#include <errno.h>
#include <assert.h>
#include <inttypes.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int safe_atou(const char *s, unsigned *ret_u) {
        char *x = NULL;
        unsigned long l;

        assert(s);
        assert(ret_u);

        /* strtoul() is happy to parse negative values, and silently
         * converts them to unsigned values without generating an
         * error. We want a clean error, hence let's look for the "-"
         * prefix on our own, and generate an error. But let's do so
         * only after strtoul() validated that the string is clean
         * otherwise, so that we return EINVAL preferably over
         * ERANGE. */

        
        errno = 0;
        l = strtoul(s, &x, 0);
        if (errno > 0)
                return -errno;
        if (!x || x == s || *x)
                return -EINVAL;
        if (s[0] == '-')
                return -ERANGE;
        if ((unsigned long) (unsigned) l != l)
                return -ERANGE;

        *ret_u = (unsigned) l;
        return 0;
}

int main(void) {
    unsigned res = 1337;
    const char *tests[] = {"0day", "day0", "42", "-0", "-1", "0", "1337", "0p3nn3t", 0};
    const char **test = tests;
    while (*test) {
        int err = safe_atou(*test, &res);
        printf("\ninput: %.6s, err: %d, val: %i", *test, err, res);
        test++;
    }

}