How to get a FILE pointer from a file descriptor?

Asked 21/12, 2009 at 17:30 Answered 21/12, 2009 at 17:32

I'm playing around with mkstemp(), which provides a file descriptor, but I want to generate formatted output via fprintf(). Is there an easy way to transform the file descriptor provided by mkstemp() into a FILE * structure that is suitable for use with fprintf()?

Abiogenetic answered 21/12, 2009 at 17:30 Comment(1)

And the inverse: How can I convert a file pointer ( FILE* ) to a file descriptor (fd)? – Fruitless 1/10, 2013 at 20:37

122

Use fdopen():

FILE* fp = fdopen(fd, "w");

Clarineclarinet answered 21/12, 2009 at 17:31 Comment(11)

And to get the file descriptor from a FILE* use fileno() : linux.die.net/man/3/fileno – Thresher 17/10, 2011 at 14:28

if fd was opened with some flags (like O_NONBLOCK) -- what will happen with them after fdopen opens it with new flags? Will they be xor-ed together or substituted? – Broadfaced 4/11, 2015 at 19:9

Not available in ANSI C. – Uncircumcised 26/3, 2016 at 23:28

@jww: File descriptors are not ANSI C to begin with. – Rachmaninoff 2/5, 2016 at 1:8

and is there a way to close a file with the fd? – Immovable 30/1, 2017 at 21:44

If you mean close(fd), that's not a good idea. fclose(fp) should be used to make sure any buffered output (if any) is written. – Clarineclarinet 31/1, 2017 at 0:10

is it bad in any way to call fdopen multiple times? (assuming it happens inside some function which gets called again and again) – Saxecoburggotha 4/7, 2017 at 23:8

You don't want to fdopen() the same file descriptor twice. Each instance will have separate buffers that will cause your input/output to get messed up. – Clarineclarinet 5/7, 2017 at 13:44

@RichardPennington I think multiple fdopen calls are fine depending on what underlies the file descriptor. For a tty or a socket this may actually be desirable since ANSI C has requirements for fseek/fgetpos/fflush calls between input and output. Here it seems different streams for input and output are the best way to go. This essentially happens with stdin/stdout/stderr anyway on a terminal device - the file descriptors refer to the same file description. See experts-exchange.com/questions/24261206/… – Skirl 21/10, 2017 at 21:16

@Broadfaced The options for the FILE* have to be within the scope of the already open file descriptor. So if you have opened fd with read only, and the FILE* with write only, it should throw an exception. For more info refer to ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/… – Imbecilic 6/12, 2017 at 7:59

@TjadClark "it should throw an exception" - that would be difficult seeing as fdopen() is a C function and C has no native exceptions mechanism. – Proa 1/10, 2018 at 15:7

FILE* f = fdopen(d, "w");

man fdopen output:

SYNOPSIS

#include <stdio.h>

FILE *
fdopen(int fildes, const char *mode);

The fdopen() function associates a stream with the existing file descriptor, fildes. The mode of the stream must be compatible with the mode of the file descriptor. When the stream is closed via fclose(3), fildes is closed also.

Azeria answered 21/12, 2009 at 17:32 Comment(2)

Not available in ANSI C. – Uncircumcised 26/3, 2016 at 23:28

For more clarity, refer to ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/… – Imbecilic 6/12, 2017 at 8:2

-8

There is no standard way of doing this (or the reverse) as the C Standard has nothing to say about file descriptors. Your specific platform may or may not provide such a mechanism.

Exportation answered 21/12, 2009 at 17:32 Comment(15)

Depends on what you mean by "standard". POSIX is a standard. – Clarineclarinet 21/12, 2009 at 17:33

though, The fdopen() function conforms to IEEE Std 1003.1-1988 (``POSIX.1''). – Azeria 21/12, 2009 at 17:34

The question was about file descriptors. ;-) – Clarineclarinet 21/12, 2009 at 17:35

@Neil > you might be right, you didn't help the OP with your answer – Azeria 21/12, 2009 at 17:35

@Gregory I helped him assuming that he wants to write portable code. And downvoting technically correct answers is not good practice. – Exportation 21/12, 2009 at 17:38

@Richard The question is tagged as C, not Posix. Posix is a platform, not a language, standard. – Exportation 21/12, 2009 at 17:39

@Neil > I didn't vote down, indeed there is nothing wrong in what you said. – Azeria 21/12, 2009 at 17:40

@Neil > the question being tagged as C doesn't mean "tell me the holly truth about the C standard" but rather "I'm coding in C, I fail at doing this, please tell me whether it is possible and how?" – Azeria 21/12, 2009 at 17:42

There is a posix tag here on SO, with 193 questions. Having said that, I think Neil's answer is correct. More people should be using posix tag. And it's a good idea to remind people what exactly "C" is. While we're at it, we should make people realize that there's nothing like "C/C++", and that these are two different languages. – Instantaneous 21/12, 2009 at 17:53

@Neil > it looks like you have fans though, as several of my answers are being downvoted right now :) – Azeria 21/12, 2009 at 17:57

I'm backing away slowly... Now turning and running away. – Clarineclarinet 21/12, 2009 at 17:59

@Gregory: For the record, it wasn't me. There are some weird downvoting issues here on SO. My posts #1898871 and https://mcmap.net/q/130293/-memory-corruption/… were downvoted, and I don't know why! – Instantaneous 21/12, 2009 at 17:59

I agree that there's no standard way of doing this, but the OP's mention of mkstemp implies that non-standard solutions are acceptable in this particular instance. In that case, however, he should have added the posix tag (which I see has since been added). – Mechling 21/12, 2009 at 17:59

@Neil, and the purpose of the Net and the meaning of life. You would make more sense if you've said that you are of different opinion. I also doubt SO is all about hairsplitting over standards. And it's not very practical to rely on tagging skills of someone with an SO-experience corresponding to the reputation of 23. – Underlet 21/12, 2009 at 18:8

Not sure why this fellow is getting downvoted. ANSI C does not have fdopen, so there's no way to convert a file descriptor to a FILE* in a base system. You'd need a different standard to get the functionality. – Uncircumcised 26/3, 2016 at 23:27

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