`int getlogin_r(char *buf, size_t len)` broken

[isavegas]

Please use the following bug reporting template to help produce actionable and reproducible issues:

• A brief description
  Attempting to use the function to get the login id of the user running the executable returns garbled strings, with a different return value each time.
• Expected results

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
        size_t len = 10;
        char buf[len];
        size_t ret = getlogin_r(buf, len);
        puts(buf);
        return 0;
}

should print out my username, with ret set to 0 to indicate success.

• Actual results (with terminal output if applicable)

The value of ret is 2, indicating an error.
Depending on the C compiler used, it outputs garbage strings.
���If compiled with GCC or Clang, it prints out a single "unknown char" box.
If compiled with TCC, it prints out a string the size of one's username, composed of aforementioned boxes and random numbers, letters, and symbols.

• Your Windows build number

14393

• Steps / commands required to reproduce the error

gcc test.c -o test && ./test

• Strace of the failing command

Command doesn't fail in this sense.

• Required packages and commands to install

sudo apt-get install build-essential

[TheWhoAreYouPerson]

Can confirm that the above sample doesn't work on WSL. Tried on a personal Ubuntu Linux Server box, it worked. Tried on WSL, and it didn't. I would also like to note that ret is set to 0 on linux, and 2 on WSL.

[isavegas]

Compiling the program on a build slave (Ubuntu 15.10) and transferring it to WSL results in the same garbage output. It works just fine on aforementioned Ubuntu instance.

[therealkenc]

It's in the strace. WSL has no /proc/self/loginuid.

open("/proc/self/loginuid", O_RDONLY)   = -1 ENOENT (No such file or directory)

[therealkenc]

WSL doesn't have /proc/self/loginuid as mentioned above, but also doesn't have /var/run/utmp, to which getlogin_r(3) will fall back in the absence of the former. This is all part-in-parcel of #352 and #573. As a workaround in the absence of those two issues being addressed:

$ sudo touch /var/run/utmp

Then:

C:\ wsl.exe sudo login -f someuser

The test case in the Summer 2016 OP is basically equivalent to logname(1), which will now succeed.

$ logname
ken

I still have this issue. The thing is, it returns \0 for debug "-O0" builds and some garbage value for others.

EDIT: Nope. So it looks like in other systems where getlogin_r() is functioning, it writes the first few bytes with the username string. On WSL where this is not working, nothing is written. So, when the buffer is uninitialized and corrupted, it returns a garbage value. However, this is not an error condition according to the Manual page [getlogin_r(3)], so it returns success. I'm deciding to overwrite the first character of the buffer with '\0' where bufsize > 0, when the username is not available. Unfortunately getlogin_r(3) doesn't return the number of characters written, so this is tricky for me

[therealkenc]

The thing is, it returns \0 for debug "-O0" builds and some garbage value for others.

The contents of the buffer passed doesn't matter (initialized or otherwise). As was mentioned in the OP, on WSL getlogin_r() returns 2 (ie ENOENT). In practice you'd test !=0. In an abundance of caution, you can also test against ERANGE. Looks like:

int main() {
    char buf[256];
    if (getlogin_r(buf, sizeof(buf)) != 0) {
        strncpy(buf, "<unknown>", sizeof(buf));
        // or handle the error however you want
    }
    printf("user is: %s\n", buf);
    return 0;
}

The thing is, it returns \0 for debug "-O0" builds and some garbage value for others.

The contents of the buffer passed doesn't matter (initialized or otherwise). As was mentioned in the OP, on WSL getlogin_r() returns 2 (ie ENOENT). In practice you'd test !=0. In an abundance of caution, you can also test against ERANGE. Looks like:

int main() {
    char buf[256];
    if (getlogin_r(buf, sizeof(buf)) != 0) {
        strncpy(buf, "<unknown>", sizeof(buf));
        // or handle the error however you want
    }
    printf("user is: %s\n", buf);
    return 0;
}

Thank you, @therealkenc . It does say it returns "nonzero" instead of -1 for error.

[Bigpet]

Still somewhat broken for me. I found this issue while using the python function os.getlogin()

The first wsl call that boots the distro opens a proper login shell and create the /var/run/utmp entries.

Every following wsl call directly opens a shell without being connected to a login session.
You can start a new login session with simply sudo login that will make the proper entry in the utmp file.

first powershell window

PS C:\Users\bigpet> wsl
Welcome to Ubuntu 22.04.5 LTS (GNU/Linux 5.15.146.1-microsoft-standard-WSL2 x86_64)
bigpet@Ubuntu:~$ python -c "import os; print(os.getlogin())"
bigpet

in another powershell window

PS C:\Users\bigpet> wsl
bigpet@Ubuntu:~$ python -c "import os; print(os.getlogin())"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
OSError: [Errno 6] No such device or address
bigpet@Ubuntu:~$ sudo login
[sudo] password for bigpet:
Ubuntu login: bigpet
Password:
Welcome to Ubuntu 22.04.5 LTS (GNU/Linux 5.15.146.1-microsoft-standard-WSL2 x86_64)
bigpet@Ubuntu:~$ python -c "import os; print(os.getlogin())"
bigpet