I'm getting a warning from my GCC compiler that says "implicit declaration of function." What could be causing this warning to appear?

General Discussions
#1

I’m getting a warning from my GCC compiler that says “implicit declaration of function.” What could be causing this warning to appear?

#2

The warning “implicit declaration of function” occurs when you use a function without providing a declaration (also known as a prototype) before the function is called.

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

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

To resolve this warning, you need to declare your function before main, either directly or in a header file:

int fun(int x, char *p); // Declaration of 'fun'

int main() {
    fun(2, "21");
    return 0;
}

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

This way, the compiler knows about the function’s signature before it is called, preventing the warning.

#3

To avoid the “implicit declaration of function” warning, you should declare the desired function before your main function. Here’s an example:

#include <stdio.h>

// Declaration of the function
int yourfunc(void);

int main(void) {
    // Call to the function
    yourfunc();
    return 0;
}

This way, the compiler knows about the function yourfunc before it’s used in main, preventing the warning.

#4

If the function is declared in a header file, make sure to include that header file at the beginning of your source file.

#include "myHeader.h"  // Include the header file that contains the function declaration

int main() {
    // Function call
    myFunction();
    return 0;
}

// Function definition
void myFunction() {
    // Function body
}


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.