Some Useful library functions

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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.