part-4

Part 4: Fixing some bugs

Row

On the definition of a Row, we allocate exactly 32 and 255 bytes for username and email respectively. The thing is that strings in C are supposed to end with a null character, which at the moment we do not have space for. So we have to allocate one additional byte:

#define COLUMN_USERNAME_SIZE 32
#define COLUMN_EMAIL_SIZE 255
typedef struct {
	uint32_t id;
	char username[COLUMN_USERNAME_SIZE + 1];
	char email[COLUMN_EMAIL_SIZE + 1];
} Row;

Insert

At the moment we are allowing the insertion of usernames or emails that are longer than the column size. To correct this our parser is needed to be upgraded (prepare_statement). The problem is that the current definition is using scanf(), which has some disadvantages. If the string it’s reading is larger than the buffer it’s reading into, it will cause a buffer overflow and start writing into unexpected places. In a better definition we want to check the length of each string before we copy it into a Row structure. To do that we need to divide the input by spaces.

PrepareResult preprare_insert(InputBuffer *input_buffer, Statement *statement) {
	statement->type = STATEMENT_INSERT;
 
	char *keyword = strtok(input_buffer->buffer, " ");
	char *id_string = strtok(NULL, " ");
	char *username = strtok(NULL, " ");
	char *email = strtok(NULL, " ");
 
	if (id_string == NULL || username == NULL || email == NULL)
		return PREPARE_SYNTAX_ERROR;
 
	int id = atoi(id_string);
	if (strlen(username) > COLUMN_USERNAME_SIZE)
		return PREPARE_STRING_TOO_LONG;
 
	if (strlen(username) > COLUMN_EMAIL_SIZE)
		return PREPARE_STRING_TOO_LONG;
 
	statement->row_to_insert.id = id;
	strcpy(statement->row_to_insert.username, username);
	strcpy(statement->row_to_insert.email, email);
 
	return PREPARE_SUCCESS;
}

Here we use strtok on the input buffer to break it into the substrings by inserting a null character whenever it reaches a delimiter (in this case the space) and then the function returns a pointer to the start of the substring. After that we call strlen() on each text value to see if it’s too long.

Negative id

Now we gotta handle the case in which the user provides a negative a negative id. That is simple.

typedef enum {
	PREPARE_SUCCESS,
	PREPARE_NEGATIVE_ID,
	PREPARE_STRING_TOO_LONG,
	PREPARE_SYNTAX_ERROR,
	PREPARE_UNRECOGNIZED_STATEMENT
} PrepareResult;

PrepareResult preprare_insert(InputBuffer *input_buffer, Statement *statement) {
	statement->type = STATEMENT_INSERT;
 
	char *keyword = strtok(input_buffer->buffer, " ");
	char *id_string = strtok(NULL, " ");
	char *username = strtok(NULL, " ");
	char *email = strtok(NULL, " ");
 
	if (id_string == NULL || username == NULL || email == NULL)
		return PREPARE_SYNTAX_ERROR;
 
	int id = atoi(id_string);
	if (id < 0)
		return PREPARE_NEGATIVE_ID;
	if (strlen(username) > COLUMN_USERNAME_SIZE)
		return PREPARE_STRING_TOO_LONG;
 
	if (strlen(username) > COLUMN_EMAIL_SIZE)
		return PREPARE_STRING_TOO_LONG;
 
	statement->row_to_insert.id = id;
	strcpy(statement->row_to_insert.username, username);
	strcpy(statement->row_to_insert.email, email);
 
	return PREPARE_SUCCESS;
}

int main(int argc, char *argv[]) {
	Table *table = new_table();
	InputBuffer *input_buffer = new_input_buffer();
	while (true) {
		print_prompt();
		read_input(input_buffer);
		if (input_buffer->buffer[0] == '.') {
			switch (do_meta_command(input_buffer, table)) {
			case (META_COMMAND_SUCCESS):
				continue;
			case (META_COMMAND_UNRECOGNIZED_COMMAND):
				printf("Unrecognized command '%s'\n", input_buffer->buffer);
				continue;
			}
		}
		Statement statement;
		switch (prepare_statement(input_buffer, &statement)) {
		case (PREPARE_SUCCESS):
			break;
		case (PREPARE_NEGATIVE_ID):
			printf("ID must be positive.\n");
			continue;
		case (PREPARE_STRING_TOO_LONG):
			printf("String is too long.\n");
			continue;
		case (PREPARE_SYNTAX_ERROR):
			printf("Syntax error. Could not parse statement.\n");
			continue;
		case (PREPARE_UNRECOGNIZED_STATEMENT):
			printf("Unrecognized keyword at start of '%s'.\n", input_buffer->buffer);
			continue;
		}
		execute_statement(&statement, table);
		switch (execute_statement(&statement, table)) {
		case (EXECUTE_SUCCESS):
			printf("Executed.\n");
			break;
		case (EXECUTE_TABLE_FULL):
			printf("Error: Table full.\n");
			break;
		}
	}
}