sharememory

Identifiers: Each IPC structure(message queue, semaphore, or share memory segment) is referred to by a non-negative integer identifier. 主要用于send/fetch the content from the IPC structure.
需要注意The identifier is an internal name for an IPC object. 在Cooperating processes之间，就需要an external namin scheme, 也就是key。

在创建IPC structure时，需要指定key，比如

1

extern int  (key_t __key, size_t __size, int __shmflg) __THROW;

那么怎么来获取key呢？

1. The server can create a new IPC structure by specifying a key of IPC_PRIVATE and store the returned identifier somewhere(such as a file ) for the client to obtain.
   注意通过key IPC_PRIVATE 只能用于create a new IPC structure, 后续无法通过这个key来access IPC structure.
   IPC_PRIVATE key 也用在parent-child relationship. 在parent中create IPC structure, 这个Identifier is available in child process after fork(). 在child process会获取parent data的副本

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#include <sys/ipc.h>
#include <iostream>
#include <string>
#include <errno.h>
#include <unistd.h>

int main(int argc, char ** argv)
{
	int id = shmget(IPC_PRIVATE, 1024, IPC_CREAT | IPC_EXCL | 0666);
	pid_t child = fork();
	if(child == 0)
	{
		std::cout << "id in child is " << id << std::endl;
		exit(0);
	}

	std::cout << "id in parent is " << id << std::endl;
	exit(0);
}

2. The client and the server can agree on a key by defining the key in a common header
3. ftok
   

   1 2	#include <sys/ipc.h> key_t ftok(const char *path, int id);