Message "warning: implicit declaration of function"

Question

My compiler (GCC) is giving me the warning:

warning: implicit declaration of function

Why is it coming?

A "why does it not give an error version": stackoverflow.com/questions/434763/… — Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com, Commented May 13, 2015 at 7:27
This can also happen if you forget to include a header file. For example if you are trying to use strlen() without including string.h you will get this error — kurdtpage
– kurdtpage, Commented Sep 11, 2016 at 8:21
I think this question should be removed, as it clutters up every search for a specific instance of this warning. e.g. "My kernel stopped building today, let's see if others are having a similar issue..." Inevitably I end up here. — fpf3
– fpf3, Commented Dec 4, 2023 at 19:51
i landed here because i tried unit testing a c source and did not see that some functions were private. i should've paid more attention to the header file. also, our compiler is set to strictly treat every warning as error. in general, it is wise to prefix private functions with an underscore, like _myPrivFunc — clockw0rk
– clockw0rk, Commented Jul 8, 2024 at 14:17

Mark M · Accepted Answer · 2024-02-18 23:33:34Z

348

You are using a function for which the compiler has not seen a declaration ("prototype") yet.

For example:

int main() {
    fun(2, "21"); /* The compiler has not seen the declaration. */       
    return 0;
}

int fun(int x, char *p) {
    /* ... */
}

You need to declare your function before main, like this, either directly or in a header:

int fun(int x, char *p);

edited Feb 18, 2024 at 23:33

Mark M

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answered Dec 9, 2011 at 3:50

cnicutar

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15 Comments

smitec Over a year ago

As an addition if you have given the prototype check that it isn't just a typo. Also if its from an external library check that you have included it.

Mien Over a year ago

I cannot run the code after I get this warning. So it behaves like an error.

How Chen Over a year ago

@Flimm, C99 and C89/C90 has different setting for this

Ciro Santilli OurBigBook.com Over a year ago

@Flimm stackoverflow.com/questions/434763/…

Nima Mousavi Over a year ago

@ZachSaw Rightly so. Else you would not have repeated yourself thrice.

|

toolic · Accepted Answer · 2025-10-11 11:57:26Z

35

The right way is to declare the function prototype in a header.

Example

main.h

#ifndef MAIN_H
#define MAIN_H

int some_main(const char *name);

#endif

main.c

#include "main.h"

int main()
{
    some_main("Hello, World\n");
}

int some_main(const char *name)
{
    printf("%s", name);
}

Alternative with one file (main.c)

static int some_main(const char *name);

int some_main(const char *name)
{
    // do something
}

edited Oct 11 at 11:57

toolic

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answered Dec 3, 2014 at 14:26

Faizal Pribadi

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Comments

Peter Mortensen · Accepted Answer · 2024-03-07 21:52:53Z

12

When you do your #includes in file main.c, add the #include reference to the file that contains the referenced function at the top of the include list.

E.g., say this is main.c and your referenced function is in file "SSD1306_LCD.h":

#include "SSD1306_LCD.h"
#include "system.h"        #include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h>       // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h"  // This has the 'BYTE' type definition

The above will not generate the "implicit declaration of function" error, but the below will:

#include "system.h"
#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h>       // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h"     // This has the 'BYTE' type definition
#include "SSD1306_LCD.h"

Exactly the same #include list, just a different order.

Well, it did for me.

edited Mar 7, 2024 at 21:52

Peter Mortensen

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answered Nov 9, 2016 at 12:04

tomc

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1 Comment

Peter Mortensen Over a year ago

Why two includes in one line? - #include "system.h" #include <stdio.h>.

Poussy · Accepted Answer · 2020-02-08 12:48:05Z

4

You need to declare the desired function before your main function:

#include <stdio.h>
int yourfunc(void);

int main(void) {

   yourfunc();
 }

answered Feb 8, 2020 at 12:48

Poussy

1014 bronze badges

Comments

Peter Mortensen · Accepted Answer · 2022-10-23 18:43:23Z

3

Don't forget, if any functions are called in your function, their prototypes must be situated above your function in the code. Otherwise, the compiler might not find them before it attempts to compile your function. This will generate the error in question.

edited Oct 23, 2022 at 18:43

Peter Mortensen

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answered Dec 15, 2020 at 9:23

Chris

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1 Comment

Adrian Mole Over a year ago

Does this add anything not already provided by the other answers?

Peter Mortensen · Accepted Answer · 2024-03-07 21:45:58Z

3

If you have the correct headers defined & are using a non glibc library (such as Musl C), GCC will also throw "error: implicit declaration of function" when GNU extensions such as malloc_trim are encountered.

The solution is to wrap the extension & the header:

#if defined (__GLIBC__)
  malloc_trim(0);
#endif

edited Mar 7, 2024 at 21:45

Peter Mortensen

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answered Aug 28, 2015 at 23:05

Stuart Cardall

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2 Comments

Peter Mortensen Over a year ago

The first link is half broken. The second link is completely broken ("Hmm. We’re having trouble finding that site. We can’t connect to the server at patchwork.alpinelinux.org.").

Stuart Cardall Over a year ago

Alpine uses Gitlab nowadays - links updated

Peter Mortensen · Accepted Answer · 2024-03-07 21:49:05Z

3

When you get the "error: implicit declaration of function", it should also list the offending function.

Often this error happens because of a forgotten or missing header file, so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top, as this section will list any header files that need to be included. Or try man. Those are the online man pages. They are hyperlinked and easy to search.

Functions are often defined in the header files, including any required header files is often the answer. Like cnicutar said,

You are using a function for which the compiler has not seen a declaration ("prototype") yet.

edited Mar 7, 2024 at 21:49

Peter Mortensen

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answered Nov 26, 2015 at 7:59

Sal

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Comments

Ndaruga · Accepted Answer · 2021-08-02 11:31:05Z

2

This error occurs because you are trying to use a function that the compiler does not understand. If the function you are trying to use is predefined in C language, just include a header file associated with the implicit function. If it's not a predefined function then it's always a good practice to declare the function before the main function.

answered Aug 2, 2021 at 11:31

Ndaruga

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Comments

Peter Mortensen · Accepted Answer · 2022-10-23 18:48:13Z

0

The GNU C compiler is telling you that it can find that particular function name in the program scope. Try defining it as a private prototype function in your header file, and then import it into your main file.

edited Oct 23, 2022 at 18:48

Peter Mortensen

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answered Jul 26, 2022 at 22:19

Gabriel soft

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1 Comment

Peter Mortensen Over a year ago

Re "...it can find": Don't you mean "...it can’t find" (the opposite)?

Peter Mortensen · Accepted Answer · 2024-03-07 21:50:45Z

0

I think the question is not 100% answered. I was searching for an issue with a missing typeof(), which is a compile-time directive.

The following links will shine light on the situation:

https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Typeof.html

https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords

As of a conclusion, try to use __typeof__() instead. Also gcc ... -Dtypeof=__typeof__ ... can help.

edited Mar 7, 2024 at 21:50

Peter Mortensen

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answered Mar 17, 2016 at 9:15

Peeter Vois

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1 Comment

Peter Mortensen Over a year ago

The first reference says "If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. "

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