c语言htoi() --将16进制数组成的字符串转换成整型

k&r习题，将16进制数(可选0x或0X前缀）组成的字符串转换成等价的整型,写了一个：

/* htoi.c
 * transform hex to decimal
 *
 * compile with my_getline.c
 */

#include<stdio.h>
#define MAXLEN 20

int my_getline(char s[], int lim);
int htoi(char s[]);
int main()
{
	char num[MAXLEN];
	int new_num;
	int len;
	while ((len = my_getline(num, MAXLEN)) > 0) {
		new_num = htoi(num);
		if (new_num != -1 )
			printf("%d\n", new_num);
		else
			printf("not a hexidecimal\n");

	}
	return 0;
}

int htoi(char s[])
{
	int n = 0;
	int i = 0;
	while (s[i] != '\0' && s[i] != '\n') {
		if (s[i] == '0') {
			if (s[i+1] == 'x' || s[i+1] == 'X')
                            i+=2;
		}
		if (s[i] >= '0' && s[i] <= '9') {
			n = n * 16 + (s[i] - '0');
		} else if (s[i] >= 'a' && s[i] <= 'f') {
			n = n * 16 + (s[i] - 'a') + 10;
		} else if (s[i] >= 'A' && s[i] <= 'F') {
			n = n * 16 + (s[i] - 'A') + 10;
		} else
			return -1;
		++i;

	}
	return n;
}

需要与my_getline.c 一块编译：gcc htoi.c my_getline.c -o htoi

/* my_getline.c
 */
#include <stdio.h>

int my_getline(char s[], int lim)
{
	int c, i;

	for (i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n';
	     ++i)
		s[i] = c;
	if (c == '\n') {
		s[i] = c;
		++i;
	}
	s[i] = '\0';
	return i;
}