stack smashing

[CLearner88]

hi all, i encountered some stack smashing error because of this set of code.
i have no idea why this happens on linux, it runs fine on code blocks on windows

the code is below

string createMatrix(string keyWord)
{
  char alphabets[24];
  int i=0;
  int asciiValue=0;
  int asciiCount=0;
  bool uChar=true;
  int wordLength=keyWord.length();
  int runCounter=1;

  for(int a=0;a<wordLength;a++)
  {
      if(keyWord[a]=='j')
      {
          keyWord[a]='i';
      }
  }

  for(runCounter=0;runCounter<wordLength;runCounter++)
  {
      for(int k=0;k<runCounter;k++)
      {
          if(keyWord[runCounter] == keyWord[k])
          {
              uChar=false;
          }
      }

      if(uChar == true)
      {
          alphabets[i]=keyWord[runCounter];
          i++;
      }
      uChar=true;
  }

  while(asciiCount<26)
  {
      for(int j=0;j<wordLength;j++)
      {
          if(asciiValue == keyWord[j] - 97)
          {
              uChar=false;
          }
      }

      if(uChar == true)
      {
          if(asciiValue != 9)
          {
              alphabets[i]=97+asciiValue;
              i++;
          }
      }
      asciiValue++;
      asciiCount++;
      uChar=true;
  }
  return alphabets;
}

may i know what`s the problem with this set of code? Thanks in advance

[Disch]

some stack smashing error

Don't paraphrase errors. What's the actual error you're getting? Copy/paste it.

[CLearner88]

my apologies on that.

here is the error

*** stack smashing detected ***: ./playfair terminated
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x48)[0x3f1de8]
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x0)[0x3f1da0]
./playfair[0x8049182]
./playfair[0x804aa92]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0x327b56]
./playfair[0x8048ee1]
======= Memory map: ========
00311000-0044f000 r-xp 00000000 08:01 295676 /lib/tls/i686/cmov/libc-2.10.1.so
0044f000-00451000 r--p 0013e000 08:01 295676 /lib/tls/i686/cmov/libc-2.10.1.so
00451000-00452000 rw-p 00140000 08:01 295676 /lib/tls/i686/cmov/libc-2.10.1.so
00452000-00455000 rw-p 00000000 00:00 0
00462000-00463000 r-xp 00000000 00:00 0 [vdso]
00b09000-00bef000 r-xp 00000000 08:01 196485 /usr/lib/libstdc++.so.6.0.13
00bef000-00bf3000 r--p 000e6000 08:01 196485 /usr/lib/libstdc++.so.6.0.13
00bf3000-00bf4000 rw-p 000ea000 08:01 196485 /usr/lib/libstdc++.so.6.0.13
00bf4000-00bfb000 rw-p 00000000 00:00 0
00ca9000-00cc4000 r-xp 00000000 08:01 294662 /lib/ld-2.10.1.so
00cc4000-00cc5000 r--p 0001a000 08:01 294662 /lib/ld-2.10.1.so
00cc5000-00cc6000 rw-p 0001b000 08:01 294662 /lib/ld-2.10.1.so
00df3000-00e0f000 r-xp 00000000 08:01 294351 /lib/libgcc_s.so.1
00e0f000-00e10000 r--p 0001b000 08:01 294351 /lib/libgcc_s.so.1
00e10000-00e11000 rw-p 0001c000 08:01 294351 /lib/libgcc_s.so.1
00f96000-00fba000 r-xp 00000000 08:01 295680 /lib/tls/i686/cmov/libm-2.10.1.so
00fba000-00fbb000 r--p 00023000 08:01 295680 /lib/tls/i686/cmov/libm-2.10.1.so
00fbb000-00fbc000 rw-p 00024000 08:01 295680 /lib/tls/i686/cmov/libm-2.10.1.so
08048000-0804c000 r-xp 00000000 08:01 140980 /home/user/playfair
0804c000-0804d000 r--p 00003000 08:01 140980 /home/user/playfair
0804d000-0804e000 rw-p 00004000 08:01 140980 /home/user/playfair
08718000-08739000 rw-p 00000000 00:00 0 [heap]
b76f7000-b76f9000 rw-p 00000000 00:00 0
b7709000-b770b000 rw-p 00000000 00:00 0
bfabe000-bfad3000 rw-p 00000000 00:00 0 [stack]
Aborted

[Disch]

Hrm.... I've never seen the term "stack smashing" before. That's weird. I'm assuming that means "stack corruption". I was actually hoping it would print the variable around which the corruption occurred. Oh well.


I see two problems related to your "alphabets" array.

#1: It's possible for you to step out of bounds of this array and write more than 24 characters to it, which would cause stack corruption.

#2: You're not null terminating it, so when you return it as a string, the string likely is reaching past the array bounds and taking corrupted memory and interpretting it as string data.



Both of these problems can be solved by using a string rather than a char array.

// Remove this:
  char alphabets[24];
  int i=0;

// ...
          alphabets[i]=keyWord[runCounter];
          i++;
// ...
              alphabets[i]=97+asciiValue;
              i++;

// Replace with this:
  string alphabets;

// ...
          alphabets += keyWord[runCounter];

// ...
              alphabets += 'a' + asciiValue; // use literal 'a' instead of 97 to be more clear 

[CLearner88]

thanks alot. The conversion did the trick! but i was wondering why the previous code works on windows but not on ubuntu linux?

[Disch]

You were getting "lucky". Accessing bad memory may or may not "work". Behavior is undefined.