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NAME
semget - get a System V semaphore set identifier ：
创建一个新的信号量或获取一个已经存在的信号量的ID

SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

/*创建一个新的信号量数组 或获取一个已经存在的信号量数组 的ID
key:
如果没有亲缘关系的进程中 创建和获取 和之前 消息队列是一样的
如果在有亲缘关系的进程中，fork()之后，每一个子进程都可以拿到父进程创建的 key值，此时不再关心key值，此时可以设置为 IPC_PRIVATE，表示该IPC 为 匿名IPC 不需要ftok

nsems: 指定当前信号量数组中有多少个元素
semflg：其他特殊要求，当key 为IPC_PRIVATE时， semflg设置为IPC_CREAT

*/

int semget(key_t key, int nsems, int semflg);

RETURN VALUE       If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise, -1 is returned, with errno indicating the error.

NAME
semctl - System V semaphore control operations
对目标信号量执行各类操作，不过最常用的是删除它。
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

/*对目标信号量数组 执行各类操作，不过最常用的是删除它。
semid : IPC ID
semnum ：目标信号量 在数组中的下标值
cmd ：命令

...IPC_RMID：从系统中删除目标信号量集合......SETVAL:设置数组中下标为semnum的成员值，即设置某个信号量的资源量......最后参数：初始值，即资源总量//设置ID为semid的信号量集合数组中 第1个信号量的资源总量为1semctl(semid,0,SETVAL,1):

*/

int semctl(int semid, int semnum, int cmd, …);

RETURN VALUE
On failure, semctl() returns -1 with errno indicating the error.
Otherwise, the system call returns a nonnegative value depending on cmd as follows:

semop() 操作信号量数组 ，P + V

NAME       semop, semtimedop - System V semaphore operationsSYNOPSIS       #include <sys/types.h>       #include <sys/ipc.h>       #include <sys/sem.h>	/*semid：目标IPC IDsops :结构体数组起始位置nsops：结构体数组中有多少个元素，即有多少个该结构体*/       int semop(int semid, struct sembuf *sops, size_t nsops);

The elements of this structure are of type struct sembuf, containing the following members:

		信号量编号，当使用单个信号量时候，为0       unsigned short sem_num;        信号量操作，取值为-1,表示P操作。归还为+1，为释放操作       short          sem_op;         short          sem_flg;  /* operation flags */

RETURN VALUE
If successful, semop() and semtimedop() return 0; otherwise they return -1 with errno indicating the error.

ERRORS
On failure, errno is set to one of the following:

   E2BIG  The argument nsops is greater than SEMOPM, the maximum number of operations allowed per system call.   EACCES The calling process does not have the permissions required to perform the specified semaphore operations, and does not have the CAP_IPC_OWNER capability.   EAGAIN 假错   EFAULT An address specified in either the sops or the timeout argument isn't accessible.   EFBIG  For some operation the value of sem_num is less than 0 or greater than or equal to the number of semaphores in the set.   EIDRM  The semaphore set was removed.   EINTR  While blocked in this system call, the thread caught a signal; see signal(7).   EINVAL The semaphore set doesn't exist, or semid is less than zero, or nsops has a nonpositive value.   ENOMEM The sem_flg of some operation specified SEM_UNDO and the system does not have enough memory to allocate the undo structure.   ERANGE For some operation sem_op+semval is greater than SEMVMX, the implementation dependent maximum value for semval.

实验：信号量使用，20个进程写同一个文件，该例程中 信号量集合中只有一个信号量，功能和互斥锁差不多。

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <pthread.h>#include <unistd.h>#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>#define PROCNUM 20#define FNAME "/home/mhr/Desktop/xitongbiancheng/super_io/out"#define LINESIZE 1024static 	int semid;//取资源量static void P(void){	struct sembuf op;		op.sem_num = 0;	op.sem_op = -1;	op.sem_flg = 0;	while(semop(semid,&op,1) < 0)	{		if(errno != EINTR || error != EAGAIN)		{			perror("semop()");			exit(1);		}	}}//归还资源量static void V(void){	struct sembuf op;		op.sem_num = 0;	op.sem_op = 1;	op.sem_flg = 0;	if(semop(semid,&op,1) < 0)	{		perror("semop()");		exit(1);	}}static void func_add(void){	FILE *fp;	int fd;	char linebuf[LINESIZE];	fp = fopen(FNAME,"r+");	if(fp == NULL)	{		perror("fopen()");		exit(1);	}P();//取资源量	fgets(linebuf,LINESIZE,fp);	fseek(fp,0,SEEK_SET);	//sleep(1);			fprintf(fp,"%d\n",atoi(linebuf)+1);	fflush(fp);V();//归还资源量		fclose(fp);return;} int main(){	int i,err;	pid_t pid;//创建信号量数组 IPC 返回值为id	semid = semget(IPC_PRIVATE,1,IPC_CREAT);	if(semid < 0)	{		perror("semget");		exir(0);	}// 设置信号量集合中 第一个信号量的资源总量为1	if(semctl(semid,0,SETVAL,1) < 0)	{		perror("semctl");		exir(0);		}	for(i = 0; i < PROCNUM; i++)	{		pid = fork();		if(pid < 0)		{			perror("fork()");			exit(1);		}				if(pid == 0)//Child 		{			func_add();			exit(0);		}	}	for(i = 0;i < PROCNUM; i++)	{		wait(NULL);	}		//从当前系统 删除该 信号量集合	semctl(semid,0,IPC_RMID);	exit(0);	}