Build your own C library

Valerio Chiodi — Wed, 27 Oct 2021 14:22:31 +0000

Hi everyone, this is my first article.

If i make any grammar mistakes, let me know.
I am not a native english speaker

Today I want to talk to you about "personal C library".
Often functions written by us are very useful and will likely need to be reused in new programs.
You can then group these functions into a useful library to facilitate their reuse.

First let's start with a simple program that we all know. HELLO WORLD!

#include <stdio.h>

void main() /*standard library */
{
    printf("HELLO WORLD!"); /*output displayed*/
    getchar(); /*Press any key to continue.*/
}

I used Visual Studio Code as a text editor, but you can use whatever you want.

So far everything ok.
But if I wanted to put my function in a personal library to be able to reuse it, how do I do it?

Each library consists of two parts: a header file and the file that contains the code.
The header file, which is denoted with a suffix .h
The header file contains constants and types, along with prototypes for the functions available in the library.

Header file

First let's write our header file and save it in a file called mylib.h

/* mylib.h */
extern void hello();

This line of code is a function prototype.
The word extern in C represents functions that will be linked at a later time

Now let's write our code in a file called mylib.c

/* mylib.c */
#include <stdio.h>
#include "mylib.h"

void hello()
{
    printf("HELLO WORLD! with my personal library ");
}

Note that the file includes its own header file (mylib.h) and this is enclosed in double quotes instead of the major and minor symbols.
The latter being used only for system libraries.

It's time to create the main program!

Then write the following program into a file and call it main.c

/* main.c */
#include <stdio.h>
#include "mylib.h"

void main()
{
    hello();
    getchar();

    printf("HELLO PROGRAMMER ! this greeting does not use your personal library ");
    getchar();
}

This code includes the utility (mylib.h) library we created.

FILL IN AND RUN THE CODE WITH A LIBRARY

If you have an IDE to program in C, you don't need to know how to compile, it will do it for you.

So, run the program and watch the magic.

Otherwise if you want to create an executable, for example if you use LINUX and you have a gcc compiler installed, here are the steps you need to do.

Open the terminal in the folder containing the three files:

mylib.h
mylib.c
main.c

and type:

gcc -c -g mylib.c
gcc -c -g main.c

This line creates a file called mylib.o and main.o which contains the machine code for the main program.
The -c option causes the compiler to produce an object file for the library

Now we need to link the two object files by typing the following command.

gcc -o myprogramname main.o mylib.o

This command links main.o with mylib.o to form an executable called myprogramname. To run the program, type myprogramname on the command line.

Or if you also use Visual Studio Code, like me, an .exe will be created in the folder have you used.

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