Wsrtjtyk

I was experimenting randomly with argc and argv in c, however this program(try.c):

/* Trying to understand argc and argv.*/



#include<stdio.h>



int main(int argc,char *argv)

{

  int i=0;



  /*

    argv[4]="arg4";

    argv[5]="arg5";

    argv[6]="arg6";

    argv[7]="arg7";

    argv[8]="arg8";

    argv[9]="arg9";;

  */



   for(i=0;i<(argc+20);i++)

   {

       printf("arg %d: %sn", i,argv[i]);

   }



  return 0;

}

when run as

./try arg1 arg2 arg3

prints out this:

arg 0: ./try

arg 1: arg1

arg 2: arg2

arg 3: arg3

arg 4: (null)

arg 5: XDG_VTNR=7

arg 6: XDG_SESSION_ID=c2

arg 7: CLUTTER_IM_MODULE=xim

arg 8: SELINUX_INIT=YES

arg 9: XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/raman

arg 10: GPG_AGENT_INFO=/run/user/1000/keyring-FAajwI/gpg:0:1

arg 11: TERM=xterm

arg 12: SHELL=/bin/bash

arg 13: VTE_VERSION=3409

arg 14: WINDOWID=58720268

arg 15: UPSTART_SESSION=unix:abstract=/com/ubuntu/upstartsession/1000/1775

arg 16: GNOME_KEYRING_CONTROL=/run/user/1000/keyring-FAajwI

arg 17: GTK_MODULES=overlay-scrollbar:unity-gtk-module

arg 18: USER=raman

arg 19: LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lz=01;31:*.xz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:

arg 20: XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0

arg 21: XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0

arg 22: SSH_AUTH_SOCK=/run/user/1000/keyring-FAajwI/ssh

arg 23: DEFAULTS_PATH=/usr/share/gconf/ubuntu.default.path`

I was expecting a segmentation fault but it worked.It worked for upto argc+63 and then gives segmentation fault.I tried googling but with no success. Someone please explain why is this happening i.e why are the environment variables(seems so) getting printed here?
If I decomment the code, I get even weirder results.

Going outside the limits of any array leads to undefined behavior. What happens in reality, is that many UNIX-like systems (like Linux) actually have a third argument to the main function, an array of string for the environment variable. So the complete prototype of main on such systems is

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

What you do when you go out of bounds of the argv array is that you cross over into the environ array.

It should be noted that this isn't actually in any standard, but it's there for backwards compatibility with old UNIX systems where this was common.

It's also mentioned in this reference, and also documented in the GNU libc manual.

There are three ways you can define entry point "main" for your program.

int main () // POSIX.1 style no command-line input

int main (int argc, char *argv) // POSIX.1 style command line inputs  

int main (int argc, char *argv, char *envp) // UNIX-specific - env. variable access pointer

Program environment pointer is accessible in UNIX programs. POSIX.1 does not allow this three-argument form, so to be portable it is best to write main to take two arguments, and use the value of environ.

Now coming to the segmentation fault error occurring in the program, it is just because your program tried to access outside the boundaries of third env. pointer.