Some of this code might not make any sense, (perhaps even I am perplexed) about how this happened and why? This code was intended for some other purpose, (i was trying to make a calculator, so the main source code [which had few problems] exists on this site as my question) i somehow broke it and so i modified it and extracted the code which initially broke it.
I tested it on Kali Linux.
Here's the code:
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
//Some of this might not make any sense, but its the way i was able to break it.. somehow
int main(int argc,char *argv[]) {
FILE *file = fopen(argv[2],"a");
if(file == NULL) {
printf("Error Opening File!");
return 1;
}
char ch;
int i;
printf("%i\n",argc);
printf("%i\n",argv);
while((ch = getopt(argc,argv,"d:"))!=EOF)
switch(ch) {
case 'd':
for(i=4; i<=48; i++) // This loop may vary on other system. On first run i used "i<=51" , and on different system, same os, its the mentioned condition
fprintf(file,"%s \n",argv[i]);
fclose(file);
break;
default:
fprintf(stderr,"No such command");
}
argc-=1;
argv+=1;
return 0;
}
The following result was produced upon running (the original source code. This code will write it to a file) :
value at argv= 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:mi=00: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:.arc=01;31:.arj=01;31:.taz=01;31:.lha=01;31:.lz4=01;31:.lzh=01;31:.lzma=01;31:.tlz=01;31:.txz=01;31:.tzo=01;31:.t7z=01;31:.zip=01;31:.z=01;31:.Z=01;31:.dz=01;31:.gz=01;31:.lrz=01;31:.lz=01;31:.lzo=01;31:.xz=01;31:.zst=01;31:.tzst=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:.alz=01;31:.ace=01;31:.zoo=01;31:.cpio=01;31:.7z=01;31:.rz=01;31:.cab=01;31:.wim=01;31:.swm=01;31:.dwm=01;31:.esd=01;31:.jpg=01;35:.jpeg=01;35:.mjpg=01;35:.mjpeg=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:.ogv=01;35:.ogx=01;35:.aac=00;36:.au=00;36:.flac=00;36:.m4a=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:.oga=00;36:.opus=00;36:.spx=00;36:.xspf=00;36:
value at argv= XDG_MENU_PREFIX=gnome-
value at argv= LANG=en_IN
value at argv= GDM_LANG=en_IN
value at argv= MANAGERPID=793
value at argv= DISPLAY=:1
value at argv= INVOCATION_ID=3e7f771adfde4001af637f9527e9cc9d
value at argv= COLORTERM=truecolor
value at argv= USERNAME=root
value at argv= XDG_VTNR=2
value at argv= SSH_AUTH_SOCK=/run/user/0/keyring/ssh
value at argv= S_COLORS=auto
value at argv= XDG_SESSION_ID=3
value at argv= USER=root
value at argv= DESKTOP_SESSION=gnome
value at argv= PWD=/media/root/F89AF7139AF6CCDE
value at argv= HOME=/root
value at argv= JOURNAL_STREAM=9:19456
value at argv= SSH_AGENT_PID=945
value at argv= QT_ACCESSIBILITY=1
value at argv= XDG_SESSION_TYPE=x11
value at argv= XDG_DATA_DIRS=/usr/share/gnome:/usr/local/share/:/usr/share/
value at argv= XDG_SESSION_DESKTOP=gnome
value at argv= DBUS_STARTER_ADDRESS=unix:path=/run/user/0/bus,guid=11c587e1337f9fb06858c2415bbac73a
value at argv= GTK_MODULES=gail:atk-bridge
value at argv= WINDOWPATH=2
value at argv= TERM=xterm-256color
value at argv= SHELL=/bin/bash
value at argv= VTE_VERSION=5002
value at argv= DBUS_STARTER_BUS_TYPE=session
value at argv= XDG_CURRENT_DESKTOP=GNOME
value at argv= GPG_AGENT_INFO=/run/user/0/gnupg/S.gpg-agent:0:1
value at argv= SHLVL=1
value at argv= XDG_SEAT=seat0
value at argv= LANGUAGE=en_IN:en
value at argv= WINDOWID=39845894
value at argv= GDMSESSION=gnome
value at argv= GNOME_DESKTOP_SESSION_ID=this-is-deprecated
value at argv= LOGNAME=root
value at argv= DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/0/bus,guid=11c587e1337f9fb06858c2415bbac73a
value at argv= XDG_RUNTIME_DIR=/run/user/0
value at argv= XAUTHORITY=/run/user/0/gdm/Xauthority
value at argv= PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
value at argv= SESSION_MANAGER=local/kali:@/tmp/.ICE-unix/894,unix/kali:/tmp/.ICE-unix/894
value at argv= _=./a.out
value at argv= (null)
My question that follows is, what actually are environment variables? And why did my give me such results?
// probably use one option at a time
//This code has some flaws, for example i didnt check if the file was opened
or not, etc, pardon me for it. This is the actual un-edited source of the
code.
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
int add(int a,int b){
return a+b;
}
int subtract(int a, int b){
return a-b;
}
int main(int argc, char *argv[]){
FILE *file1 = fopen("Results.txt","a");
char ch;
int res;
while((ch=getopt(argc,argv,"a:s:"))!=EOF)
switch (ch){
case 'a':
res = add(atoi(optarg),atoi(argv[3]));
fprintf(file1,"%i\n",res);
break;
case 's':
res = subtract(atoi(optarg),atoi(argv[3]));
printf("%i \n",res);
fprintf(file1,"%i\n",res);
break;
default:
fprintf(stderr,"No such option");
return 1;
}
//THe commented out section produces weird behaviour...
//printf("Opind = %i, argc = %i, argv = %i \n",optind,argc,argv);
argc-=optind;
//printf("value at optarg = %i value at argv_3= %s \n",optarg,argv[3]);
argv+=optind;
//printf("Opind = %i, argc = %i, argv = %i \n",optind,argc,argv);
//printf("value at optarg = %i value at argv_3= %s \n",optarg,argv[1]);
fprintf(file1,"\nWritten to file\n");
fclose(file1);
return 0;
}
First of all, going out of bounds of an array leads to undefined behavior. You probably do that when you iterate over the argv array without checking for e.g. i < argc (unless you actually provide over 48 arguments on the command line).
Secondly, as for why you print the environment variables when going out of bounds of argv, it's because on POSIX systems (like Linux or macOS) there's a third argument to the main function, usually called environ. Similar to argv it's an array of strings (declared the same way, e.g. char *environ[]) that contains the environment variables. By going out of bounds of argv you go into the memory for the environ array.
On a related note, you use argv[2] without any checking that the argument actually exists, or that it's not any other argument you parse with getopt. Don't do that.
Also note that the getopt function returns an int. This is actually very significant and important for the check against -1 (not EOF!).