An example of the parsing component of a compiler for reference.

tokenizer.c

#include <stdio.h>
#include <stdlib.h>

/* constants */
#define NUM 257

/* global */
int currentToken;
int currentAttribute;

/* Prototypes */
/* The error routine is void, so it needs a prototype */
void error( char* );
void match( int );

/* MAIN
 * This is the initial implementation of a tokenizer
 * used when designing and implementing a compiler.  It
 * tokenizes input from stdin (lexical analysis), and parses
 * it to determines if the input is syntactically correct
 * (grammar analysis).  It then, evaluta
 */
main () {
  int c, value;
  
  /* initialized currentToken */
  currentToken = getToken();
  
  /* call parser */
  value = expr();
  
  /* output value */
  fprintf(stderr, "\n\nValue = %d\n", value);
}

/* 
 * expr -> term expr'
 * expr' -> '+' term expr' | empty
 */
int expr() { 
 int value = term();
 while(1) {
 	
 	if ( currentToken == '+' ) {
 		match('+');
 		value += term(); /* this gives meaning to the input '+' symbol */
 	} else return value;
 }
}

/* term -> factor term'
 * term' -> '*' factor term' | empty
 */
int term() { 
 int value = factor();
 while(1) {
 	
 	if ( currentToken == '*' ) {
 		match('*');
 		value *= factor();
 	} else return value;
 }
}

/* 
 * factor -> '(' expr ')' | NUM
 */
int factor() {

int value;
if ( currentToken == '(') {
	match('(');
	value = expr();
	match(')');
	return value;
} else if ( currentToken == NUM ) {
	value = currentAttribute;
	match(NUM);
	return value;
} else error("Factor: expecting ( or NUM");
}  

/* tokenizer or scanner:
* returns the next token
*/
int getToken() {
 int c, value;
 
 while ( (c = getchar()) != EOF ) { // EOF is an integer, so you need an int to catch it

 	switch(c) {
 		case '+':
 		case '*':
 		case '(':
 		case ')':
 			fprintf(stderr, "[%c]", c);
 			return c;	/* return operators and brackets as is */
 		case ' ':
 		case '\t':
 		case '\n':
 			fprintf(stderr, "%c", c);
 			continue;
 		default:
 			if ( isdigit(c) ) {
 				value = 0;
 				do {
 					value = value*10 + (c - '0'); /* used for converting ASCII to int) */
 				} while(isdigit( c = getchar()) );
 				ungetc(c, stdin);
 				currentAttribute = value;
 				fprintf(stderr, "[NUM:%d]", value);
 				return NUM;
 			} else {
 				error("getToken: unknown character");
 			}
 	}
 }
 return c;
}

void match(int expectedToken) { 
 if ( currentToken == expectedToken ) {
 	currentToken = getToken();
 } else {
 	error("Match: unexpected token");
 }
}

void error( char* message ) { 
 fprintf(stderr, "Error: %s\n", message);
 exit(1);
}