Can not wait on shared semaphore - c

Question

I have to program Readers-Writers Problem in C without using threads. I have used this to initialize shared semaphores:
/* initialize semaphores for shared processes */
resource = sem_open ("/rSem", 0644, O_CREAT | O_EXCL, 1);
sem_unlink ("/rSem");
mutex = sem_open ("/mSem", 0644, O_CREAT | O_EXCL, 1);
sem_unlink ("/mSem");
but while running child process
sem_wait(resource);
terminates the program.
here is my complete code:
/* Includes */
#include <unistd.h> /* Symbolic Constants */
#include <sys/types.h> /* Primitive System Data Types */
#include <errno.h> /* Errors */
#include <stdio.h> /* Input/Output */
#include <stdlib.h> /* General Utilities */
#include <pthread.h> /* POSIX Threads */
#include <string.h> /* String handling */
#include <semaphore.h> /* Semaphore */
#include <sys/mman.h>
#include <fcntl.h> /* O_CREAT, O_EXEC */
/* prototype for thread routine */
void writer ( int , int *);
void reader ( int , int *, int *);
void multipleFork ( int );
/* global variables */
sem_t *mutex;
sem_t *resource;
int main(int argc, char **argv)
{
int * Buffer = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
int * readcount = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
/* initialize semaphores for shared processes */
resource = sem_open ("/rSem", 0644, O_CREAT | O_EXCL, 1);
sem_unlink ("/rSem");
mutex = sem_open ("/mSem", 0644, O_CREAT | O_EXCL, 1);
sem_unlink ("/mSem");
pid_t pid = fork();
if (pid == 0)
writer(1, Buffer);
else
reader(3, Buffer, readcount);
sem_destroy(resource);
sem_destroy(mutex);
exit(0);
} /* main() */
void writer ( int n , int * Buffer)
{
multipleFork(n);
sleep(1);
while(1){
usleep(rand()%3000);
sem_wait(resource);
//<CRITICAL Section>
printf("The writer (%d) ‫‪acquires‬‬ ‫‪the‬‬ ‫‪lock.\n‬‬", getpid());
(*Buffer)++;
printf("The writer (%d) writes ‫‪the‬‬ ‫‪value %d.\n‬‬", getpid(), *Buffer);
sleep(1);
printf("The writer (%d) releases ‫‪the‬‬ ‫‪lock.‬‬\n", getpid());
//<CRITICAL Section>
sem_post(resource);
sleep(1);
}
}
void reader( int n , int * Buffer, int * readcount) {
multipleFork(n);
while(1){
usleep(rand()%3000);
sem_wait(mutex);
usleep(rand()%3000);
if ((*readcount) == 0)
{
sem_wait(resource);
printf("The first reader (%d) acquires the lock.%d\n", getpid(), *readcount);
(*readcount)++;
}
else if ((*readcount) > 0)
{
printf("The reader (%d) acquires the lock.%d\n", getpid(), *readcount);
(*readcount)++;
}
sem_post(mutex);
//<CRITICAL Section>
printf("Reader (%d) reads the value %d.%d\n", getpid(), *Buffer, *readcount);
sleep(1);
//<CRITICAL Section>
sem_wait(mutex);
(*readcount)--;
if ((*readcount) == 0)
{
printf("The last reader (%d) releases the lock.\n", getpid());
sem_post(resource);
}
else
printf("The reader (%d) releases the lock.\n", getpid());
sem_post(mutex);
sleep(1);
}
}
void multipleFork (int n)
{
while(n-1 > 0){
pid_t pid = fork();
if (pid == 0){
return;
}
else
n--;
}
return;
}

Answer 1

I have used
resource = sem_open ("/rSem", 0644, O_CREAT | O_EXCL, 1);
instead of
resource = sem_open ("/rSem", O_CREAT | O_EXCL, 0644, 1);