3

As the title states, I am getting the following error: "implicit declaration of function ‘fileno’" when I try to compile on Linux but not on a Mac. I know I can fix it with the simple line int fileno(FILE *stream);, but I want to know why this happening.

helppls
  • 51
  • 1
  • 2
  • Use strict warnings in both environments. I'd be surprised if you get the warning on missing prototype on only one of them then. – Yunnosch Jun 19 '17 at 05:30
  • 1
    And where is the code which shows this problem? – Sourav Ghosh Jun 19 '17 at 05:32
  • 1
    Related: [How do I remove the following 'implicit declaration of function' warnings?](https://stackoverflow.com/a/9427286/4520911) – iRove Jun 19 '17 at 05:32
  • 2
    What compilation options are you using? `-std=c11` or `-std=c99`? Try `-std=gnu11` or `-std=gnu99` instead. – Jonathan Leffler Jun 19 '17 at 05:35
  • Sorry, but I don't know what ISO C and POSIX have to do with Linux and Mac or what they are. The line of code is this int f = fileno(outputFile);. I'm not looking for a fix, I already have one. What I'm trying to understand is why this is error is happening – helppls Jun 19 '17 at 05:38
  • 4
    Officially, you have to request POSIX functions by having `#define _XOPEN_SOURCE 700` or `#define _POSIX_C_SOURCE 200809L` in effect before the first (POSIX) standard header is included. If you specify `-std=c11`, then those functions are not declared by defaul. – Jonathan Leffler Jun 19 '17 at 05:57
  • [As n.m. suggested](https://stackoverflow.com/a/44623177/4520911), try adding a `#define _POSIX_C_SOURCE 1` above `#include ` to explicitly state you are using POSIX. – iRove Jun 19 '17 at 06:04
  • "I don't know what ISO C and POSIX have to do with Linux and Mac or what they are" You probably want to learn these things if you plan to produce meaningful C code for these systems. – n. m. could be an AI Jun 19 '17 at 06:13

3 Answers3

9

You should not declare functions provided by a system library. You should properly include an appropriate header.

Since fileno is not a standard C function, it is not normally declared by in <stdio.h> . You have to define a suitable macro in order to enable the declaration.

man fileno

Requirements for glibc (see feature_test_macros(7)):

fileno(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

Defining any of the the three macros before inclusion of should do the trick.

n. m. could be an AI
  • 112,515
  • 14
  • 128
  • 243
3

If you remembered to include <stdio.h>, this warning indicates that your <stdio.h> does not declare this function.

fileno is not a standard function. It is perfectly expected that some implementations do not provide it, especially if you are compiling in strict standard-compliant mode.

If your code still links successfully, this almost certainly means that your Linux compiler settings direct your compiler to work in "strict" mode and thus force <stdio.h> to "hide" declaration of fileno.

AnT stands with Russia
  • 312,472
  • 42
  • 525
  • 765
-1

I wrote a simple program in my Ubuntu 16.04 machine. And it compiles well.

#include <stdio.h>

int main ()
{
    printf ("%d\n", fileno(stdout));

    return 0;
}

I hope you know that fileno is present in <stdio.h>. If you have not included the header file, please do so and re-compile.

I am not so sure why Mac version works.

EDIT:

I have not used C99 standard. I compiled using: gcc -Wall -Werror test.c

NVS Abhilash
  • 574
  • 7
  • 24