Why is the usage of printf wrong in this program?

Question

I saw this sample code in an answer to How do function pointers in C work?

#import <stdlib.h>
#define MAX_COLORS  256

typedef struct {
    char* name;
    int red;
    int green;
    int blue;
} Color;

Color Colors[MAX_COLORS];


void eachColor (void (*fp)(Color *c)) {
    int i;
    for (i=0; i<MAX_COLORS; i++)
        (*fp)(&Colors[i]);
}

void printColor(Color* c) {
    if (c->name)
        printf("%s = %i,%i,%i\n", c->name, c->red, c->green, c->blue);
}

int main() {
    Colors[0].name="red";
    Colors[0].red=255;
    Colors[1].name="blue";
    Colors[1].blue=255;
    Colors[2].name="black";

    eachColor(printColor);
}

The code returns the following error:

test.c: In function ‘printColor’:
test.c:21: warning: incompatible implicit declaration of built-in function ‘printf’

Does it work if you include ``? – chris Jul 24 '12 at 15:44 — chris, Jul 24 '12 at 15:44

score 5 · Accepted Answer · answered Jul 24 '12 at 15:45

5

printf lives in stdio.h, not stdlib.h.

answered Jul 24 '12 at 15:45

Ed S.

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score 3 · Answer 2 · answered Jul 24 '12 at 15:48

Just to add to what others have said, if the C compiler comes across a function for which it has not seen a prototype it will make an assumption about the signature of that function that is generally going to be wrong.

Including stdio.h includes a prototype of the function so that the compiler does not have to guess at its signature.

score 2 · Answer 3 · answered Jul 24 '12 at 15:45

2

Add include of stdio.h:

#include <stdio.h>

answered Jul 24 '12 at 15:45

René Kolařík

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score 1 · Answer 4 · answered Jul 24 '12 at 15:47

1

you have included stdlib.h instead of stdio.h. It stdio.h where printf is defined not stdlib.h. So if you chnage, the warning may be resolved.

answered Jul 24 '12 at 15:47

heretolearn

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Why is the usage of printf wrong in this program?

4 Answers4