C – Creating library functions

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User defined functions in C:

  • 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.

# 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);


Total = 30


Continue on C – Command line arguments….

Continue on C – Variable length arguments….

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