parse_int.c

#include <errno.h>
#include <limits.h>
#include <stddef.h>
#include <stdint.h>

int
parse_signed(const char src[static 1], intmax_t min, intmax_t max,
		intmax_t dest[static 1], char **endptr) {
	int sign;
	intmax_t value;
	int got_sign;
	int got_digit;
	const char *p;
	int digit;

	if (min > max || min > INTMAX_MAX - 9 || max < INTMAX_MIN + 9) {
		errno = EINVAL;
		return 0;
	}

	sign = 1;
	value = 0;
	got_sign = 0;
	got_digit = 0;

	for (p = src; *p != '\0'; p++) {
		if (*p >= '0' && *p <= '9') {
			digit = *p - '0';
			got_digit = 1;

			if ((sign == 1 && value > (max - digit) / 10) ||
					(sign == -1 && value < (min + digit) / 10)) {
				errno = ERANGE;
				return 0;
			}

			value *= 10;
			value += sign * digit;
		}
		else if (!got_digit && !got_sign && (*p == '-' || *p == '+')) {
			if (*p == '-')
				sign = -1;
			got_sign = 1;
		}
		else {
			break;
		}
	}

	if (!got_digit)
		return 0;

	*dest = value;

	if (endptr != NULL)
		*endptr = (char *)p;

	return 1;
}

int
parse_unsigned(const char src[static 1], uintmax_t min, uintmax_t max,
		uintmax_t dest[static 1], char **endptr) {
	uintmax_t value;
	int got_digit;
	const char *p;
	int digit;

	if (min > max || min > INTMAX_MAX - 9 || max < 9) {
		errno = EINVAL;
		return 0;
	}

	value = 0;
	got_digit = 0;

	for (p = src; *p != '\0'; p++) {
		if (*p >= '0' && *p <= '9') {
			digit = *p - '0';
			got_digit = 1;

			if (value > (max - digit) / 10) {
				errno = ERANGE;
				return 0;
			}

			value *= 10;
			value += digit;
		}
		else {
			break;
		}
	}

	if (!got_digit)
		return 0;

	if (value < min) {
		errno = ERANGE;
		return 0;
	}

	*dest = value;

	if (endptr != NULL)
		*endptr = (char *)p;

	return 1;
}

read_int.c

#include <stdio.h>
#include <limits.h>
#include <errno.h>

int
read_int(FILE* src, int dest[static 1]) {
	int sign;
	int value;
	int got_sign;
	int got_digit;
	int c;
	int digit;

	sign = 1;
	value = 0;
	got_sign = 0;
	got_digit = 0;

	while ((c = fgetc(src)) != EOF) {
		if (c >= '0' && c <= '9') {
			digit = c - '0';
			got_digit = 1;

			if ((sign == -1 && value < (INT_MIN + digit) / 10) ||
					(sign == 1 && value > (INT_MAX - digit) / 10)) {
				errno = ERANGE;
				return 0;
			}

			value *= 10;
			value += sign * digit;
		}
		else if (!got_digit && !got_sign && (c == '-' || c == '+')) {
			if (c == '-')
				sign = -1;
			got_sign = 1;
		}
		else {
			ungetc(c, src);
			break;
		}
	}

	if (ferror(src))
		return 0;

	*dest = value;

	return 1;
}