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My compiler (gcc) throws warnings (not errors!) on the line which declares fp:

int fd = open("filename.dat", O_RDONLY); FILE* fp = fdopen(fd, "r"); // get a file pointer fp from the file descriptor fd

These are the warnings:

main.c: In function ‘main’: main.c:606: warning: implicit declaration of function ‘fdopen’ main.c:606: warning: initialization makes pointer from integer without a cast

I do not understand these warnings since the return value of fopen is a FILE*. What is the mistake I am making here?

EDIT: I am including stdio.h (and I am also on Linux).

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  • @robert: stdio.h, stdlib.h, string.h, errno.h, unistd.h, sys/types.h, sys/stat.h, fcntl.h Commented Sep 2, 2012 at 13:01
  • Does man fdopen list any headers that you don't have? Commented Sep 2, 2012 at 13:02
  • @robert: no, it lists on stdio.h. Commented Sep 2, 2012 at 13:03
  • Which compiler flags are you using? Commented Sep 2, 2012 at 13:06
  • 2
    I'm using gcc 4.4.3 with -g -O0 -std=c99 -Wall. Commented Sep 2, 2012 at 13:08

5 Answers 5

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Short answer: use -std=gnu99 when compiling, the usual standard is non-POSIX and does not have fdopen.

warning: implicit declaration of function ‘fdopen’

Means you have forgot to include the header file which the declaration of fdopen() resides in. Then an implicit declaration by the compiler occurs - and that means the return value of the unknown function will be assumed to be int - thus the second warning. You have to write

#include <stdio.h>

Edit: if you properly include stdio.h, then fdopen() might not be available on the system you're targeting. Are you on Windows? This function is POSIX-only.

Edit 2: Sorry, I really should have perceived this. C99 means the ANSI C99 standard - and standard C doesn't force the concept of file descriptors in order to support non-POSIX systems, so it provides fopen() only. fdopen() is related to file descriptors, so it's POSIX-only, so it's not part of standard C99. If you use the -std=gnu99 switch for GCC, it gets rid of the standard's restrictions and lets in the POSIX and GNU-only extensions, essentially fixing your problem.

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6 Comments

Sorry, I forgot to mention that I am including stdio.h (see edit).
@pap42 you might be using windows. If that's the case, I suggest you to cease and desist.
@pap42 are you using a compiler option that restricts you to C89 and/or ANSI-C only? I'm looking at gcc -std=c89 -ansi...
I'm compiling with gcc using the flags: -g -O0 -std=c99 -Wall.
@pap42 if you use -std=gnu99 instead, does it help?
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#define _XOPEN_SOURCE 600 #include <stdio.h>

This conforms perfectly with strict c99

gcc -std=c99 -pedantic -Wall -Wextra -Werror
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You are not including #include <stdio.h> in C the compiler therefore "guesses" the declaration of the function you're trying to call. (Taking the parameters you've based and using int as return value). Usually you don't want such guesses therefore the compiler warns you.

Solution: Add proper #includes.

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4 Comments

Sorry, but this answer is incorrect. I am including stdio.h (see edit).
What system are you on? - POSIX says fdopen() has to be in stdio.h (or in a header included by stdio.h) - pubs.opengroup.org/onlinepubs/007908775/xsh/fdopen.html
I'm on Linux (and I'm equally baffled by this warning since I am including stdio.h).
fdopen is conditionally declared in <stdio.h>; see the other answers.
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The fdopen function is not part of the C standard and is not available as part of the standard headers if you compile in standard C mode. So you either need to use -std=gnu99 instead of -std=c99 to compile your source or declare the function yourself.

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There's a good explanation for the compiler's diagnostic in @H2CO3's answer, so let's only look on the why of things: if you're using glibc (and you probably are), certain POSIX functions may require specific feature test macros to show up.

In particular, you may need to put the following line:

#define _POSIX_SOURCE // or #define _XOPEN_SOURCE

before

#include <stdio.h>

Certain compilers (such as gcc) also have command line options to the same effect (all the gnu* standards options in gcc).

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1 Comment

This is right in principle, but _POSIX_SOURCE is from the outdated early-90s version of the POSIX standard. The modern name is _POSIX_C_SOURCE and it can't just be defined to a blank definition; it has to be defined to the version you want. 200809L is the correct value for the current standard.