Linux 信号量之Posix基于内存的信号量

信号量（semaphore），也和互斥锁一样提供了线程间或者进程间的同步功能。

信号量有三种：

• Posix有名字的信号量
• Posix基于内存的信号量
• System V信号量

信号量比互斥锁高级，互斥锁只允许一个线程访问临界区，信号量可以多个，可以把信号量看作成互斥锁的升级版，但是如果能用互斥锁解决，就用互斥锁，互斥锁比信号量节省资源。

这篇文章只介绍Posix基于内存的信号量

1，单个生产者和单个消费者

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>#define NBUFF 10int nitems;
struct {
int buff[NBUFF];
sem_t mutex, nempty, nstored;
} shared;void* produce(void *args);
void* consume(void* args);int main(int argc, char** argv){ pthread_t tid_produce, tid_consume; if(argc != 2){
printf("usage error\n");
exit(1);
} nitems = atoi(argv[1]); //create 3 semaphore
sem_init(&shared.mutex, 0, 1);
sem_init(&shared.nempty, 0, NBUFF);
sem_init(&shared.nstored, 0, 0); pthread_create(&tid_produce, NULL, produce, NULL);
pthread_create(&tid_consume, NULL, consume, NULL); pthread_join(tid_produce, NULL);
pthread_join(tid_consume, NULL); sem_destroy(&shared.mutex);
sem_destroy(&shared.nempty);
sem_destroy(&shared.nstored);
exit(0);
}void* produce(void *args){
int i;
for(i = 0; i < nitems; ++i){
sem_wait(&shared.nempty);
sem_wait(&shared.mutex);
shared.buff[i % NBUFF] = i;
sem_post(&shared.mutex);
sem_post(&shared.nstored);
} return NULL;
}void* consume(void* args){
int i;
for(i = 0; i < nitems; ++i){
sem_wait(&shared.nstored);
sem_wait(&shared.mutex);
shared.buff[i % NBUFF] = i;
sem_post(&shared.mutex);
sem_post(&shared.nempty);
} return NULL;
}

2，多个生产者和单个消费者

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>#define NBUFF 10
#define MAXTHRS 100
#define min(x,y) ( x > y ? y:x )int nitems, nproducers;struct {
int buff[NBUFF];
int idx;
int val;
sem_t mutex, nempty, nstored;
} shared;void* produce(void *args);
void* consume(void* args);int main(int argc, char** argv){ int i, count[MAXTHRS];
pthread_t tid_produce[MAXTHRS], tid_consume; if(argc != 3){
printf("usage error\n");
exit(1);
} nitems = atoi(argv[1]);
nproducers = min(atoi(argv[2]), MAXTHRS); //create 3 semaphore
sem_init(&shared.mutex, 0, 1);
sem_init(&shared.nempty, 0, NBUFF);
sem_init(&shared.nstored, 0, 0); for(i = 0; i < nproducers; ++i){
count[i] = 0;
pthread_create(&tid_produce[i], NULL, produce, &count[i]);
}
pthread_create(&tid_consume, NULL, consume, NULL); for(i = 0; i < nproducers; ++i){
pthread_join(tid_produce[i], NULL);
printf("count[%d] = %d\n", i, count[i]);
}
pthread_join(tid_consume, NULL); sem_destroy(&shared.mutex);
sem_destroy(&shared.nempty);
sem_destroy(&shared.nstored);
exit(0);
}void* produce(void *arg){
int i;
for(i = 0; i < nitems; ++i){
sem_wait(&shared.nempty);
sem_wait(&shared.mutex); if(shared.idx >= nitems){
sem_post(&shared.nempty);//注意点
sem_post(&shared.mutex);
return NULL;// all done
} shared.buff[shared.idx % NBUFF] = shared.val;
shared.idx++;
shared.val++;
sem_post(&shared.mutex);
sem_post(&shared.nstored);
*((int*) arg) += 1;
} return NULL;
}void* consume(void* args){
int i;
for(i = 0; i < nitems; ++i){
sem_wait(&shared.nstored);
sem_wait(&shared.mutex);
if(shared.buff[i % NBUFF] != i){
printf("error:buff[%d] = %d\n", i, shared.buff[i % NBUFF]);
}
sem_post(&shared.mutex);
sem_post(&shared.nempty);
} return NULL;
}

3，多个生产者和多个消费者

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>#define NBUFF 10
#define MAXTHRS 100
#define min(x,y) ( x > y ? y:x )int nitems, nproducers, nconsumers;struct {
int buff[NBUFF];
int idx;
int val;
int gidx;
int gval;
sem_t mutex, nempty, nstored;
} shared;void* produce(void *args);
void* consume(void* args);int main(int argc, char** argv){ int i, prodcount[MAXTHRS], conscount[MAXTHRS];
pthread_t tid_produce[MAXTHRS], tid_consume[MAXTHRS]; if(argc != 4){
printf("usage error\n");
exit(1);
} nitems = atoi(argv[1]);
nproducers = min(atoi(argv[2]), MAXTHRS);
nconsumers = min(atoi(argv[3]), MAXTHRS); //create 3 semaphore
sem_init(&shared.mutex, 0, 1);
sem_init(&shared.nempty, 0, NBUFF);
sem_init(&shared.nstored, 0, 0); for(i = 0; i < nproducers; ++i){
prodcount[i] = 0;
pthread_create(&tid_produce[i], NULL, produce, &prodcount[i]);
}
for(i = 0; i < nconsumers; ++i){
conscount[i] = 0;
pthread_create(&tid_consume[i], NULL, consume, &conscount[i]);
} for(i = 0; i < nproducers; ++i){
pthread_join(tid_produce[i], NULL);
printf("prodcount[%d] = %d\n", i, prodcount[i]);
}
for(i = 0; i < nconsumers; ++i){
pthread_join(tid_consume[i], NULL);
printf("conscount[%d] = %d\n", i, conscount[i]);
} sem_destroy(&shared.mutex);
sem_destroy(&shared.nempty);
sem_destroy(&shared.nstored);
exit(0);
}void* produce(void *arg){
int i;
for(i = 0; i < nitems; ++i){
sem_wait(&shared.nempty);
sem_wait(&shared.mutex); if(shared.idx >= nitems){
sem_post(&shared.nstored);//注意点
sem_post(&shared.nempty);//注意点
sem_post(&shared.mutex);
return NULL;// all done
} shared.buff[shared.idx % NBUFF] = shared.val;
shared.idx++;
shared.val++;
sem_post(&shared.mutex);
sem_post(&shared.nstored);
*((int*) arg) += 1;
} return NULL;
}void* consume(void* arg){
int i;
for(; ;){
sem_wait(&shared.nstored);
sem_wait(&shared.mutex); if(shared.gidx >= nitems){
sem_post(&shared.nstored);//注意点
sem_post(&shared.mutex);
return NULL;// all done
}
i = shared.gidx % NBUFF;
if(shared.buff[i] != shared.gval){
printf("error:buff[%d] = %d\n", i, shared.buff[i]);
}
shared.gidx++;
shared.gval++; sem_post(&shared.mutex);
sem_post(&shared.nempty);
*((int*) arg) += 1;
} return NULL;
}

c/c++ 学习互助QQ群：877684253

本人微信：xiaoshitou5854