## Pre-defined function

Library function:
• C language provides built-in functions called library function.
• The C standard library consists of wide verity of header files which can be included into a programmer’s project with a single directive.
• Each header file contains one or more function declarations, data type definition, and macros.
• The following library functions are,
1. sqrt(x):
This function evaluates the square root of ‘x’. Note that the argument must be non-negative.
Example: sqrt(25)
Here the square root of 25 evaluated as 5.
2. log(x):
Where the argument ‘x’ must be real. Here loge x is evaluated.
Example:   log(7.5-x*y) this evaluate loge(7.5-xy)
3. exp(x):
This function evaluates ex. The argument must be of real quantity.
Example:   exp(7.5) this evaluate e7.5
4. pow(a,b):
Where the arguments ‘a’ and ‘b’ are maybe integer or float values.
This evaluates the value of ab.
Example:   pow(4,2) this evaluate (4)2.
5. ceil(x):
It is used to rounding x to the next integer value.
Example.    Ceil(5.6) evaluates 6
6. fmod(x,y):
It reterurn the remainder of x/y.
Example:   fmod(7,3) evaluates 1, because, when 7 is divided by 3 the remainder is 1.
7. sin(x):
This evaluates the trigonometric sine value.
Example:   sin(30) evaluate 0.5
8. cos(x):
This evaluates the trigonometric cosine value.
Example:    cos(30) evaluate 0.75
9. tan(x):
This evaluate the trigonometric tangent(  )value.
Example:    tan(30) evaluate 0.577
10. toascii(x):
Returns the integer value for the articular character.
Example:    toascii(a)     evaluates 97
11. tolower(x):
It is used to convert the character capital letter into a small letter.
Example:    tolower(‘Z’) evaluates z
12. toupper(x):
It is used to convert the character small letter into capital letter.
Example:    toupper(‘z’) evaluates Z.
13. ctime(t):
It is used to show the current time in your system.
Using #include<time.h> header file and time_t datatype.
Example program for library function:

Output: