i can't find why i m getting segmentation error

hey i want to increment a shared counter using shared memory and semaphores it is actually the code of richard stevens unp v2

Code:

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/fcntl.h>
#include <semaphore.h>

#define DEBUG 1
#define SHM_MODE (S_IRUSR,S_IWUSR,S_IRGRP,S_IROTH)
#define SHM_FLAGS (O_RDWR,O_CREAT,O_EXCL)
struct shmstruct
{
	int count;
};//the structure used to share information in the shared memory

sem_t* mutex;

int main(int argc,char** argv)
{
	int fd,i,nloop;
	pid_t pid;
	struct shmstruct *ptr;
	
	if(argc!=4)
	{
		fprintf(stderr,"usage: shmsem_read <arg1=shmname> <arg2=semname> <arg3=#loops>");
		}
		nloop=atoi(argv[3]);
	fd=shm_open(argv[1],O_RDWR,SHM_MODE);
	ptr=mmap(NULL,sizeof(struct shmstruct),PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
	
	close(fd);
	
		
	mutex=sem_open(argv[2],1);
	if(mutex==-1)
	{
		perror("sem_open");
		}
	pid=getpid();
	
	for(i=0;i<nloop;i++)
	{
		sem_wait(mutex);
		if(DEBUG)
		{
			fprintf(stdout,"i m here for the %dth time t\n",i);
		}
		fprintf(stdout,"PId 	%ld:%d",(long)pid,ptr->count++);
		sem_post(mutex);
		}
		
	exit(0);
	}

gdb says that i m getting segmentation error from the sem_wait function i can't sort out why

>>if (argc != 4)
You should exit the program if you don't have the correct number of args.

i forgot the exit statement in the if check for the number of arguments but this doesnot give segmentation error that is for sure