进程间通信-共享内存

1.共享内存(shared memory)是最快的IPC,多个进程同时对一段内存地址操作,它没有同步机制,
所以需要同步,常把信号量和共享内存结合起来使用。
2.相关函数：#include <sys/shm.h>
创建共享内存:int shmget(key_t key, size_t size, int flag);
创建或获取一个共享内存：成功返回共享内存ID，失败返回-1
连接共享内存:void *shmat(int shm_id, const void *addr, int flag);
连接共享内存到当前进程的地址空间:成功返回指向共享内存的指针,失败返回-1
断开共享连接:int shmdt(void *addr);
断开与共享内存的连接:成功返回0,失败返回-1
控制共享连接:int shmctl(int shm_id, int cmd, struct shmid_ds *buf);
控制共享内存的相关信息:成功返回0,失败返回-1
struct shmid_ds  
{  
    uid_t shm_perm.uid;  
    uid_t shm_perm.gid;  
    mode_t shm_perm.mode;  
};  


#define TEXT_SZ 2048  
struct shared_use_st  
{  
    int written;//作为一个标志，非0：表示可读，0表示可写  
    char text[TEXT_SZ];//记录写入和读取的文本  

};  

//shmread.c#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <sys/shm.h>#include "shmdata.h"int main(){int running = 1;//程序是否继续运行的标志void *shm = NULL;//分配的共享内存的原始首地址struct shared_use_st *shared;//指向shmint shmid;//共享内存标识符//创建共享内存shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666|IPC_CREAT);if(shmid == -1){fprintf(stderr, "shmget failed\n");exit(EXIT_FAILURE);}//将共享内存连接到当前进程的地址空间shm = shmat(shmid, 0, 0);if(shm == (void*)-1){fprintf(stderr, "shmat failed\n");exit(EXIT_FAILURE);}printf("\nMemory attached at %X\n", (int)shm);//设置共享内存shared = (struct shared_use_st*)shm;shared->written = 0;while(running)//读取共享内存中的数据{//没有进程向共享内存定数据有数据可读取if(shared->written != 0){printf("You wrote: %s", shared->text);sleep(rand() % 3);//读取完数据，设置written使共享内存段可写shared->written = 0;//输入了end，退出循环（程序）if(strncmp(shared->text, "end", 3) == 0)running = 0;}else//有其他进程在写数据，不能读取数据sleep(1);}//把共享内存从当前进程中分离if(shmdt(shm) == -1){fprintf(stderr, "shmdt failed\n");exit(EXIT_FAILURE);}//删除共享内存if(shmctl(shmid, IPC_RMID, 0) == -1){fprintf(stderr, "shmctl(IPC_RMID) failed\n");exit(EXIT_FAILURE);}exit(EXIT_SUCCESS);}

//shmwrite#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <string.h>#include <sys/shm.h>#include "shmdata.h"int main(){int running = 1;void *shm = NULL;struct shared_use_st *shared = NULL;char buffer[BUFSIZ + 1];//用于保存输入的文本int shmid;//创建共享内存shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666|IPC_CREAT);if(shmid == -1){fprintf(stderr, "shmget failed\n");exit(EXIT_FAILURE);}//将共享内存连接到当前进程的地址空间shm = shmat(shmid, (void*)0, 0);if(shm == (void*)-1){fprintf(stderr, "shmat failed\n");exit(EXIT_FAILURE);}printf("Memory attached at %X\n", (int)shm);//设置共享内存shared = (struct shared_use_st*)shm;while(running)//向共享内存中写数据{//数据还没有被读取，则等待数据被读取,不能向共享内存中写入文本while(shared->written == 1){sleep(1);printf("Waiting...\n");}//向共享内存中写入数据printf("Enter some text: ");fgets(buffer, BUFSIZ, stdin);strncpy(shared->text, buffer, TEXT_SZ);//写完数据，设置written使共享内存段可读shared->written = 1;//输入了end，退出循环（程序）if(strncmp(buffer, "end", 3) == 0)running = 0;}//把共享内存从当前进程中分离if(shmdt(shm) == -1){fprintf(stderr, "shmdt failed\n");exit(EXIT_FAILURE);}sleep(2);exit(EXIT_SUCCESS);}