这个系统调用需要shm_open返回的文件描述符，它返回指向内存的指针。

This system call requires the file descriptor from shm_open and returns a pointer to memory.

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执行fork之后，父进程和子进程获得打开的所有文件描述符和数据值的拷贝，所以指针对于它们都是有效的。

After the fork, the parent and child have copies of all open file descriptors and data values, so the pointer works for both.

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fd 是文件描述符，是指向文件的一个指针。

fd is the file descriptor, a pointer to the file. For this example, use the following

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