Return to Answer

/**
 * Returns the contents of the specified file as a NUL-terminated string.
 * If any error occurs (e.g. file not found, permission denied, no free memory),
 * returns NULL, and errno will be tell the reason.
 */
char *readSourceCode(const char *filename) {
 /* Hints: Use stat() or fstat() to find the file size.
 Allocate that many bytes, plus 1 for the '0円'.
 Remember to fclose() when done!
 */
 ...
}

/**
 * Returns the contents of the specified file as a NUL-terminated string.
 * If any error occurs (e.g. file not found, permission denied, no free memory),
 * returns NULL, and errno will tell the reason.
 */
char *readSourceCode(const char *filename) {
 /* Hints: Use stat() or fstat() to find the file size.
 Allocate that many bytes, plus 1 for the '0円'.
 Remember to fclose() when done!
 */
 ...
}

Suggestion

I'll try to provide some guidance without giving away a complete solution to your homework .

The key insight is that it is possible to construct an automaton that works at a higher level than character-by-character.

I'll start with main().

/* Advice: The C standard says that main() must return an int, not void. */
int main(int argc, char *argv[]) {
 /* Advice: Don't hard-code input filename. Accept a command-line parameter instead */
 if (argc < 2) {
 fprintf(stderr, "Usage: %s FILENAME\n", argv[0]);
 return 1;
 }
 const char *filename = argv[1];
 /* Advice: Avoid global variables. Return a pointer instead. */
 char *program = readSourceCode(filename);
 if (!program) {
 perror(filename);
 return 1;
 }
 const char *cursor = program;
 while (*cursor) {
 /* Advice: What does your automaton do? Pick an informative name. */
 switch (consumeKeyword(&cursor)) {
 /* Advice: Avoid magic numbers. #define constants or use an enum. */
 case CASE: printf("CASE\n"); break;
 case DO: printf("DO\n"); break;
 case ELSE: printf("ELSE\n"); break;
 case ELSEIF: printf("ELSE IF\n"); break;
 default:
 if (isIdentifier(&cursor)) {
 printf("IDENT\n"); break;
 } else {
 printf("Invalid! %s", cursor);
 }
 free(program);
 return 1;
 }
 }
 free(program);
 return 0;
}

The first thing to get out of the way is readSourceCode(). See if you can implement this:

/**
 * Returns the contents of the specified file as a NUL-terminated string.
 * If any error occurs (e.g. file not found, permission denied, no free memory),
 * returns NULL, and errno will be tell the reason.
 */
char *readSourceCode(const char *filename) {
 /* Hints: Use stat() or fstat() to find the file size.
 Allocate that many bytes, plus 1 for the '0円'.
 Remember to fclose() when done!
 */
 ...
}

The main challenge is consumeKeyword(). As you can see, main() passes a "cursor" to consumeKeyword() — it does so by reference, so that consumeKeyword() can increment main's cursor.

enum keyword {
 NOT_A_KEYWORD,
 ...
};
/**
 * Skips over any whitespace characters at the cursor, then looks for any
 * recognizable keywords. If a keyword is found, advances the cursor to
 * just beyond the end of the keyword. If no keyword is found, returns
 * NOT_A_KEYWORD and leaves the cursor in place.
 */
enum keyword consumeKeyword(const char **cursor) {
 while (isspace(**cursor)) {
 (*cursor)++;
 }
 size_t advance;
 if ((advance = isKeyword(cursor, "case"))) {
 *cursor += advance;
 return CASE;
 } else if ... {
 ...
 } else {
 return NOT_A_KEYWORD;
 }
}

You should be able to fill in the rest:

/**
 * If the specified keyword is found at the specified cursor position,
 * returns the length of the keyword. Otherwise, returns 0.
 */
size_t isKeyword(const char *const *cursor, const char *keyword) {
 /* Hint: With a for-loop, the entire function should take no more
 than four lines. */
}