Difference between #include and #include” ” in C/C++ with Examples

The difference between the two types is in the location where the preprocessor searches for the file to be included in the code.

#include <filename>  // Standard library header

#include “filename”  // User defined header

#include<filename>

#include<> is for pre-defined header files. If the header file is predefined then simply write the header file name in angular brackets. 

Example:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

The preprocessor searches in an implementation-dependent manner, normally in search directories pre-designated by the compiler/IDE. This means the compiler will search in locations where standard library headers are residing. The header files can be found at default locations like /usr/include or /usr/local/include. This method is normally used to include standard library header files.

Example: Below is the C++ program to demonstrate the above concept:

• C

// C program to demonstrate // the above concept #include <stdio.h>   // Driver code int main() {   printf("GeeksforGeeks | ");   printf("A computer science portal for geeks");   return 0; }

Output:

GeeksforGeeks | A computer science portal for geeks

#include “FILE_NAME”

#include ” “ is for header files that programmer defines. If a programmer has written his/ her own header file, then write the header file name in quotes. 

Example:

#include “mult.h” 
Here, mul.h is header file written by programmer.

The preprocessor searches in the same directory as the file containing the directive. The compiler will search for these header files in the current folder or -I defined folders. This method is normally used to include programmer-defined header files. 

mul.h Header file:

// mul.h
int mul(int a, int b)
{
    return(a * b);
}

Below is the C program to include and use the header file mul.h:

• C

// C program to demonstrate // the above approach   // Include user-defined // header file #include "mul.h"   // Driver code int main() {   int a = 10;   int b = 20;       // Invoke the function defined in   // header file   int c = mul(a, b);   printf("%d", c);     return 0; }

Output:

200

#include<> vs #include””

S No.	#include<filename>	#include”filename”
1	The preprocessor searches in the search directories pre-designated by the compiler/ IDE.	The preprocessor searches in the same directory as the file containing the directive.
2	The header files can be found at default locations like /usr/include or /usr/local/include.	The header files can be found in -I defined folders.
3	This method is normally used for standard library header files.	This method is normally used for programmer-defined header files.
4	<> It indicates that file is system header file	” ” It indicates that file is user-defined header file

Case 1: Include standard library header using notation #include””.

• C

// C program to demonstrate // the difference // Header file #include "stdio.h"   // Driver code int main() {   int a = 10;   printf("%d", a);   return 0; }

Output:

10

Explanation:
#include ” ” will search ./ first. Then it will search the default include path. One can use the below command to print the include path.

gcc -v -o a filename.c

Case2: Include standard header file using the notation #include<>

• C

// C program to demonstrate // the difference // Header file #include <stdio.h>   // Driver code int main() {   int a = 10;   printf("%d", a);   return 0; }

Output:

10

Case 3: Include standard header file using both notation #include”” and #include<>, such as stdio.h

// stdio.h
int add(int a, int b)
{
    return(a + b);
}

• C

// C program to demonstrate // the difference between // ""  and <>   // This will include the standard // header file #include <stdio.h>   // This will include the user-defined // header file #include "stdio.h"   // Driver code int main() {   int a = 10;   int b = 20;       // Invoke the function defined in   // header file   int c = add(a, b);   printf("Result of addition is: %d", c);     return 0; }