Thread: strtol and const char*

Code:

void validate(int *arg, const char *optarg, ...)
{
    errno = 0;
    char **invalid = NULL;
    *arg = (int) strtol(optarg, invalid, 10);

I'm having trouble with the above code. The standard says that invalid will only be updated if it is not NULL (7.22.1.4(5) and 7.22.1.4(7)).

However I can't seem to be able to define a working pointer to optarg:

const char **invalid = &optarg -> fails on the strtol call. invalid is expected to be a char **.

char **invalid = &optarg -> is obviously wrong.

The alternative is to drop the const qualifier from the validate function definition. But I'd rather it stayed. How can I solve this?

It's normally used like this.

Code:

#include <stdio.h>
#include <stdlib.h>

void validate(int *arg, const char *optarg) {
    char *endptr = NULL;
    *arg = (int) strtol(optarg, &endptr, 10);
    printf("%p %p\n", optarg, endptr);
}

int main(void) {
    int n = 0;
    validate(&n, "123xyz");
    printf("%d\n", n);
    return 0;
}

algorism beat me to it. Yep, you just provide it an address of a proper pointer if you want the function to change it. Otherwise you just pass in NULL there and the function knows it's not a proper address and thus won't try to write to it. So what the standard says is you can do what algorism did, or: result = strtol(s, NULL, 10);

Makes all sense. Thank you.