/*********************************************************************************
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* Copyright: (C) 2025 LingYun IoT System Studio
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* All rights reserved.
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*
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* Filename: semaphore.c
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* Description: This file semaphore example program
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*
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* Version: 1.0.0(11/07/2025)
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* Author: Guo Wenxue <guowenxue@gmail.com>
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* ChangeLog: 1, Release initial version on "11/07/2025 05:37:20 PM"
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*
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********************************************************************************/
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#include <stdio.h>
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#include <string.h>
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#include <errno.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#define FTOK_PATH "/dev/zero"
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#define FTOK_PROJID 0x22
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union semun
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{
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int val;
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struct semid_ds *buf;
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unsigned short *arry;
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};
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int semaphore_init(void);
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int semaphore_p(int semid);
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int semaphore_v(int semid);
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void semaphore_term(int semid);
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int main(int argc, char **argv)
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{
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int semid;
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pid_t pid;
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int i;
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if( (semid=semaphore_init()) < 0)
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{
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printf("semaphore initial failure: %s\n", strerror(errno));
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return -1;
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}
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if( (pid=fork()) < 0)
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{
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printf("fork() failure: %s\n", strerror(errno));
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return -2;
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}
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else if( 0 == pid) /* child process */
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{
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printf("Child process start working now...\n");
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sleep(1);
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printf("Child process done and V operator\n");
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semaphore_v(semid);
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printf("Child process exit...\n");
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exit(0);
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}
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/* 前面的semaphore_init()函数里将信号量的值设为0，如果这时候父进程先执行的话，
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P操作会阻塞。直到子进程执行V操作后，父进程的P操作才能返回继续执行
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*/
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printf("Parent process P operator to wait child process done\n");
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semaphore_p(semid);
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printf("Parent process start working now...\n");
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sleep(1);
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printf("Perent process done and exit...\n");
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semaphore_term(semid);
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return 0;
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}
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int semaphore_init(void)
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{
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key_t key;
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int semid;
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union semun sem_union;
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if( (key=ftok(FTOK_PATH, FTOK_PROJID)) < 0 )
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{
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printf("ftok() get IPC token failure: %s\n", strerror(errno));
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return -1;
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}
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semid = semget(key, 1, IPC_CREAT|0644);
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if( semid < 0)
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{
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printf("semget() get semid failure: %s\n", strerror(errno));
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return -2;
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}
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sem_union.val = 0;
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if( semctl(semid, 0, SETVAL, sem_union)<0 )
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{
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printf("semctl() set initial value failure: %s\n", strerror(errno));
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return -3;
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}
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printf("Semaphore get key_t[0x%x] and semid[%d]\n", key, semid);
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return semid;
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}
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void semaphore_term(int semid)
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{
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union semun sem_union;
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if( semctl(semid, 0, IPC_RMID, sem_union)<0 )
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{
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printf("semctl() delete semaphore ID failure: %s\n", strerror(errno));
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}
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return ;
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}
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int semaphore_p(int semid)
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{
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struct sembuf _sembuf;
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_sembuf.sem_num = 0;
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_sembuf.sem_op = -1;
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_sembuf.sem_flg = SEM_UNDO; // IPC_NOWAIT SEM_UNDO
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if( semop(semid, &_sembuf, 1) < 0 )
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{
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printf("semop P operator failure: %s\n", strerror(errno));
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return -1;
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}
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return 0;
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}
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int semaphore_v(int semid)
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{
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struct sembuf _sembuf;
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_sembuf.sem_num = 0;
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_sembuf.sem_op = 1;
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_sembuf.sem_flg = SEM_UNDO; // IPC_NOWAIT SEM_UNDO
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if( semop(semid, &_sembuf, 1) < 0 )
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{
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printf("semop V operator failure: %s\n", strerror(errno));
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return -1;
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}
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return 0;
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}
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