Linux操作系统 使用共享内存实现进程通信和同步

共享内存使用

//main.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/shm.h>
#include <string.h>
int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
  strcpy(s,"hello");
   shmdt(s);

   exit(0);
}


//test.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/shm.h>
#include <string.h>
int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
   printf("%s",s);
   shmdt(s);

   shmctl(shmid,IPC_RMID,NULL);
   exit(0);
}

进程a从键盘循环获取数据并拷贝到共享内存中，进程b从共享内存中获取并打印数据。要求a输入一次，b输出一次。a不输入，b也不输出

//main.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/shm.h>
#include <string.h>
int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);//这里加上IPC_EXCL也可以,去掉也可以
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
  // strcpy(s,"hello");
  while(1)
  {
      printf("input:\n");
      char buff[128]={0};
      fgets(buff,128,stdin);

      strcpy(s,buff);
      if(strncmp(buff,"end",3)==0)
      {   
          break;
      }   
  }
   shmdt(s);

   exit(0);
}


//test.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/shm.h>
#include <string.h>
int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
  // printf("%s",s);
  while(1)
  {
      if(strncmp(s,"end",3)==0)
      {   
          break;
      }   
      printf("read:%s\n",s);
      sleep(1);
  }
   shmdt(s);

   shmctl(shmid,IPC_RMID,NULL);
   exit(0);
}

 上面代码会出现问题，会发现如果不sleep（1），他就会一直打印下去。所以共享内存，必须结合信号量一起使用

//sem.h
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/sem.h>
#define SEM1  0
#define SEM2  1
#define SEM_MAX 2
union semun
{
    int val;
};

void sem_init();
void sem_p(int index);
void sem_v(int index);
void sem_destroy();


//sem.c
#include "sem.h"
static int semid=-1;
void sem_init()
{
    semid=semget((key_t)1234,SEM_MAX,IPC_CREAT|IPC_EXCL|0600);
    if(semid==-1)
    {
        semid=semget((key_t)1234,SEM_MAX,0600);
        if(semid==-1)
        {
            printf("sem failed!\n");
            return ;
        }
    }
    else
    {
        union semun a;
        int arr[SEM_MAX]={1,0};
        for(int i=0;i<SEM_MAX;i++)
        {
            a.val=arr[i];
            if(semctl(semid,i,SETVAL,a)==-1)
            {
                printf("semctl setval failed\n");
            }
        }
    }
}

void sem_p(int index)
{
    if(index<0||index>=SEM_MAX)
    {
        return ;
    }
    struct sembuf a;
    a.sem_num=index;
    a.sem_op=-1;
    a.sem_flg=SEM_UNDO;

    if(semop(semid,&a,1)==-1)
    {
        printf("semop p error!\n");
    }
}
void sem_v(int index)
{
    if(index<0||index>=SEM_MAX)
    {
        return ;
    }
    struct sembuf a;
    a.sem_num=index;
    a.sem_op=1;
    a.sem_flg=SEM_UNDO;

    if(semop(semid,&a,1)==-1)
    {
        printf("semop v error!\n");
    }
}

void sem_destroy()
{
    if(semctl(semid,0,IPC_RMID)==-1)
    {
        printf("destroy error!\n");
    }
}


//main.c
#include <string.h>
#include "sem.h"
#include <sys/shm.h>

int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
  // strcpy(s,"hello");
  char buff[128];
  sem_init();
   while(1)
  {
      printf("input:\n");
      memset(buff,128,0);
      fgets(buff,128,stdin);
    
      sem_p(SEM1);
      strcpy(s,buff);
      sem_v(SEM2);
      if(strncmp(buff,"end",3)==0)
      {   
          break;
      }   
  }
   shmdt(s);

   exit(0);
}


//b.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/shm.h>
#include <string.h>
#include "sem.h"
int main()
{
    int shmid=shmget((key_t)1234,256,IPC_CREAT|0600);
    assert(shmid!=-1);

    char *s=(char *)shmat(shmid,NULL,0);
   // assert(s!=NULL);
   if(s==(char *)-1)
   {   
       exit(1);
   }   
  // printf("%s",s);
  sem_init();
  while(1)
  {
      sem_p(SEM2);
      if(strncmp(s,"end",3)==0)
      {   
          break;
      }   
      printf("read:%s\n",s);
      sem_v(SEM1);
  }
   shmdt(s);
   sem_destroy();
   shmctl(shmid,IPC_RMID,NULL);
   exit(0);
}