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shm
	int 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);
}