c - Removing comments (/* and //) and strings - c

Question

Can anyone tell me whats wrong with the following code that's suppose to remove comments and strings from an input (but not comments that's why it recognizes comments)? This is related to a prior question of mine: Removing comments with a sliding window without nested while loops
#include <stdio.h>
int main()
{
int c, c1 = 0, c2 = 0 ,state = 0, next = 0;
while(1)
{
switch(state)
{
case 0: next = ((c2 == '*' && c1 == '/') ? 1 : (c2 == '\"') ? 2 : (c2 == '/' && c1 == '/') ? 3 : (c2 == '\'') ? 4: 0); break;
case 1: next = ((c2 == '/' && c1 == '*') ? 0 : 1); break;
case 2: next = ((c2 == '\"' && c1 != '\\') ? 0 : 2); break;
case 3: next = ((c2 == '\n') ? 0 : 3); break;
case 4: next = ((c2 == '\'' && c1 != '\\') ? 0 : 4); break;
default: next = state;
}
c = getchar(); if( c < 0) break;
c1 = c2; c2 = c; // slide window
if(state == 1)
{
if(c2 == '*')
{
c = getchar();
c1 = c2; c2 = c;
if(c2 != '/')
putchar(c1);
}
else
putchar(c2);
}
else if(state == 2)
{
if(c2 != '"' || (c2 == '\"' && c1 != '\\'))
putchar(c2);
}
else if(state == 3)
{
putchar(c2);
}
else
state = next;
// c2 is the current input byte and c1 is the previous input byte
}
return 0;
}

Answer 1

I don't think you actually need a sliding window for your task of removing C and C++ comments. You can expand your state machine to include a few additions states for tracking escapes, etc... With more states the code gets a bit bigger, but it might make it conceptually simpler since you only have one state to track. So converting the spirit of your code to the new state machine formula I'd suggest, you get the code below (and I also agree with Basile's suggestion of using enums and included it).
#include <stdio.h>
int main()
{
enum {
START, SLASH,
STRING, CHAR, STRING_ESCAPE, CHAR_ESCAPE,
SINGLE_LINE_COMMENT, MULTI_LINE_COMMENT, MULTI_LINE_END,
} state = START;
int c;
while ((c = getchar()) != EOF) {
switch (state) {
case START:
state_START:
if (c == '/') { state = SLASH; break; }
putchar(c);
if (c == '\"') state = STRING;
else if (c == '\'') state = CHAR;
break;
case SLASH:
if (c == '/') state = SINGLE_LINE_COMMENT;
else if (c == '*') state = MULTI_LINE_COMMENT;
else { state = START; goto state_START; }
break;
case STRING:
putchar(c);
if (c == '"') state = START;
else if (c == '\\') state = STRING_ESCAPE;
break;
case CHAR:
putchar(c);
if (c == '\'') state = START;
else if (c == '\\') state = CHAR_ESCAPE;
break;
case SINGLE_LINE_COMMENT:
if (c == '\n') state = START;
break;
case MULTI_LINE_COMMENT:
state_MULTI_LINE_COMMENT:
if (c == '*') state = MULTI_LINE_END;
break;
case STRING_ESCAPE:
putchar(c);
state = STRING;
break;
case CHAR_ESCAPE:
putchar(c);
state = CHAR;
break;
case MULTI_LINE_END:
if (c == '/') state = START;
else { state = MULTI_LINE_COMMENT; goto state_MULTI_LINE_COMMENT; }
break;
}
}
return 0;
}

Answer 2

just to give you an advice without knowing your purposes... Did you thought about regular expressions to solve your problem? It is may faster and your code will be cleaner, assuming you understand regular expressions.
I found a neat site for your problem by the way... It explains how to get those comments out of the code...
How get comments with regex
And here is a library for regex in C.