Linux 信号量之Posix基于内存的信号量

Posted 2022-02-02 xiaoshiwang

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 10

int 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;

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