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The smallest parts of program which are recognized as atomic unit by compiler during parsing are tokens. All a C program contain is bunch of tokens. Each and Individual words and punctuation marks are called tokens which are also called lexical units. Similarly, the smallest individual unit in a c program is known as a token or a lexical unit. C tokens can be of following types:

1. Keywords
2. Identifiers
3. Operators
4. Constants
5. Special Symbols(punctuators)
6. Strings

Every thing in a c program are tokens. Lets do anatomy of following code.

int fact(int n){
if(n <=1){
return 1;
}
else {
return n * fact(n-1);
}

• int, if, else, return :- Keywords
• fact, n :- Identifiers
• (, {, ;, ), } :- symbol, punctuation
• <=, *, – :- Operators
• 1 :- Constant

In this way compiler recognize each token from a program.