7  

You're correct that the system will close the write end of the pipe once the child exits. However there could be another write end of that pipe open, if the child forks or passes a duplicate of the write end to another process.

你是正确的,一旦孩子退出,系统将关闭管道的写端。但是,如果子进程将fornd的副本分叉或传递给另一个进程,则可能会打开该管道的另一个写入结束。

It is still true that the system would be able to tell when all the descriptors at one end of a pipe have been closed (either explicitly or because the owning process exited). It still doesn't make sense to close those on the other end of the pipe, as that would lead to confusion when the parent process tries to close the descriptor on its end of the pipe; either:

系统仍然能够判断管道一端的所有描述符何时被关闭(显式或因为退出拥有过程),这仍然是正确的。关闭管道另一端的那些仍然没有意义,因为当父进程试图关闭管道末端的描述符时会导致混淆;之一:

• the fd has been closed by the system, in which case there is an error as it tries to close an already closed fd; or

fd已被系统关闭,在这种情况下,当它试图关闭已经关闭的fd时会出现错误;要么

• the fd has been reused, which is even worse as it is now closing a completely unrelated fd.

fd已被重用,这更糟糕,因为它现在正在关闭一个完全不相关的fd。

From the point of view of the system, it might well have discarded the pipe once all the descriptors at one end have been closed, so you don't need to worry about inefficiency there. What matters more is that the user space process should have a consistent experience, which means not closing the descriptor unless it is specifically requested.

从系统的角度来看,一旦所有描述符都被关闭,它可能会丢弃管道,所以你不必担心那里的效率低下。更重要的是,用户空间进程应具有一致的体验,这意味着除非特别请求,否则不会关闭描述符。

5  

File descriptors are not closed by the system, until the process exits. This is true for pipes, as well as any other file descriptor.

系统不会关闭文件描述符,直到进程退出。对于管道以及任何其他文件描述符都是如此。

There's a big difference between a pipe (or any other file) with no data in it and a closed file descriptor.
When a file descriptor is closed, the system can reuse its number for a new file descriptor. Then, when you read, you get something else. So after you've closed a file descriptor, you must no longer use it.

没有数据的管道(或任何其他文件)与封闭文件描述符之间存在很大差异。关闭文件描述符时,系统可以将其编号重用于新的文件描述符。然后,当你阅读时,你会得到别的东西。因此,在关闭文件描述符后,您必须不再使用它。

Now imagine that once there's no more data, the system would automatically close the file descriptor. This would make the number available for reuse, and a subsequent unrelated open may get it. Now the reader, who doesn't know yet that there's no more data, will read from what it thinks is the pipe, but will actually read from another file.

现在假设一旦没有更多数据,系统将自动关闭文件描述符。这将使该数量可供重用,并且随后的无关打开可以获得它。现在,那个还不知道没有更多数据的读者会从它认为是管道的内容中读取,但实际上会读取另一个文件。