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Close a FILE pointer without closing the underlying file descriptor

Answer 1

+6 A:

If you're on a POSIXy system (which I assume you are, since you have fileno()), you can use dup() to clone the file descriptor:

int newfd = dup(fileno(stream));
fclose(stream);

Or you can hand fdopen() a duplicate file descriptor:

FILE *stream = fdopen(dup(fd), "r");

Either way, the other copy of the fd won't close with the FILE *. However, keep in mind the location pointer is shared, so be careful if you are using both at the same time.

bdonlan 2009-10-21 22:09:32

windows and other platforms support fileno()!i can't believe i didn't think of this dup() solution, very elegant, and obvious

Matt Joiner 2009-10-21 22:17:02

Don't you also need to dup2() the new fd back to the original fd and then close() the new fd so everything is back the way it was before, except the file stream is closed? :D Which means you need oldfd = fileno(stream) before you do fclose().

Jonathan Leffler 2009-10-21 22:17:52

Jonathan, doing do is racy in multithreaded applications. It's better to `dup` before giving the fd to `fdopen` so you can be sure it never touches your original.

bdonlan 2009-10-21 22:35:34

@bdonlan, explain: i don't see how it's racy

Matt Joiner 2009-10-22 04:09:54

Thread A: close(42); Thread B: open(...) (returns 42) Thread A: dup2(42, 54); Now thread B's fd has been closed and replaced with whatever thread A was working with.

bdonlan 2009-10-22 04:20:57

yeah but that's unavoidable regardless of how you pass the fd around. no threads are introduced between the dup, and fdopen, you just need to make sure that no thread closes it in that time. the code as given above is in no way racy on it's own.

Matt Joiner 2009-10-22 09:44:22

Answer 2

+1 A:

If everything else fails, dup(2) could help.

ndim 2009-10-21 22:10:16

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Close a FILE pointer without closing the underlying file descriptor

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