CODEWITHSUNDEEP
cstdio
Wernsey
Wernsey

Reputation: 5491

How do I detect if stdio.h is included?

I have a header foo.h file that declares a function prototype 

void foo(FILE *f); 

/* ... Other things that don't depend on FILE ... */

among other things.

Now obviously, to use this header, I need to do the following

#include <stdio.h>
#include "foo.h"

I would like to surround this particular prototype with something like the following:

#ifdef _STDIO_H
void foo(FILE *f);
#endif

/* ... Other things that don't depend on FILE ... */

so that I can #include "foo.h" without worrying about #include <stdio.h> in cases where I don't need that particular function.

Is the #ifdef _STDIO_H the way to go if I want my code to be portable and standards compliant?

I could find no mention of _STDIO_H in the standards document, but I see it is used in a variety of C libraries. Should I rather use something that I know to be defined in stdio.h, like EOF?

A related question: What do you do for other standard C headers, like stdlib.h?

Upvotes: 3

Views: 2459

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

<stdio.h> and <stdlib.h> are part of the C99 (and C11) standards. So every (hosted) standard conforming C implementation have them.

On most practical implementations, they are header files with some include guards.

A standard conforming implementation might process #include <stdio.h> very specifically, e.g. by using some database. I know no such implementation.

So simply add

#include <stdio.h>

near the top of your header file, something like

// file foo.h
#ifndef FOO_INCLUDED
#define FOO_INCLUDED
#include <stdio.h>
// other includes ...
// ...
// other stuff
#endif /* FOO_INCLUDED */

Alternatively, you could not care and document that #include "foo.h" requires a previous #include <stdio.h>; any sensible developer using a good-enough C implementation would be able to take care of that.

Actually, I was wrong in my comment on Alter Mann's deleted answer. It looks like stdin is required to be some macro, and then you might use #ifdef stdin ... endif as Alter Mann correctly answered. I believe it is not very readable, and you just want to have <stdio.h> included, either by including it yourself in your foo.h or by requiring it in your documentation.

Contrarily to C++ standard headers, C standard headers are in practice quite quick to be compiled, so I don't think it is worth to optimize the unusual case when <stdio.h> has not been included.

Upvotes: 1

i486
i486

Reputation: 6573

Open your stdio.h file (for your compiler) and see whether it has _STDIO_H or similar definition.

Upvotes: 0

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