C++ argv indices larger than argc

So I've been trying to access command line arguments with indices larger than argc in C++ (because why not) and discovered that there are actually some parameters passed on to the program. The following code produces the following result in my Ubuntu 14.04:

#include <cstdio>
int main (int argc, char** argv) {
    for (int i=argc+1; argv[i]!=0; i++)
        printf("%3d %s\n", i, argv[i]);
}

This is the output:

  2 XDG_VTNR=7
  3 LC_PAPER=tr_TR.UTF-8
  4 LC_ADDRESS=tr_TR.UTF-8
  5 XDG_SESSION_ID=c2
  6 XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/kubuzetto
  7 SELINUX_INIT=YES
  8 LC_MONETARY=tr_TR.UTF-8
  9 CLUTTER_IM_MODULE=xim
  (and so on)

What is this and is it distro-dependent?

Solution

Don't run past argc, it's not defined.

In this case, we know what happened. You see, main() is declared and called as

extern int main(int argc, char **argv, char **envp);

and it just so happens that the the way argv and envp are built leaves envp right after argv, so running off the end encounters envp.

It might change someday, so don't depend on this working. If you wanna depend on the kernel's passing method, provide the kernel's entry point yourself (which must be done in asm [or possibly a naked function] as it doesn't look like a function call).