Library Functions:


There are certain set of general purpose operations which are quite frequently used by the programmers in their programs. For example: To calculate the square root of a number, to calculate power of a number, to calculate the length of the string, to perform input/output and many more. Making functions for performing these operations in every program is unnecessary and a time consuming job. So these general purpose operations are programmed and stored in C library so that they can be called through any program in the form of functions. These functions are the library functions.
            
The library functions are predefined and precompiled functions that are designed to perform some specific task. Information is passed to these functions via arguments and return via the return statement. Before using library function in the program, it is necessary to include the corresponding header file using #include directive. The header file contains the information about library functions (function prototype) and actual coding (function definition) is available in the object code form in the library file.
There are a number of library functions available in C. Some of the library functions and their corresponding header files are given below.


a.      Library functions that carry out standard input/output operations. Some of these include scanf ( ), printf ( ), getchar ( ), fclose ( ), fopen ( ) etc. These functions are included in stdio.h header file.


Beside these, some other library functions that are part of stdio.h library file are fclose ( ), feof ( ), fgetc ( ), fopen ( ), fopen ( ), fprintf ( ), fputs ( ), fread ( ).

b.     Library functions that carry out operations on argument of type character. Some of these include tolower ( ), toupper ( ) etc. Theseare collectively grouped under ctype.h header file.

Some other library functions available in this header file are isalnum ( ), iscntrl ( ), isgraph ( ), islower ( ), islower ( ), isodigit ( ). All these functions return non-zero value if true, 0 otherwise.

c.      Library functions that perform operations on strings: Some of these include strcmp ( ), strcpy ( ), strlen ( ) etc. These are collectively grouped in string.h header file.



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