getlogin(), prints ((NULL)) when printing multiple times in a row.

["J." <mailing-lists@xs4all.nl>]

Hello,

I have ran into undefined behaviour. Not from my girl-friend this time,
but from my C program. (It's not that time of month, yet... :()

But, Everytime when I execute the following C code (bottom) from within a
shell loop as in:

while : ; do ./program ; done

it prints: ((NULL))

But when I excute it in the normal way:
./program
It prints perfectly ok: myusername

C Code.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main(void) {
 char *username = NULL;

 username = getlogin();
 /* for(;;) */
  printf("%s\n", username);

 return 0;
}

When I call the printf from a for loop in the native C code it also
starts to print ((NULL)) after a couple of loops.

I am writing a network program that gives a lot of status output when
working verbose. It doesn't look very good ((NULL)) everytime when the
output is above average.

Anyone ?

Thnkx 

J.