Creating library functions in C Language

User defined functions in C language:

As you know, there are 2 types of functions in C. They are, library functions and user defined functions.
Library functions are inbuilt functions which are available in common place called C library. Where as, User defined functions are the functions which are written by us for our own requirement.

Adding user defined functions in C library:

Do you know that we can add our own user defined functions in C library?
Yes. It is possible to add, delete, modify and access our own user defined function to or from C library.

The advantage of adding user defined function in C library is, this function will be available for all C programs once added to the C library.
We can use this function in any C program as we use other C library functions.
In latest version of GCC compilers, compilation time can be saved since these functions are available in library in the compiled form.
Normal header files are saved as ”file_name.h” in which all library functions are available. These header files contain source code and this source code is added in main C program file where we add this header file using “#include <file_name.h>” command.
Where as, precompiled version of header files are saved as “file_name.gch”.

Steps for adding our own functions in C library:

Step 1:

For example, below is a sample function that is going to be added in the C library. Write the below function in a file and save it as “addition.c”

addition(int i, int j)
{
int total;
total = i + j;
return total;
}

Step 2:

Compile “addition.c” file by using Alt + F9 keys.

step 3:

addition.obj file would be created which is the compiled form of addition.c file.

Step 4:

Use the below command to add this function to library.
c:\> tlib math.lib + c:\ addition.obj
+ means adding c:\addition.obj file in the math library.
We can delete this file using – (minus).

Step 5:

Create a file “addition.h” & declare prototype of addition() function like below.
int addition (int i, int j);
Now addition.h file containing prototype of function “addition”.

Step 6:

Let us see how to use our newly added library function in a C program.

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# include <stdio.h>

// Including our user defined function.

# include “c:\\addition.h”     

int main ()

{

       int total;

       // calling function from library

       total = addition (10, 20); 

       printf ("Total = %d \n", total);

}

Output:

Total = 30

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