`setlocale` sets `errno` to `0`

The setlocale C/C++ function writes the global variable errno to 0. In Linux it doesn't have this behavior and the documentation doesn't says anything about setting errno (as I understand, no function should set errno to 0

The following code allows to reproduce it

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
  ifstream myStream;
  myStream.open("NONEXISTENT.txt", ios::in);
  cout << errno << endl;
  setlocale(LC_ALL, "en_US.UTF-8");
  cout << errno << endl;
}
// Output:
// 2
// 0

I don't think is a compiler issue as the error also happens with gcc but rather a libc issue.

As a general rule, errno is only valid immediately after a call that’s documented to set it. If you call into library code that says nothing about errno, you can’t assume that it preserves its value.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

If you call into library code that says nothing about errno, you can’t assume that it preserves its value.

But POSIX says that no function sets it to zero:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/errno.html

No function in this volume of POSIX.1-2017 shall set errno to 0

Does macOS claim to comply with (this version of) POSIX?

Oh hey, woulda you look at that. What a weird requirement. Sure, you should feel free to file a bug about that.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

`setlocale` sets `errno` to `0`
 
 
Q