Semaphores in Linux
Pages: 1, 2, 3, 4, 5, 6
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:
<u>File: sysvsem_demo.c</u>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
//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;
}
