[Bug libc/19978] New: [glibc] getlogin() returns user name 'NULL'

["jfrieben at hotmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19978

            Bug ID: 19978
           Summary: [glibc] getlogin() returns user name 'NULL'
           Product: glibc
           Version: 2.23
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: jfrieben at hotmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

This bug was originally reported as
https://bugzilla.redhat.com/show_bug.cgi?id=1328947.

Using the sample code from
https://bugzilla.redhat.com/show_bug.cgi?id=621959#c0, namely

----
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    char *username = getlogin();

    printf("[%s]\n", username);
    return 0;
}
----

It turns out that the getlogin() call in Fedora 24 which features the current
glibc 2.23 erroneously returns the user name 'NULL' whereas in Fedora 23 which
has glibc 2.22, it returns the actual user name. Also Ubuntu 16.04 LTS exhibits
this issue which thus appears to be generic.

On both systems, running the executable 'test*' leads to the following output:

$ ./test
[(null)]

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