C Tutorial - Header File and Library Functions Uses

Header file

Introduction

The C provides a large number of C functions as libraries. Some of these implement frequently used operations, while others are very specialized in their application.

Wise programmers will check whether a library function is available to perform a task before writing their own version.

This will reduce program development time. The library functions have been tested, so they are more likely to be correct than any function which the programmer might write.

This will save time when debugging the program. For using these files ,certain files are needed to be included in the program which make call to these functions.

These files are known as Header files and they contain macro definition ,type definition, and function declarations.

These header files usually have an extension .h as stdio.h, ctype.h, string.h, math.h, stdlib.h, stdarg.h, time.h etc.

Use of library functions

To use a function, ensure that you have made the required #includes in your C file. Then the function can be called as though you had defined it yourself.

It is important to ensure that your arguments have the expected types; otherwise the function will probably produce strange results.

Some libraries require extra options before the compiler can support their use.

For example, to compile a program including functions from the math.h library the command might be cc mathprog.c -o mathprog –lm

The final -lm is an instruction to link the maths library with the program. The manual page for each function will usually inform you if any special compiler flags are required.

Some Useful library functions

There is a vast collections of function .Some of them are grouped together and listed below.

String Function:

strcpy copies one string into another. strcat appends one string to another. strcmp compare one string to another. strcmpi compare one string to another without case sensitive. strlen calculates the length of a string. strrev reserve a string.

Mathematical Function:

abs returns absolute value of an integer. sin calculate the sine. cos calculate the arc cos. tan calculate the arc tangent. acos calculate the arc cosine. asin calculate the arc sine. atan calculate the arc tangent. ceil rounds up. floor rounds down. log calculate the natural logarithm of x. pow calculate x to the power of y. sqrt calculate the positive square root of input value.

Date & Time  Function:

asctime converts date and time to ASCII. clock determine process time. getdate gets system date gettime gets system time. setdate sets DOS date settime sets system time. time gets time of day.

Utility Function:

abort() abnormally terminates a process. bsearch() binary search of an array. tolower() translate character to lower case. toupper() translate character to upper case. qsort() sorting using the quick sort algorithm. exit() terminate execution of a program. free() frees allocated block.

Character Class Test Functions:

isupper() check and returns non-zero if c is an upper case letter (A-Z). islower() check and returns non-zero if c is a lower case letter (a-z). isspace(() check and returns non-zero if c is a space tab, carriage return, newline, vertical tab, form feed etc. isascii() tests whether a character is an ASCII (0 to 127) character. isalpha() check and returns non-zero if c is a letter (A-Z or a-z). iscntrl() tests whether a character is a control character. toascii() translate character to ASCII format. tolower() translate character to upper case.

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