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#include Directive in C

The #include directive tells the preprocessor to copy the contents of another file into the current source file before compilation. This is how you bring standard library functions and your own header files into a program.

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Syntax

For standard (system) headers — use angle brackets: 
#include <header_file.h>
For your own (user-defined) headers — use double quotes: 
#include "my_header.h"
The difference: angle brackets search the system include directories; double quotes search the current directory first, then system directories.

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Common Standard Header Files

HeaderContents
<stdio.h>printf(), scanf(), fopen(), file I/O
<stdlib.h>malloc(), free(), exit(), rand()
<string.h>strlen(), strcpy(), strcat(), strcmp()
<math.h>sqrt(), pow(), abs(), ceil(), floor()
<ctype.h>isalpha(), isdigit(), toupper(), tolower()
<time.h>time(), clock()
<stdbool.h>bool, true, false (C99+)

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Example: Including Multiple Headers

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

int main() {
    // stdio.h
    printf("Hello!\n");

    // math.h
    printf("sqrt(16) = %.0f\n", sqrt(16));

    // string.h
    char name[] = "Vishal";
    printf("Length = %d\n", strlen(name));

    // stdlib.h
    int r = rand() % 100;
    printf("Random = %d\n", r);

    return 0;
}

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Creating Your Own Header File

mymath.h: 
#ifndef MYMATH_H
#define MYMATH_H

int square(int x);
int cube(int x);

#endif
mymath.c: 
#include "mymath.h"

int square(int x) { return x * x; }
int cube(int x)   { return x * x * x; }
main.c: 
#include <stdio.h>
#include "mymath.h"   // user header — double quotes

int main() {
    printf("Square of 4 = %d\n", square(4));
    printf("Cube   of 3 = %d\n", cube(3));
    return 0;
}

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Include Guards

Header files use include guards to prevent being included more than once (which would cause redefinition errors): 
#ifndef MY_HEADER_H
#define MY_HEADER_H

// declarations here

#endif
Or the modern equivalent (most compilers support it): 
#pragma once