信号量—— semaphore

// 信号量：锁机制,进程并发处理（内核全局）

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>

#define PROCNUM 20
#define FNAME "/tmp/out"
#define LINESIZE 1024

static int semid;
// 提取资源 －1
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 && errno != EAGAIN)
{
perror("semop()");
exit(1);
}
}
}
// 归还资源
static void V(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 && errno != EAGAIN)
{
perror("semop()");
exit(1);
}
}

}

static void func_add()
{
FILE *fp;
int fd;
char linebuf[LINESIZE];

fp = fopen(FNAME,"r+");
if(fp == NULL)
{
perror("fopen()");
exit(1);
}
//fd = fileno(fp);

P();
fgets(linebuf,LINESIZE,fp);
fseek(fp,0,SEEK_SET);
sleep(1);
fprintf(fp,"%d\n",atoi(linebuf)+1);
fflush(fp);
V();

fclose(fp);

return ;
}

int main()
{
int i;
pid_t pid;

semid = semget(IPC_PRIVATE,1,0600);
if(semid < 0)
{
perror("semget()");
exit(1);
}

if(semctl(semid,0,SETVAL,1) < 0)
{
perror("semctl()");
exit(1);
}

for(i = 0; i < PROCNUM; i++)
{
pid = fork();
/*if error*/
if(pid == 0)
{
func_add();
exit(0);
}
}

for(i = 0; i < PROCNUM; i++)
wait(NULL);

semctl(semid,0,IPC_RMID);

exit(0);
}