Semaphores in Linux page 3

futuregod

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

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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
               
               
               

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