用system V信号量实现进程同步的例子

以下是一个system V的信号量同步例子，共5个进程，P1，P2，P3，P4，P5。同步关系是P1最先执行；P2、P3、P4都是等待P1完成之后才能执行，P5等待P2，P3，P4全部完成之后才能执行。

#include <stdio.h>

#include <sys/sem.h>

#include <sys/ipc.h>

#include <sys/types.h>

#define KEY1 1211

#define KEY2 1212

#define KEY3 1213

#define KEY4 1214

#define KEY5 1215

#define KEY6 1216

union semun{

int val;

struct semid_ds *buf;

unsigned short *array;

};

int crt_sem(key_t key, int val)

{

union semun arg;

int id = semget(key, 1, IPC_CREAT);

arg.val = val;

semctl(id, 0, SETVAL, arg);

return id;

}

void P(int semid)

{

struct sembuf p[1];

p[0].sem_num = 0;

p[0].sem_op = -1;

p[0].sem_flg = 0;

semop(semid, p, 1);

}

void V(int semid)

{

struct sembuf v[1];

v[0].sem_num = 0;

v[0].sem_op = 1;

v[0].sem_flg = 0;

semop(semid, v, 1);

}

int

main(void)

{

int i = 1;

pid_t pid;

int t;

int p12 = crt_sem(KEY1, 0);

t = semctl(p12, 0, GETVAL);

printf("init value of p12 is %d./n");

int p13 = crt_sem(KEY2, 0);

t = semctl(p13, 0, GETVAL);

printf("init value of p13 is %d./n");

int p14 = crt_sem(KEY3, 0);

t = semctl(p14, 0, GETVAL);

printf("init value of p14 is %d./n");

int p25 = crt_sem(KEY4, 0);

t = semctl(p25, 0, GETVAL);

printf("init value of p25 is %d./n");

int p35 = crt_sem(KEY5, 0);

t = semctl(p35, 0, GETVAL);

printf("init value of p35 is %d./n");

int p45 = crt_sem(KEY6, 0);

t = semctl(p45, 0, GETVAL);

printf("init value of p45 is %d./n");

while (i < 6){

if ((pid = fork()) < 0){

printf("process fork error./n");

return(0);

}else if(pid == 0){

break;

}else{

i++;

}

}

if (i == 1){

sleep(1);

printf("----------------process1./n");

//printf("-----------before:%d/n",semctl(p12,0,GETVAL,0));

V(p12);

V(p13);

V(p14);

printf("p12 after process1:%d/n",semctl(p12,0,GETVAL,0));

printf("p13 after process1:%d/n",semctl(p13,0,GETVAL,0));

printf("p14 after process1:%d/n",semctl(p14,0,GETVAL,0));

}

if (i == 2){

sleep(2);

P(p12);

t = semctl(p12, 0, GETVAL, 0);

printf("--------------process2./n");

V(p25);

printf("p12 after process2:%d/n",semctl(p12,0,GETVAL,0));

printf("p25 after process2:%d/n",semctl(p25,0,GETVAL,0));

}

if (i == 3){

sleep(2);

P(p13);

printf("--------------process3./n");

V(p35);

printf("p13 after process3:%d/n",semctl(p13,0,GETVAL,0));

printf("p35 after process3:%d/n",semctl(p35,0,GETVAL,0));

}

if (i == 4){

sleep(2);

P(p14);

printf("--------------process4./n");

V(p45);

printf("p14 after process4:%d/n",semctl(p14,0,GETVAL,0));

printf("p45 after process4:%d/n",semctl(p45,0,GETVAL,0));

}

if (i == 5){

sleep(4);

P(p25);

P(p35);

P(p45);

printf("--------------process5./n");

printf("p25 after process5:%d/n",semctl(p25,0,GETVAL,0));

printf("p35 after process5:%d/n",semctl(p35,0,GETVAL,0));

printf("p45 after process5:%d/n",semctl(p45,0,GETVAL,0));

}

if (i == 6)

sleep(5);

return(0);

}