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:
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)
total = i + j;
Compile “addition.c” file by using Alt + F9 keys (in turbo C).
“addition.obj” file would be created which is the compiled form of “addition.c” file.
Use the below command to add this function to library (in turbo C).
c:\> tlib math.lib + c:\ addition.obj
+ means adding c:\addition.obj file in the math library.
We can delete this file using – (minus).
Create a file “addition.h” & declare prototype of addition() function like below.
int addition (int i, int j);
Now, addition.h file contains prototype of the function “addition”.
Note : Please create, compile and add files in the respective directory as directory name may change for each IDE.
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 ()
// calling function from library
total = addition (10, 20);
printf ("Total = %d \n", total);
Total = 30
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