mb@dennis ~ $ perl -E'say getlogin'
mb
mb@dennis ~ $ cat gl.c
#include <unistd.h>
#include <stdio.h>

int main(void) {
   puts(getlogin());
   return 0;
}
mb@dennis ~ $ gcc gl.c
mb@dennis ~ $ ./a.out
mb
[download]

Looks like an OS or library bug.

$ cat >gl.c
#include <unistd.h>
#include <stdio.h>

int main(void) {
   puts(getlogin());
   return 0;
}
---- rintintin ~ $ gcc gl.c
---- rintintin ~ $ ./a.out
Segmentation fault (core dumped)
[download]

Maybe it's the same thing as this? https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1067779

Maybe it's the same thing as this? https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1067779

Strange. My netbook has the very same distribution, my /proc/self/loginuid contains UINT_MAX, but stuff still works. strace tells me that instead of reading /proc/self/loginuid (which on some Linuxes such as my Gentoo doesn't even exist) it checks the /proc/self/fd/0 link to find the terminal the program is running on and then checks /var/run/utmpx or /var/run/utmp for the user logged in on that terminal. The weird thing is that there is no trace of getutent() or something in my glibc's getlogin() implementation. I haven't found whether it gets vendor-patched in somewhere or why else that might be.