linux共享内存简析

共享内存是IPC的一种机制，允许两个不相关的进程共享同一块内存

//共享内存可以双向通信，但其本身没有相应机制,需要程序编写者设计，本例为单向通信（分为读端和写端）。

共享内存读端：

#include <stdio.h>

#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>

//自定义数据结构，flag为同步机制标志：flag 为1表示读端可读， flag为1表示写端可写;str为数据存储的字符数组
struct my_shared
{
int flag;
char str[1024];
};


int main(void)
{
void *shared_memory = NULL;
struct my_shared *shared_buff = NULL;
int running = 1;
int shmid;

//创建共享内存标识
shmid = shmget((key_t)1234, sizeof(struct my_shared), 0666|IPC_CREAT);
if(shmid < 0)
{
perror("fail to shmget");
exit(1);
}

//连接共享内存到进程地址空间
shared_memory = shmat(shmid, NULL, 0);
if(shared_memory == NULL)
{
perror("fail to shmat");
exit(1);
}


shared_buff = (struct my_shared *)shared_memory;
shared_buff->flag = 0;


while(running)

{

//flag为1，读数据

if(shared_buff->flag)
{
printf("shared_memory message is: %s ", shared_buff->str);
shared_buff->flag = 0;


if(strncmp(shared_buff->str, "end", 3) == 0)
{
running = 0;
}


}

}

//把共享从本进程空间分离
if(shmdt(shared_memory) == -1)
{
perror("fail to shmdt");
exit(1);
}

//删除共享内存
if(shmctl(shmid, IPC_RMID, 0) == -1)
{
perror("fail to shmctl");
exit(1);
}


return 0;

}

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>


struct my_shared
{
int flag;
char str[1024];
};


int main(void)
{
void *shared_memory = NULL;
struct my_shared *shared_buff = NULL;
int running = 1;
int shmid;
char buffer[512];


shmid = shmget((key_t)1234, sizeof(struct my_shared), 0666|IPC_CREAT);
if(shmid < 0)
{
perror("fail to shmget");
exit(1);
}


shared_memory = shmat(shmid, NULL, 0);
if(shared_memory == NULL)
{
perror("fail to shmat");
exit(1);
}


shared_buff = (struct my_shared *)shared_memory;
shared_buff->flag = 0;


while(running)
{

//flag为1，等等读端读数据
while(shared_buff->flag)
{
printf("wait for other process's reading ");
sleep(2);
}

printf("Please input some data ");
fgets(buffer, 512, stdin);


shared_buff->flag = 1;
strncpy(shared_buff->str, buffer, 1024);


if(strncmp(shared_buff->str, "end", 3) == 0)
{
running = 0;
}



}


if(shmdt(shared_memory) == -1)
{
perror("fail to shmdt");
exit(1);
}




return 0;

}