warning: implicit declaration of function

Question

My compiler (GCC) is giving me the warning:

warning: implicit declaration of function

Please help me understand why is it coming

Answer 1

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

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 conculsion try to use __typeof__() instead. Also gcc ... -Dtypeof=__typeof__ ... can help.

Answer 2

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

Answer 3

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

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

int main(void) {

   yourfunc();
 }

Answer 4

The right way is to declare function prototype in 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
}

Answer 5

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);

Answer 6

When you do your #includes in main.c, put 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 "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 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 different order.

Well, it did for me.