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:

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);
}

Ok ok No one can answer me please!!!!!!!!!!!!!!!!!!!!!!

http://en.wikipedia.org/wiki/Semaphore_(programming)

I know that i'm a beginner about semaphore i train myself with a little exercises and i just want to know what this code do sorry to disturb you!!!!

Then ask a specific question about specific lines of code.

Don't just dump the whole thing and expect a complete explanation(*). Show what you've managed to figure out for yourself, then perhaps we'll help with the next step.

(*) Any explanation would suffer from the "3 bears" problem of either being
- too hard for you to understand
- tell you things you already know
- just right.
A 66% of wasting our time isn't productive.

Salam salem
Thanks a lot to reply me
actually i don't want you to describe all the line
i just want an explaination of what they do this code and in what case we can use it?
Thank you and sorry for the few details

OK, so what do you expect from us which isn't in the manual pages for those functions?

Originally Posted by MacGyver

http://en.wikipedia.org/wiki/Semaphore_(programming)

You missed the ')' . . . http://en.wikipedia.org/wiki/Semaphore_(programming)

What i understand:

critical section created by p and v
3 childs created
only 1 child can access to the critical section between p and v

Yes, that is what a semaphore is designed to do - ensure restricted access to a critical resource.

http://www.ecst.csuchico.edu/~beej/guide/ipc/

heres a link to a good semaphore page.....I honestly don't know how I still have this, this is a link from the class I leared semaphores from and use them all the time. May be outdated from 5 years ago.

This is excellent for anyone trying to learn too :
http://www.cs.mtu.edu/~shene/NSF-3/e-Book/index.html

I've found its explainations better than most so far.

Code:

  if(fork() == 0)

fork makes me smile