Adding user defined functions in C Library

Adding User-Defined Functions in C Library

In this article, I will discuss How to Add User-Defined Functions in C Library with an example. Please read our previous articles, where we discussed the Block Scope in C Language.

How do you Add User-Defined Functions in C Library?

Adding user-defined functions to the C standard library isn’t something you can do directly, as the C standard library is a set of pre-compiled functions provided by the compiler and its associated runtime. However, you can create your own library in C, which can be used alongside the standard library. Here’s how you can do it:

  • Create a Header File: This file will contain the declarations of your functions. It usually has a .h extension.
  • Implement the Functions: Write the actual code for your functions in a separate source file, usually with a .c extension.
  • Compile Your Library: Compile your source file into an object file.
  • Use Your Library: Include your header file in your projects and link the object file when compiling your projects.
Steps to Add User-Defined Functions in C Library:
Write Your Function:

First, you need to write the function you want to add. This involves writing the function definition in a C source file (.c file). For example:

// mylib.c
#include "mylib.h"

int my_function(int a, int b) {
    return a + b;
Create a Header File:

A header file (.h file) is used to declare the functions and data types your library provides. This makes it easier to include your functions in other programs.

// mylib.h
#ifndef MYLIB_H
#define MYLIB_H

int my_function(int a, int b);

Compile Your Library:

You can compile your library as a static or dynamic library. A static library is a collection of object files (.o files), and a dynamic library is a single shared object file (.so on Unix-like systems or .dll on Windows).

For a static library:
gcc -c mylib.c -o mylib.o
ar rcs libmylib.a mylib.o

For a dynamic library:
gcc -shared -o mylib.c

Use Your Library in Other Programs:

To use your library in another program, include the header file and link against the library.

In your C source file:

#include "mylib.h"

int main() {
    int result = my_function(5, 3);
    // use result

When compiling the program, link it with your library:

For a static library:
gcc -o my_program my_program.c -L. -lmylib

For a dynamic library, make sure the library is in a directory known to the linker or update the LD_LIBRARY_PATH environment variable.

Distribute Your Library: You can distribute your header file and the compiled library file to others, who can then use your functions in their programs.

Example to Add User-Defined Functions in C Library:
Define Your Functions

First, write your functions in C. For example, let’s create a simple function to add two numbers:

int add(int a, int b) {
    return a + b;
Create a Header File

Next, create a header file (.h) for your functions. This file declares the functions you’ve implemented. For the above function, the header file (myfunctions.h) would look like this:


int add(int a, int b);

Implement Your Functions in a Source File

Now, implement your functions in a separate source file (.c). Include the header file at the top:

#include "myfunctions.h"

int add(int a, int b) {
    return a + b;
Compile Your Library

You can compile your source file into an object file. For GCC, the command would be:
gcc -c myfunctions.c -o myfunctions.o

Create a Static or Shared Library

You can create a static library (.a) or a shared library (.so or .dll, depending on the OS) from the object file.
For a static library: ar rcs libmyfunctions.a myfunctions.o
For a shared library: gcc -shared -o myfunctions.o

Use Your Library in a Program

To use your library, include the header file in your C program and link the library when compiling:

#include "myfunctions.h"

int main() {
    int result = add(5, 3);
    return 0;

Compile the program with your library: gcc main.c -L. -lmyfunctions -o main
(Ensure your library path is correctly specified with -L and the library name with -l.)

Example Program Using the Custom Library

Here’s a simple example demonstrating how to use the add function from the custom library:

#include <stdio.h>
#include "myfunctions.h"

int main() {
    int sum = add(10, 20);
    printf("Sum: %d\n", sum);
    return 0;

Note: These functions are not part of the C standard library but act as a custom library you can use in your projects.

When should we add user-defined functions in the C Library?

Deciding when to add user-defined functions to a custom C library (since you cannot modify the standard C library itself) typically revolves around a few key considerations:

  • Reusability: If you find yourself writing the same function in multiple programs or projects, it indicates that this function could be a candidate for your custom library. This helps reuse code without having to rewrite or copy-paste it each time.
  • Complexity: Complex functions that perform specific, intricate tasks are good candidates for inclusion in a custom library. This is because encapsulating complex operations in a function can simplify your main program code, making it more readable and maintainable.
  • Modularity: When you’re working on larger projects, breaking down the code into smaller, modular functions that can be easily managed and tested individually is a best practice. If these functions have potential use in other projects, adding them to a library is beneficial.
  • Abstraction: If a function provides a layer of abstraction, hiding the complexity of a task (like intricate file operations, complex calculations, etc.), it’s a good candidate for a library. This allows library users to perform complex tasks without needing to understand the underlying details.
  • Specialization: Functions that perform specialized tasks, which might not be available in the standard library or third-party libraries, are ideal for a custom library. For example, domain-specific calculations in science, engineering, or finance.
  • Performance Optimizations: If you have developed a version of a function that is more efficient than commonly used methods, it may be beneficial to include it in a custom library for ease of access and efficiency in future projects.
  • Cross-Project Consistency: If you are working on multiple projects with similar functionalities, having a custom library ensures consistency across these projects. It helps in maintaining a standard approach and reduces errors.

In the next article, I will discuss Recursion in C Language with Examples. In this article, I try to explain how to add user-defined functions to C Library. I hope you enjoy this “adding user-defined functions to the C Library” article. I would like to have your feedback. Please post your feedback, questions, or comments about this article.

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