Thread: semaphore

Please i found some examples of semaphore but i don't understand what they do?
if someone can explain to me what they do with details thank you!
Code:

Code:

pv.h
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>

extern int errno;

#define SEMPERM 0600
#define TRUE 1
#define FALSE 0
#define SEMKEY 0*200
#define SEMKEYSTR "0*200"


union _semun{
 int val;
 struct semid_ds *stat;
 unsigned short *array;
} 

int p (int semid);
int v (int semid);
int initsem (key_t semkey);
void handlesem (key_t semkey);

***********************
/* initsem.c -- semaphore initialisation */

#include "pv.h"

int initsem(key_t semkey)
{
    int status = 0, semid;
    union semun arg;
    arg.val = 1;

    if((semid = semget(semkey, 1, SEMPERM|IPC_CREAT|IPC_EXCL)) == -1)
    {
	if (errno ==EEXIST)
	    semid = semget(semkey, 1, 0);
    }
    else /* if created ... */
    {
	status = semctl(semid, 0, SETVAL, arg);
    }

    if (semid == -1 || status == -1)
    {
	perror("initsem failed");
	return(-1);
    }

    /* all okay */
    else
    return(semid);
}
void removesem (char *key)
{
    execlp ("ipcrm", "ipcrm", "-S", key,(char *) 0);

*************************
pc.c
#include "pv.h"
int p(int semid){
    struct sembuf p_buf;

    p_buf.sem_num = 0;
    p_buf.sem_op = -1;
    p_buf.sem_flg = SEM_UNDO; /* undoes operation if process exits! */

    if (semop(semid, &p_buf, 1) == -1)
    {
	perror("p(semid) failed");
	exit(1);
    } 
else
	return (0);
}
//p()

***************************
vc.c
#include "pv.h"
int v(int semid){
    struct sembuf v_buf;

    v_buf.sem_num = 0;
    v_buf.sem_op = 1;
    v_buf.sem_flg = SEM_UNDO;

    if (semop(semid, &v_buf, 1) == -1){
	perror("v(semid) failed");
	exit(1);
    } else
	return(0);
}//v()

****************************
testsem.c
/* program for testing p() and v() -- gets mutual exclusion */

#include "pv.h"

int main(void){
  key_t semkey = SEMKEY;

  if(fork() == 0) handlesem(semkey);
  else if(fork() == 0) handlesem(semkey);
  else if(fork() == 0) handlesem(semkey);
  
  while (wait ((int *) 0) >=0)
  removesem (semkeystr);
  return 0;
  }//main()

void handlesem(key_t skey){
  int semid, pid = getpid();

  /* initialize the semaphore if we just created it */
  
  if ((semid = initsem(skey)) < 0) exit(1);

  printf("\nprocess %d BEFORE critical section\n", pid);
  
  p(semid);

  printf("\nprocess %d in critical section \n", pid);

  sleep(10);
   
  printf("\nprocess %d leaving critical section \n", pid);
  
  v(semid); /* call semaphore SIGNAL operation */
  
  printf("\nprocess %d is OUTSIDE critical section and quitting!\n", pid);
  exit(0);
}