Undefined behavior when adding a const with const_cast?

Question

This question is regarding the behavior I observed while using const_cast for making a char * const char *. I am aware that this casting is done implicitly and t working for me when the cast is being done implicitly.

The problematic code is:

#include <cstdlib>
int main() {
    const char * org_str = NULL;
    org_str = const_cast<const char*>(getenv("ENV_VAR")); // PROBLEM !!
}

As per the Linux man page getenv() takes const char * and returns char*. So, as per my understanding of const-correctness, I can do a const cast on char* without any issues.

So, my question is, why const_cast here giving me a UB (code is crashing) but as expected without const_cast(implicit casting) its working fine(So the problem has to be with the use of const_cast) ?

Please note, I know implicit cast is the way to go here, through this post I require the answer specifically for the behavior observed here.

EDIT:

Since the bug is non reproducible by fellow So'ers, I am assuming this as some weird runtime/compiler issue. But, do let me know if there is any mention of problems such as this in the standard.

For the time being I am accepting Mike's answer.

Are you sure this works at all? Since getenv is a function, this shouldn't compile because you can't cast a function pointer to a const char* by adding or removing const. Similarly, this const_cast is adding const, not removing it, so it should be safe. Can we see your actual code? — templatetypedef
– templatetypedef, Commented Feb 8, 2012 at 5:18
There is nothing wrong with that code. There must be something wrong with either your compiler, or some other part of your program. — Mankarse
– Mankarse, Commented Feb 8, 2012 at 7:19
Yeah, even I thought it must be elsewhere.But it is working very fine since I removed the const_cast. — Arunmu
– Arunmu, Commented Feb 8, 2012 at 7:21

Mike DeSimone · Accepted Answer · 2012-02-08 13:31:52Z

3

You are casting the function pointer, not the pointer returned by the function. Call the function first with (), then cast the result.

EDIT: I can't reproduce the problem. Here's the code I used:

#include <cstdlib>
#include <iostream>

using namespace std;
int main() {
    const char * org_str = NULL;
    org_str = const_cast<const char*>(getenv("PATH"));
    cout << "Got: " << org_str << endl;
}

Here's what I got:

$ g++ foo.cc -o foo.app
$ ./foo.app
Got: /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/X11R6/bin
$

BTW, the assignment to NULL is unnecessary; recommended practice is to use one of:

const char *org_str = const_cast<const char*>(getenv("PATH"));

const char *org_str(const_cast<const char*>(getenv("PATH")));

const char *org_str(getenv("PATH"));

edited Feb 8, 2012 at 13:31

answered Feb 8, 2012 at 5:24

Mike DeSimone

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2 Comments

templatetypedef Over a year ago

If the OP tried to cast a function pointer with const_cast, it wouldn't even compile. The code posted is not what the OP is actually running.

Arunmu Over a year ago

I have edited the code. Please check, I want to get an env var "ENV_VAR".

Dominik Grabiec · Accepted Answer · 2012-02-08 05:25:27Z

2

You don't need a const_cast<> to make something const, you only need it to take away the const-ness.

Also I don't believe the code you have there is correct at all, since getenv is a function and it looks like you're using it as a variable. Perhaps something like this would work:

const char * org_str = getenv("name-of-env");

answered Feb 8, 2012 at 5:25

Dominik Grabiec

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3 Comments

Arunmu Over a year ago

I know I dont need cons_cast, but whats wrong when I use it, thats the question. Also, I have edited the code.

Dominik Grabiec Over a year ago

I tried your sample code in Visual Studio 2010 but nothing went wrong. My suspicion is that something else is going wrong.

Arunmu Over a year ago

@It work for me too when build in debug mode. Its with optimized mode that it is not working.

martiert · Accepted Answer · 2012-02-08 08:02:48Z

0

It's, as far as I understand, not the return value of getenv you should cast, but the const char you have. As org_str is constant, you can't assign to it without using const_cast, which means you would need to do something like:

#include <cstdlib>
int main() {
    const char * org_str = NULL;
    const_cast<char*>(org_str) = getenv("ENV_VAR"); // NO PROBLEM !!
}

EDIT: As for having const_cast on the getenv, it makes no sense, as you don't assign to that and therefor will not have any violations of the const expression, as

org_str = getenv("ENV_VAR") will give you.

answered Feb 8, 2012 at 8:02

martiert

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3 Comments

martiert Over a year ago

It's kind of weird, I agree. But this is what is done in: en.cppreference.com/w/cpp/language/const_cast Another use is of course to cast a const object, to change it's values. At least that's the only two use cases I have seen.

Mike DeSimone Over a year ago

I tried this and got: warning: target of assignment not really an lvalue; this will be a hard error in the future.

Arunmu Over a year ago

@martiet: I think lvalue casts can only be used with references. Need to dig the standard though ;)

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