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, Mar 10 '14 at 08:53
@ThoAppelsin: Nice catch!! But won't compiler complain as no matching `endif` for this error. — 0xF1, Mar 10 '14 at 08:55
Thank you all - typo galore - it's clearly too early in the morning — Jodes, Mar 10 '14 at 08:57
What's the purpose of using `printf` with just one argument? Shouldn't `puts` be more suitable in most such cases? See also http://stackoverflow.com/a/16813480/908515 — undur_gongor, Mar 10 '14 at 09:05
@undur_gongor `puts` prints an additional new line when it reaches the end of the string pushed into. So they aren't truly analogous even for the cases where `printf` has only one argument. Maybe `#define macrofn(str) for ( int i = 0; str[i]; i++ ) putchar( str[i] );` — Utkan Gezer, Mar 10 '14 at 12:45
@ThoAppelsin: I'm aware about that. But even if you want to avoid the line break, you should rather define the macro as `printf("%s", str)`. — undur_gongor, Mar 10 '14 at 13:49
@undur_gongor How about `#define macrofn( str, ... ) printf( str, __VA_ARGS__ )` then? — Utkan Gezer, Mar 10 '14 at 14:27

score 5 · Accepted Answer · answered Mar 10 '14 at 08:57

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 '14 at 08:57

Utkan Gezer

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True, but it is common sense to have the includes on top. And you need it before the invocation of the macro. – undur_gongor Mar 10 '14 at 09:00
1

@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. – Utkan Gezer Mar 10 '14 at 09:02

score 3 · Answer 2 · edited May 23 '17 at 10:28

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 '17 at 10:28

Community

1
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answered Mar 10 '14 at 08:50

Benesh

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This is wrong, if it means what it looks like it means. One does have to `#include `, but not necessarily before the macro definition. – Utkan Gezer Mar 10 '14 at 08:59
1

Force of a habit. Thanks! – Benesh Mar 10 '14 at 09:00

undur_gongor · Answer 3 · 2014-03-10T08:50:37.737

2

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

edited Mar 10 '14 at 08:50

answered Mar 10 '14 at 08:49

undur_gongor

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score 2 · Answer 4 · answered Mar 10 '14 at 08:55

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 '14 at 08:55

developer

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C preprocessor: macro function to call printf()

4 Answers4