semaphore implementation

问题:

I am getting error in the following program. I want to demonstrate how two processes can share a variable using semaphore. Can anyone guide me?

I am not able to debug the errors...

#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<semaphore.h>
int main()
{
  int pid,mutex=1;
  int semid;               /* semid of semaphore set */
  key_t key = 1234; /* key to pass to semget() */
  int nsems = 1; /* nsems to pass to semget() */
  semid=semget(key,nsems,IPC_CREAT|0666);
  if (semid<0) 
  { 
    perror("Semaphore creation failed ");
  }
  if ((pid = fork()) < 0) 
  {
    perror("fork");
    return 1;
  }
  else if(pid==0)
  {
    sem_wait(&semid);
    printf("IN CHILD PROCESS :\n");
    mutex++; 
    printf("value of shared variable =%d",mutex);
    sem_post(&semid);
    return 0;
  }
  sem_wait(&semid);
  printf("IN PARENT PROCESS :\n");
  mutex--;
  printf("value of shared variable =%d",mutex);
  sem_post(&semid);
  return 0;
} 

回答1:

Your Fundamentals are wrong, the program won't work, so go through the basics and rewrite the program.

Some of the corrections you must make are:

1) You must make a variable of semaphore type

sem_t semvar;

2) The functions sem_wait(), sem_post() require the semaphore variable but you are passing the semaphore id, which makes no sense.

sem_wait(&semvar);
   //your critical section code
sem_post(&semvar);

3) You are passing the semaphore to sem_wait() and sem_post() without initializing it. You must initialize it to 1 (in your case) before using it, or you will have a deadlock.

ret = semctl( semid, 1, SETVAL, sem);
if (ret == 1)
     perror("Semaphore failed to initialize");

Study the semaphore API's from the man page and go through this example.

回答2:

The fundamental issue with your code is that you mix two APIs. Unfortunately online resources are not great at pointing this out, but there are two semaphore APIs on UNIX-like systems:

• POSIX IPC API, which is a standard API
• System V API, which is coming from the old Unix world, but practically available almost all Unix systems

Looking at the code above you used semget() from the System V API and tried to post through sem_post() which comes from the POSIX API. It is not possible to mix them.

To decide which semaphore API you want you don't have so many great resources. The simple best is the "Unix Network Programming" by Stevens. The section that you probably interested in is in Vol #2.

These two APIs are surprisingly different. Both support the textbook style semaphores but there are a few good and bad points in the System V API worth mentioning:

• it builds on semaphore sets, so once you created an object with semget() that is a set of semaphores rather then a single one
• the System V API allows you to do atomic operations on these sets. so you can modify or wait for multiple semaphores in a set
• the SysV API allows you to wait for a semaphore to reach a threshold rather than only being non-zero. waiting for a non-zero threshold is also supported, but my previous sentence implies that
• the semaphore resources are pretty limited on every unixes. you can check these with the 'ipcs' command
• there is an undo feature of the System V semaphores, so you can make sure that abnormal program termination doesn't leave your semaphores in an undesired state

回答3:

Vary the consumer-rate and the producer-rate (using sleep), to better understand the operation of code. The code below is the consumer-producer simulation (over a max-limit on container).

Code for your reference:

#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>

sem_t semP, semC;
int stock_count = 0;
const int stock_max_limit=5;

void *producer(void *arg) {
    int i, sum=0;
    for (i = 0; i < 10; i++) {

        while(stock_max_limit == stock_count){
            printf("stock overflow, production on wait..\n");
            sem_wait(&semC);
            printf("production operation continues..\n");
        }

        sleep(1);   //production decided here
        stock_count++;
        printf("P::stock-count : %d\n",stock_count);
        sem_post(&semP);
        printf("P::post signal..\n");
    }
 }

void *consumer(void *arg) {
    int i, sum=0;
    for (i = 0; i < 10; i++) {

        while(0 == stock_count){
            printf("stock empty, consumer on wait..\n");
            sem_wait(&semP);
            printf("consumer operation continues..\n");
        }

        sleep(2);   //consumer rate decided here
        stock_count--;
        printf("C::stock-count : %d\n", stock_count);
        sem_post(&semC);
        printf("C::post signal..\n");
        }
}

int main(void) {

    pthread_t tid0,tid1;
    sem_init(&semP, 0, 0);
    sem_init(&semC, 0, 0);

        pthread_create(&tid0, NULL, consumer, NULL);
        pthread_create(&tid1, NULL, producer, NULL);
        pthread_join(tid0, NULL);
        pthread_join(tid1, NULL);

    sem_destroy(&semC);
    sem_destroy(&semP);

    return 0;
}

回答4:

Please check this out below sample code for semaphore implementation(Lock and unlock).

    #include<stdio.h>
    #include<stdlib.h>
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include<string.h>
    #include<malloc.h>
    #include <sys/sem.h>
    int main()
    {
            int key,share_id,num;
            char *data;
            int semid;
            struct sembuf sb={0,-1,0};
            key=ftok(".",'a');
            if(key == -1 ) {
                    printf("\n\n Initialization Falied of shared memory \n\n");
                    return 1;
            }
            share_id=shmget(key,1024,IPC_CREAT|0744);
            if(share_id == -1 ) {
                    printf("\n\n Error captured while share memory allocation\n\n");
                    return 1;
            }
            data=(char *)shmat(share_id,(void *)0,0);
            strcpy(data,"Testing string\n");
            if(!fork()) { //Child Porcess
                 sb.sem_op=-1; //Lock
                 semop(share_id,(struct sembuf *)&sb,1);

                 strncat(data,"feeding form child\n",20);

                 sb.sem_op=1;//Unlock
                 semop(share_id,(struct sembuf *)&sb,1);
                 _Exit(0);
            } else {     //Parent Process
              sb.sem_op=-1; //Lock
              semop(share_id,(struct sembuf *)&sb,1);

               strncat(data,"feeding form parent\n",20);

              sb.sem_op=1;//Unlock
              semop(share_id,(struct sembuf *)&sb,1);

            }
            return 0;
    }