How to create your own header file in ‘C’

Here we have three files

1. demo.h includes the header file and function declaration
2. demo.c includes the function definition
3. main.c includes the header to call function and function call

demo.h

#ifndef DEMO_H_   /* Include guard */
#define DEMO_H_

int functiontest(int x);  /* An example function declaration */

#endif

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demo.c

#include "demo.h"  /* Include the header (not strictly necessary here) */

int functiontest(int x)    /* Function definition */
{
    return x + 5;
}

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main.c

#include <stdio.h>
#include "demo.h"  /* Include the header here, to obtain the function declaration */

int main(void)
{
    int y = functiontest(4);  /* Function call */
    printf("%d\n", y);
    return 0;
}