What is function in C programming?

Definition of function:

Function is block or part of program. When any program is very long or same code is repeating many times then we try to cut the program in different parts (or blocks) so that whole program became more understandable, easier to debug (error checking) and size of code will be lesser.

Syntax of function in c programming

Simple example of function structure

int sum (int,int); //function declaration

int main(){

int p;

p=sum(3,4); //function call

printf(“%d”,sum);
return 0;

}

int sum( int a,int b){ //function definition

int s; //function body

s=a+b;

return s; //function returning a value

}

Detail explanation of syntax of function

(1) Function_name :
Function naming rule in c programming:


Rule 1. Name of function includes alphabets, digit and underscore.
Valid name: world, addition23, sum_of_number etc.
Invalid name: factorial#, avg value, display*number etc.
More example click here

Rule 2. First character of name of any function must be either alphabets or underscore.

Valid name: _calulate, _5,a_, __ etc.
Invalid name: 5_, 10^_function, 123 etc.
More example click here

Rule 3. Name of function cannot be any keyword of c program.

Invalid name: interrupt, float, asm, enum etc.
More example click here


Rule 4. Name of function cannot be global
identifier.
Valid name: __TOTAL__, __NAME__ ,__TINY__etc.
Invalid name: __TIME__,__DATE__, __FILE__,__LINE__,__STDC__
More example click here

Note: It is good practice to not write the variable name in the above format.

Rule 5: Name of function cannot be register Pseudo variables

Register Pseudo variables are:

_AX _AL _AH _SI _ES

_BX _BL _BH _DI _SS

_CX _CL _CH _BP _CS

_DX _DL _DH _SP _DS

_FLAGS

For examples click here.


Rule 6. 

Name of function cannot be exactly same as of name of other function or identifier within the scope of the function.

Good example click here


Rule 7. Name of function is case sensitive.
Example

Rule 8. Only first 32 characters are significant of the function’s name.

Example:

abcdefghijklmnopqrstuvwxyz123456aaa,

abcdefghijklmnopqrstuvwxyz123456bbb,

abcdefghijklmnopqrstuvwxyz123456cad

All three function name are same because only first 32 characters has meaning. Rest has not any importance.