/* File: semaphore.c */	#include <stdio.h>	#include <stdlib.h>	#include <unistd.h>	#include <time.h>	#include <sys/types.h>	#include <sys/wait.h>	#include <linux/sem.h>	#define NUM_PROCS  5	#define SEM_ID    250	#define FILE_NAME "/tmp/sem_mutex"	#define DELAY     400000	void update_file(int sem_set_id, char* file_path, int number)	{		struct sembuf sem_op;		FILE* file;		//等待信號燈的數值變為非負數		sem_op.sem_num = 0;		sem_op.sem_op = -1;		sem_op.sem_flg = 0;		semop(sem_set_id, &sem_op, 1);		//寫如文件, 寫入的數值是當前進程的進程號		file = fopen(file_path, "w");		if (file) {			fprintf(file, "%d\n", number);			printf("%d\n", number);			fclose(file);		}		//發送信號, 把信號燈的數值加1		sem_op.sem_num = 0;		sem_op.sem_op = 1;		sem_op.sem_flg = 0;		semop(sem_set_id, &sem_op, 1);	}	//子進程寫文件	void do_child_loop(int sem_set_id, char* file_name)	{		pid_t pid = getpid();		int i, j;		for (i=0; i<3; i++) {			update_file(sem_set_id, file_name, pid);			for (j=0; j<400000; j++) ;		}	}	int main(int argc, char **argv)	{		int sem_set_id;           //信號燈集的ID		union semun sem_val;      //信號燈的數值, 用于 semctl()		int child_pid;            //子進程的進程號		int i;		int rc;		//建立信號燈集, ID是250, 其中只有一個信號燈		sem_set_id = semget(SEM_ID, 1, IPC_CREAT | 0600);		if (sem_set_id == -1) {			perror("main: semget");			exit(1);		}		//把第一個信號燈的數值設置為1		sem_val.val = 1;		rc = semctl(sem_set_id, 0, SETVAL, sem_val);		if (rc == -1) {			perror("main: semctl");			exit(1);		}		//建立一些子進程, 使它們可以同時以競爭的方式訪問信號燈		for (i=0; i<NUM_PROCS; i++) {			child_pid = fork();			switch(child_pid) {				case -1:					perror("fork");					exit(1);				case 0:		//子進程					do_child_loop(sem_set_id, FILE_NAME);					exit(0);				default:	//父進程接著運行					break;			}		}		//等待子進程結束		for (i=0; i<NUM_PROCS; i++) {			int child_status;			wait(&child_status);		}		printf("main: we're done\n");		fflush(stdout);		return 0;	}