This is my second attempt at K&R 1-23,

Write a program to remove all comments from a C program. Don't forget to handle quoted strings and character constants properly. C comments don't nest.

I was previously packing characters into various io buffers, and someone suggested this was not a good choice. Thought I'd try something more along the lines of a state machine:

#include <stdio.h>

#define NORMAL          0
#define SINGLE_QUOTE    1
#define DOUBLE_QUOTE    2
#define SLASH           3
#define MULTI_COMMENT   4
#define INLINE_COMMENT  5
#define STAR            6

int state_from_normal(char prev_symbol, char symbol)
{
    int state = NORMAL;

    if (symbol == '\'' && prev_symbol != '\\') {
        state = SINGLE_QUOTE;
    } else if (symbol == '"') {
        state = DOUBLE_QUOTE;
    } else if (symbol == '/') {
        state = SLASH;
    }
    return state;
}

int state_from_single_quote(char prev_symbol, char symbol)
{
    int state = SINGLE_QUOTE;

    if (symbol == '\'' && prev_symbol != '\\') {
        state = NORMAL;
    }
    return state;
}

int state_from_double_quote(char prev_symbol, char symbol)
{
    int state = DOUBLE_QUOTE;

    if (symbol == '"' && prev_symbol != '\\') {
        state = NORMAL;
    }
    return state;
}

int state_from_slash(char symbol)
{
    int state = SLASH;

    if (symbol == '*') {
        state = MULTI_COMMENT;
    } else if (symbol == '/') {
        state = INLINE_COMMENT;
    } else {
        state = NORMAL;
    }
    return state;
}

int state_from_multi_comment(char symbol)
{
    int state = MULTI_COMMENT;

    if (symbol == '*') {
        state = STAR;
    }
    return state;
}

int state_from_star(char symbol)
{
    int state = STAR;

    if (symbol == '/') {
        state = NORMAL;
    } else {
        state = MULTI_COMMENT;
    }
    return state;
}

int state_from_inline_comment(char symbol)
{
    int state = INLINE_COMMENT;

    if (symbol == '\n') {
        state = NORMAL;
    }
    return state;
}

int state_from(int prev_state, char prev_symbol, char symbol)
{
    switch(prev_state) {
        case NORMAL :
            return state_from_normal(prev_symbol, symbol);
        case SINGLE_QUOTE :
            return state_from_single_quote(prev_symbol, symbol);
        case DOUBLE_QUOTE :
            return state_from_double_quote(prev_symbol, symbol);
        case SLASH :
            return state_from_slash(symbol);
        case MULTI_COMMENT :
            return state_from_multi_comment(symbol);
        case INLINE_COMMENT :
            return state_from_inline_comment(symbol);
        case STAR :
            return state_from_star(symbol);
        default :
            return -1;
    }
}

int main(void)
{
    char input;
    char symbol = '\0';
    char prev_symbol;

    int state = NORMAL;
    int prev_state;

    while ((input = getchar()) != EOF) {
        prev_symbol = symbol;
        prev_state  = state;
        symbol = input;
        state  = state_from(prev_state, prev_symbol, symbol);

        if (prev_state == SLASH && state == NORMAL) {
            putchar(prev_symbol);
        }
        if (prev_state != STAR && state < SLASH) {
            putchar(symbol);
        }
    }
}