source: bootcd/isolinux/syslinux-6.03/com32/lib/strntoumax.c

/*
 * strntoumax.c
 *
 * The strntoumax() function and associated
 */

#include <stddef.h>
#include <stdint.h>
#include <ctype.h>

static inline int digitval(int ch)
{
    if (ch >= '0' && ch <= '9') {
        return ch - '0';
    } else if (ch >= 'A' && ch <= 'Z') {
        return ch - 'A' + 10;
    } else if (ch >= 'a' && ch <= 'z') {
        return ch - 'a' + 10;
    } else {
        return -1;
    }
}

uintmax_t strntoumax(const char *nptr, char **endptr, int base, size_t n)
{
    int minus = 0;
    uintmax_t v = 0;
    int d;

    while (n && isspace((unsigned char)*nptr)) {
        nptr++;
        n--;
    }

    /* Single optional + or - */
    if (n && *nptr == '-') {
        minus = 1;
        nptr++;
        n--;
    } else if (n && *nptr == '+') {
        nptr++;
    }

    if (base == 0) {
        if (n >= 2 && nptr[0] == '0' && (nptr[1] == 'x' || nptr[1] == 'X')) {
            n -= 2;
            nptr += 2;
            base = 16;
        } else if (n >= 1 && nptr[0] == '0') {
            n--;
            nptr++;
            base = 8;
        } else {
            base = 10;
        }
    } else if (base == 16) {
        if (n >= 2 && nptr[0] == '0' && (nptr[1] == 'x' || nptr[1] == 'X')) {
            n -= 2;
            nptr += 2;
        }
    }

    while (n && (d = digitval(*nptr)) >= 0 && d < base) {
        v = v * base + d;
        n--;
        nptr++;
    }

    if (endptr)
        *endptr = (char *)nptr;

    return minus ? -v : v;
}