In the previous chapter we learned about shared memory. And there is a problem of race condition when there are multiple process are trying to access and change the shared memory.
Now, we have 2 different API to use semaphores. System V semaphore are the older version and POSIX semaphore API which are newer version.
In this chapter we shall learn about System V semaphore.
So to solve this problem we should use semaphores to avoid the race condition.
We shall learn about semaphores by taking series of simple steps and then taking a full example.
1. How to get a System V semaphore?
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To get a semaphore you need to use “semget()” system call.
The function prototype is:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
Here:
key : It is a unique identifier, if any process wants to connect to the queue, it should have to use the same key.
As key is a “long” data type, you can use any integer to set the key.
Or you can also use “ftok()” known as “file to key”. The function accepts 2 arguments, first is file path and the next is a id. “ftok()” will use these 2 arguments and then will create a unique key. The other program trying to access this queue should use the same parameters.
Function prototype for ftok is:
key_t ftok(const char *path, int id);nsems : It is the number of semaphores in the present semaphore set.
Below is the way to create a semaphore:
#include <sys/ipc.h>
#include <sys/sem.h>
key_t key;
int semid;
key = ftok("/home/aj/myFile", 'E');
semid = semget(key, 10, 0666 | IPC_CREAT);
Once a semaphore is created, they’re all uninitialized; to make them free, you need to use semop() or semctl(). We shall see in further in this chapter.
2. How to control a System V semaphore?
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To control a semaphore use “semctl()” system call.
“semctl()” will allow to initialize a positive value the semaphores, to make the resources available.
It allows you set for individual semaphore or complete semaphore set.
Function prototype is:
int semctl(int semid, int semnum, int cmd, ... /*arg*/);
union semun {
int val; /* used for SETVAL only */
struct semid_ds *buf; /* used for IPC_STAT and IPC_SET */
ushort *array; /* used for GETALL and SETALL */
};
3. How to perform operations a System V semaphore?
int semop(int semid, struct sembuf *sops, unsigned int nsops);4. How to destroy a System V semaphore?
semctl(semid, 0, IPC_RMID);
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
//for more tutorials on C, C++, STL, DS visit www.ProDeveloperTutorial.com
int main(void)
{
key_t key;
int semid;
union semun arg;
if ((key = ftok("semdemo.c", 'J')) == -1)
{
perror("ftok");
exit(1);
}
/* create a semaphore set with 1 semaphore: */
if ((semid = semget(key, 1, 0666 | IPC_CREAT)) == -1)
{
perror("semget");
exit(1);
}
/* initialize semaphore #0 to 1: */ arg.val = 1;
if (semctl(semid, 0, SETVAL, arg) == -1)
{
perror("semctl");
exit(1);
}
return 0;
}
SemDemo.c
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
//for more tutorials on C, C++, STL, DS visit www.ProDeveloperTutorial.com
int main(void)
{
key_t key;
int semid;
struct sembuf sb = {0, -1, 0}; /* set to allocate resource */
if ((key = ftok("semdemo.c", 'J')) == -1)
{ perror("ftok");
exit(1);
}
/* grab the semaphore set created by seminit.c: */
if ((semid = semget(key, 1, 0)) == -1)
{
perror("semget");
exit(1);
}
printf("Press return to lock: ");
getchar();
printf("Trying to lock...\n");
if (semop(semid, &sb, 1) == -1)
{
perror("semop");
exit(1);
}
printf("Locked.\n");
printf("Press return to unlock: ");
getchar();
sb.sem_op = 1; /* free resource */
if (semop(semid, &sb, 1) == -1)
{
perror("semop");
exit(1);
}
printf("Unlocked\n");
return 0;
}SemRemove.c
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
//for more tutorials on C, C++, STL, DS visit www.ProDeveloperTutorial.com
int main(void)
{
key_t key;
int semid;
union semun arg;
if ((key = ftok("semdemo.c", 'J')) == -1)
{
perror("ftok");
exit(1);
}
/* grab the semaphore set created by seminit.c: */
if ((semid = semget(key, 1, 0)) == -1)
{
perror("semget");
exit(1);
}
/* remove it: */
if (semctl(semid, 0, IPC_RMID, arg) == -1)
{
perror("semctl");
exit(1);
}
return 0;
}