Semaphores in Linux page 3

The IPC_RMID flag here signifies the type of operation that needs to be carried out on the semaphore identified by semid.

The following working example uses these concepts:

File: sysvsem_demo.c
#include 
#include 
#include 
#include 

//create user defined semun for initializing the semaphores

void *Thread1(void *arg)
{
  int semid;
  semid = (int)arg;

  //in order to perform the operations on semaphore
  // first need to define the sembuf object
  struct sembuf op1,op2;

  //operation for 0th semaphore
  op1.sem_num = 0; //signifies 0th semaphore
  op1.sem_op = -1; //reduce the semaphore count to lock
  op1.sem_flg = 0; //wait till we get lock on semaphore

  //operation for 1th semaphore
  op2.sem_num = 1; //signifies 0th semaphore
  op2.sem_op = -1; //reduce the semaphore count to lock
  op2.sem_flg = 0; //wait till we get lock on semaphore

  //locking the 0th semaphore
  if (semop(semid,&op1,1) == -1)
    {
      perror("Thread1:semop failure Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread1:Successfully locked 0th semaphore\n");
  //lock the 1th semaphore
  if (semop(semid,&op2,1) == -1)
    {
      perror("Thread1:semop failure Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread1:Successfully locked 1th semaphore\n");



  //release the 0th semaphore
  op1.sem_num = 0; //signifies 0th semaphore
  op1.sem_op = 1; //reduce the semaphore count to lock
  op1.sem_flg = 0; //wait till we get lock on semaphore

  if (semop(semid,&op1,1) == -1)
    {
      perror("Thread1:semop failure Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread1:Successfully unlocked 0th semaphore\n");

  //release the 1th semaphore
  op2.sem_num = 1; //signifies 0th semaphore
  op2.sem_op = 1; //reduce the semaphore count to lock
  op2.sem_flg = 0; //wait till we get lock on semaphore

  if (semop(semid,&op2,1) == -1)
    {
      perror("Thread1:semop failure Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread1:Successfully unlocked 1th semaphore\n");
}

void *Thread2(void *arg)
{
  int semid;
  semid = (int)arg;

  //in order to perform the operations on semaphore
  // first need to define the sembuf object
  struct sembuf op1,op2;

  //operation for 0th semaphore
  op1.sem_num = 0; //signifies 0th semaphore
  op1.sem_op = -1; //reduce the semaphore count to lock
  op1.sem_flg = 0; //wait till we get lock on semaphore

  //operation for 1th semaphore
  op2.sem_num = 1; //signifies 0th semaphore
  op2.sem_op = -1; //reduce the semaphore count to lock
  op2.sem_flg = 0; //wait till we get lock on semaphore

  //lock the 0th semaphore
  if (semop(semid,&op1,1) == -1)
    {
      perror("Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread2:Successfully locked 0th semaphore\n");

  //lock the 1th semaphore
  if (semop(semid,&op2,1) == -1)
    {
      perror("Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread2:Successfully locked 1th semaphore\n");

  //release 0th semaphore

  op1.sem_num = 0; //signifies 0th semaphore
  op1.sem_op = 1; //reduce the semaphore count to lock
  op1.sem_flg = 0; //wait till we get lock on semaphore

  if (semop(semid,&op1,1) == -1)
    {
      perror("Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread2:Successfully unlocked 0th semaphore\n");

  //release the 1th semaphore
  op2.sem_num = 1; //signifies 0th semaphore
  op2.sem_op = 1; //reduce the semaphore count to lock
  op2.sem_flg = 0; //wait till we get lock on semaphore

  if (semop(semid,&op2,1) == -1)
    {
      perror("Reason:");
      exit(-1);
    }
  else
    fprintf(stderr,"Thread2:Successfully unlocked 1th semaphore\n");

}

int main()
{
  pthread_t tid1,tid2;
  int semid;

  //create user defined semun for initializing the semaphores

  typedef union semun
  {
    int val;
    struct semid_ds *buf;
    ushort * array;
  }semun_t;

  semun_t arg;
  semun_t arg1;

  //creating semaphore object with two semaphore in a set
  //viz 0th & 1th semaphore
  semid = semget(IPC_PRIVATE,2,0666|IPC_CREAT);
  if(semid<0)
    {
      perror("semget failed Reason:");
      exit(-1);
    }

  //initialize 0th semaphore in the set to value 1
  arg.val = 1;
  if ( semctl(semid,0,SETVAL,arg)<0 )
    {
      perror("semctl failure Reason:");
      exit(-1);
    }
  //initialize 1th semaphore in the set to value 1
  arg1.val = 1;
  if( semctl(semid,1,SETVAL,arg1)<0 )
    {
      perror("semctl failure Reason: ");
      exit(-1);
    }

  //create two threads to work on these semaphores
  if(pthread_create(&tid1, NULL,Thread1, semid))
    {
      printf("\n ERROR creating thread 1");
      exit(1);
    }
  if(pthread_create(&tid2, NULL,Thread2, semid) )
    {
      printf("\n ERROR creating thread 2");
      exit(1);
    }
  //waiting on these threads to complete
  pthread_join(tid1, NULL);
  pthread_join(tid2, NULL);

  //once done clear the semaphore set
  if (semctl(semid, 1, IPC_RMID ) == -1 )
    {
      perror("semctl failure while clearing Reason:");
      exit(-1);
    }
  //exit the main threads
  pthread_exit(NULL);
  return 0;
}