inter process locking

Hi,

I am using mutex for inter process locking. In the following code, I
expect that when a process is runing, its another instance should wait
until the previous instance completes. However, this does not work. why?

code
-----
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include </usr/pkg/include/pthread.h>

typedef struct {
  pthread_mutex_t Interprocess_mutex;
  int state;
}share_t;
share_t *share;

void readline(FILE *fp);

int main(int argc, char *args[]) {
  pthread_mutexattr_t mattr;
  int read_byte, i;
  int shmid;
  key_t key = 5347;
  char *shm, *s;

  //create a share memory
  if((shmid = shmget(key, sizeof(share_t), IPC_CREAT|0666)) < 0) {
    perror("shmget");
    exit(1);
  }

  if((shm = shmat(shmid, NULL, 0)) == (char *)-1) {
    perror("shamt");
    exit(1);
  }

  share = (share_t *)shm;

  if(share->state != 1 ) {
    printf("init mutex\n");
    share->state = 1;
    pthread_mutexattr_init(&mattr);
    pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED);
    pthread_mutex_init(&share->Interprocess_mutex, &mattr);
  }

  printf("state = %d\n", share->state);

  pthread_mutex_lock(&share->Interprocess_mutex);

  for(i = 0; i < 200000; i++) {
    printf("%d\n", i);
  }

  pthread_mutex_unlock(&share->Interprocess_mutex);
  shmdt((const void *)share);

  exit(0);

}