Use semaphores to synchronize shared memory - c

Question

I have the following code which works fine with the sleep time. But I want to remove sleep and use semaphores to synchronize the threads ( so that the reader waits till data is available in shmptr.) How do I do that? What do I use:
sem_t mutex or
pthread_mutex_t mutex?
P.S : I tried using both above options separately (not shown in above code though) but didn't get desired result.Most of the times I was getting error in shmget in reader - "shmget in reader: No such file or directory".
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
#include <semaphore.h>
#include <errno.h>
void *write(void *t)
{ char str[100] = "\0";
char str1[50];
key_t key;
char *shmptr;
int shmid;
if( (key = ftok("shmSemphore.c",'B')) == -1)
{
perror("ftok in writer");
pthread_exit(NULL);
}
if( (shmid = shmget(key, 1024, 0666 | IPC_CREAT)) == -1)
{
perror("shmget in writer");
pthread_exit(NULL);
}
shmptr = shmat(shmid,0,0);
if( (shmptr == (char *)(-1)))
{
perror("shmget in writer");
pthread_exit(NULL);
}
printf("ENter your text :- ");
//scanf("%s",str);
fgets(str,sizeof(str),stdin);
strcpy(shmptr,str);
sleep(2);
shmdt(shmptr);
if( shmctl(shmid, IPC_RMID, NULL) == -1 )
{
perror("shmctl");
exit(-1);
}
pthread_exit(NULL);
}
void *read(void *t)
{ sleep(2);
key_t key;
char *shmptr;
int shmid;
if( (key = ftok("shmSemphore.c",'B')) == -1)
{
perror("ftok in writer");
pthread_exit(NULL);
}
if( (shmid = shmget(key, 1024, 0)) == -1)
{
perror("shmget in reader");
pthread_exit(NULL);
}
shmptr = shmat(shmid,0,0);
if( (shmptr == (char *)-1))
{
perror("shmget in writer");
pthread_exit(NULL);
}
printf("your text:- %s \n",shmptr);
pthread_exit(NULL);
}
int main()
{
pthread_t reader,writer;
int rt;
long t = 0;
rt = pthread_create(&writer,NULL,write,(void *)t);
/*if(rt == -1)
{ perror("writer thread");
pthread_exit(NULL);
}*/
rt = pthread_create(&reader,NULL,read,(void *)t);
/*if(rt == -1)
{ perror("writer thread");
pthread_exit(NULL);
}*/
pthread_join(writer,NULL);
pthread_join(reader,NULL);
return 0;
}