Implicit declaration means

alaios · 08-29-2005, 03:39 AM

Hi... what should i understand reading these things?
monitor.c: In function `local_get_htb_parameters':
monitor.c:133: warning: implicit declaration of function `execl'
monitor.c: In function `get_htb_parameters':
monitor.c:144: warning: implicit declaration of function `sleep'

dub.wav · 08-29-2005, 03:49 AM

The compiler has no knowledge of the signature of those functions, therefore it thinks they return int.
You should include the corresponding headers, in this case <unistd.h>.

The man pages for the functions usually says which include files they require.

EDIT: To be more precise, "implicit declaration" means that the compiler accepts whichever signature you're using for the functions, and that it expects them to return int.

alaios · 08-29-2005, 05:14 AM

Thx a lot.. I have created those functions... Can u explain me why the unistd.h library can make it work? What this file provides?

dub.wav · 08-29-2005, 06:47 AM

unistd.h provides declarations of those two functions as well as other things, see 'man unistd.h' for more details.
A practical example:

Code:

#include <stdio.h>

int main() {
	printf("%4.2lf\n", multiply(2.4, 3.5));
	return 0;
}

double multiply(double x, double y) {
	return x * y;
}

The multiply function comes after main, so when used in main it is declared implicitly.
The compiler accepts my usage, and expect the function to return int. When run, the program prints 0.00.
Note that this example doesn't compile in gcc 3.4 and 4.0, but it does in 3.2 and possibly 3.3, and in icc.

Then, we try this:

Code:

#include <stdio.h>
double multiply(double x, double y);

int main() {
	printf("%4.2lf\n", multiply(2.4, 3.5));
	return 0;
}

double multiply(double x, double y) {
	return x * y;
}

The only difference is that I've provided a declaration of the multiply function. Now the program prints 8.40.

schneidz · 08-29-2005, 10:35 AM

why are functions declared after main() in c (i think i remember the oppisite in c++).
i did this and it worked with cc

Code:

#include <stdio.h>

double multiply(double x, double y) {
        return x * y;
}

int main() {
        printf("%4.2lf\n", multiply(2.4, 3.5));
        return 0;
}

is there anything inherantly wrong with defining it above main()?

dub.wav · 08-29-2005, 11:30 AM

No, of course not. It was just a quick example to demonstrate implicit declarations in C.

devkpict · 12-05-2008, 12:51 AM

I have the same warning after searching I found out that the function which i am calling from some other file is a static function

jiikka · 03-09-2009, 06:20 AM

i got the same error n i included unistd.h header file ..
now its working ..