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);

The right way is to declare function prototype in header.

Example

main.h

#ifndef MAIN_H#define MAIN_Hint 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}

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.