What is Directive, Character Set and trigraph characters?

character sets:

when you write a program , you express c source file as text lines containing characters from the source character set . when a program execute in the target enviroment ,it uses character set .these character sets are related , but need not have the same encoding or all the same members.

trigraphs:

a trigraphs is a sequence of three characters that begins with tow question marks (??).you use trigraphs to write c sources files with a character set that dose not contain convenient graphic representation for some punctuation characters . (the resultant c source file is not necessarily more readable , but it is unambiguous.)