Can I close a FILE without closing the fd?

user_9205399 0
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I've opened a FILE *f with fdopen(fd, "w+") and I would like to keep the fd open after closing with fclose(f).

Is there an elegant way to do that?

Can I simply call fflush(f); free(f); or is that dangerous?

Or is there a way to change the internal fd to an invalid value -1 so that fd cannot be closed by fclose()?

linux c
2 Answers

If you want to get the file descriptor and disassociate it from the the FILE*, there is one supported way:

  1. Get the file descriptor with fileno(). (POSIX)
  2. Duplicate it using dup(). (POSIX)
  3. Close the FILE the normal way with fclose(). (c)

Now you only have a file descriptor.

user_32045510

If you'd really like to use a FILE object rather than a file descriptor, you needn't worry about close()ing the file descriptor: fclose() does this for you. Also, if performance is important, you should worry about all of the fdopen() and wrapped system calls defined in f*() standard I/O functions. Big thanks to all the comments!

Also see the fclose man page:

The fclose() function flushes the stream pointed to by fp (writing any buffered output data using fflush(3)) and closes the underlying file descriptor.

user_144308440
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