0; i < 1000; i++)
{
sem_init(sSem, 0, 1);
sem_wait (sSem);
iCounter++;
sem_post (sSem);
}
exit (0);
}
}
for (i = 0; i < NUMBER_OF_PROCESSES; i++)
waitpid (pid[i], &iStatus, WUNTRACED);
printf ("Value of iCounter = %d\n", iCounter);
sem_close (sSem);
sem_unlink (SEM_NAME);
}

Answer 1

You have two big issues:
You're not using named semaphores correctly. sem_init() is only meant to be used on anonymous ones, and only once per semaphore, where you're calling it on an already initialized one many times. That's undefined behavior.
Your iCounter variable isn't shared between the processes you create.
The following reworking of your program allocates it using POSIX shared memory (And has some general cleanup):
#include <errno.h>
#include <fcntl.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#define SEM_NAME "/sync"
#define SHM_NAME "/counter"
#define NUMBER_OF_PROCESSES 2
int main() {
int *iCounter = MAP_FAILED;
pid_t pid[NUMBER_OF_PROCESSES];
int iStatus;
int ret = 0;
int memfd = -1;
sem_t *sSem = sem_open(SEM_NAME, O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, 1);
if (sSem == SEM_FAILED) {
perror("sem_open");
ret = 1;
goto cleanup;
}
memfd = shm_open(SHM_NAME, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
if (memfd < 0) {
perror("shm_open");
ret = 1;
goto cleanup;
}
if (ftruncate(memfd, sizeof *iCounter) < 0) {
perror("ftruncate");
ret = 1;
goto cleanup;
}
iCounter = mmap(NULL, sizeof *iCounter, PROT_READ | PROT_WRITE, MAP_SHARED,
memfd, 0);
if (iCounter == MAP_FAILED) {
perror("mmap");
ret = 1;
goto cleanup;
}
close(memfd);
*iCounter = 0;
for (int i = 0; i < NUMBER_OF_PROCESSES; i++) {
pid[i] = fork();
if (pid[i] < 0) {
fprintf(stderr, "Could not create process: %s\n", strerror(errno));
ret = 1;
goto cleanup;
} else if (pid[i] == 0) {
for (i = 0; i < 1000; i++) {
sem_wait(sSem);
(*iCounter)++;
sem_post(sSem);
}
sem_close(sSem);
return 0;
}
}
for (int i = 0; i < NUMBER_OF_PROCESSES; i++)
waitpid(pid[i], &iStatus, WUNTRACED);
printf("Value of iCounter = %d\n", *iCounter);
cleanup:
if (sSem != SEM_FAILED) {
sem_close(sSem);
sem_unlink(SEM_NAME);
}
if (memfd >= 0)
shm_unlink(SHM_NAME);
return ret;
}
Example:
$ gcc -g -O -Wall -Wextra example.c -lrt -lpthread
$ ./a.out
Value of iCounter = 2000