C preprocessor: macro function to call printf()

Question

I want to conditionally use either printf() or a statement:

#define USE_PRINTF

#ifdef USE_PRINTF
#define macrofn(str) printf(str)
#else
#define macrofn(str) some_statement
#ifndef USE_PRINTF

But I'm getting the following error:

incompatible implicit declaration of built-in function 'printf'

What am I doing wrong? Thanks

This is not correct... You should have been using an #endif instead of the #ifndef USE_PRINTF which means "if USE_PRINTF is not defined", which already is being handled by the #else part. You absolutely need an #endif for each #if. — Utkan Gezer
– Utkan Gezer, Commented Mar 10, 2014 at 8:53
@ThoAppelsin: Nice catch!! But won't compiler complain as no matching endif for this error. — 0xF1
– 0xF1, Commented Mar 10, 2014 at 8:55
What's the purpose of using printf with just one argument? Shouldn't puts be more suitable in most such cases? See also stackoverflow.com/a/16813480/908515 — michael_wu
– michael_wu, Commented Mar 10, 2014 at 9:05

Utkan Gezer · Accepted Answer · 2014-03-10 08:57:44Z

5

You don't necessarily have to include the <stdio.h> before the macro definition. What you really need is #endif for the #if you have started. For example, the following programme will work all fine:

#define USE

#ifdef USE
#define asd printf("asd")
#else
#define asd puts("kek")
#endif

#include<stdio.h>

int main( ) {
    asd;
    getchar( );
    return 0;
}

So... yeah.

answered Mar 10, 2014 at 8:57

Utkan Gezer

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2 Comments

michael_wu Over a year ago

True, but it is common sense to have the includes on top. And you need it before the invocation of the macro.

Utkan Gezer Over a year ago

@undur_gongor I also wouldn't put things in this order, but I wouldn't mind a bit if someone did, that's what being flexible is.

Community · Accepted Answer · 2017-05-23 10:28:30Z

3

You need to add #include <stdio.h> to your file.

Take a look here for more information about this error message.

edited May 23, 2017 at 10:28

CommunityBot

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answered Mar 10, 2014 at 8:50

Benesh

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2 Comments

Utkan Gezer Over a year ago

This is wrong, if it means what it looks like it means. One does have to #include <stdio.h>, but not necessarily before the macro definition.

Benesh Over a year ago

Force of a habit. Thanks!

michael_wu · Accepted Answer · 2014-03-10 08:50:37Z

2

You need to include stdio.h if you want to use printf.

edited Mar 10, 2014 at 8:50

answered Mar 10, 2014 at 8:49

michael_wu

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Comments

Embedded Software Engineer · Accepted Answer · 2014-03-10 08:55:34Z

2

You should use this syntax:

#include <stdio.h>

#define USE_PRINTF

#ifdef USE_PRINTF
#define macrofn(str) printf(str)
#else
#define macrofn(str) some_statement
#endif

answered Mar 10, 2014 at 8:55

Embedded Software Engineer

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