Question

This is my first attempt on semaphores and threads.
I constructed this code from examples and the man pages found on the Net.
I have two doubts with this code.

1. Why do I get a Bus error whenever I try semctl( I know this is the root of the problem because of the debug line 3 does not get printed) and how to prevent it?

2. Why am I not able to acquire a lock on my critical section inspite of removing semctl()?

I am trying to execute the following code:

#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/sem.h>

#define NUM 3
char c='a';
int semId=-1;
struct sembuf lockOperation = { 0, -1, 0};
struct sembuf unlockOperation = { 0, -1, 0};

void* PrintLetter()
{
  int rc;

  rc=semop(semId, &lockOperation, 1);

  if(rc)
    {
      printf("\n Unable to lock\n");
      exit(1);
    }
  printf("%c",c);    //CRITICAL SECTION
  c=c+1;             //CRITICAL SECTION  
  rc = semop(semId, &unlockOperation, 1);
  if(rc)
    {
      printf("\n Unable to unlock\n");
      exit(1);
    }

  pthread_exit(NULL);

}
int main()
{
  pthread_t threads[NUM];

  int rc=1;
  long t;
   printf("debug line 1\n");
  semId = semget(IPC_PRIVATE, 1, IPC_CREAT|IPC_EXCL);
  if( semId == -1)
    {
      printf("\n Error in making the sem\n");
      exit(1);
    }
  printf("debug line 2\n");
  rc = semctl(semId, 0, SETVAL, (int)1); // Comment from here

  if(rc)
    {
      printf("\n Unable to set val to 1\n");
      exit(1);
      }    ////////////////////////////////// till here
   printf("debug line 3\n");


  for(t=0; t<NUM; t++){
    printf("In main: creating thread %ld\n", t);
    rc = pthread_create(&threads[t], NULL, PrintLetter, NULL);
    if (rc){
      printf("ERROR; return code from pthread_create() is %d\n", rc);
      exit(-1);
    }
    sleep(3);
  }
  pthread_exit(NULL);
}

NOTE:
I added the following to the code after suggestion:

union semun {
  int              val;    /* Value for SETVAL */
  struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
  unsigned short  *array;  /* Array for GETALL, SETALL */
  struct seminfo  *__buf;  /* Buffer for IPC_INFO
                              (Linux specific) */
};

Also added the following:

union semun s_u;
s_u.val=1;

Changed the semctl line to

rc = semctl(semId, 0, SETVAL, s_u);

and made all rc checks to:

if(rc == -1)

semctl() line still does not execute sucessfully.
The errno now states :
Permission Denied

UPDATE: I am able to get rid off the “Permission Denied” error using the following change:

semId = semget(IPC_PRIVATE, 1, IPC_CREAT|IPC_EXCL|0660);

Now , the new problem is that, I am unable to print “abc” on the console.
The program just prints “a” and hangs. Not sure why.

Final UPDATE:
I messed up in the Unlock code:
i used -1 instead of 1
Here is the new code:

struct sembuf unlockOperation = { 0, 1, 0};

I thank you all for your help and patience.

Answer 1

The fourth argument to semctl (if present) should be of type union semun:

union semun {
            int val;
            struct semid_ds *buf;
            unsigned short  *array;
} arg;

Maybe passing an int there instead is causing alignment problems.