信號量（semaphore），也和互斥鎖一樣提供了線程間或者進程間的同步功能。

信號量有三種：

• Posix有名字的信號量
• Posix基於記憶體的信號量
• System V信號量

信號量比互斥鎖高級，互斥鎖只允許一個線程訪問臨界區，信號量可以多個，可以把信號量看作成互斥鎖的升級版，但是如果能用互斥鎖解決，就用互斥鎖，互斥鎖比信號量節省資源。

這篇文章只介紹Posix基於記憶體的信號量

1，單個生產者和單個消費者

#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，多個生產者和單個消費者

#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);//註意點
      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，多個生產者和多個消費者

#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);//註意點
      sem_post(&shared.nempty);//註意點
      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);//註意點
      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;
}