string code showing error?

Question

#include <stdio.h>    

int main()  
{  
  char s[] = "churchgate: no church no gate";  
  char t[25];  
  char *ss, *tt;  
  ss = s;  
  while (*ss != '\0')  
    *tt++ = *ss++;  
  printf("%s\n", t);  
  return 0;  
}

what is the problem with this code? when I try to run it. Its showing up some garbage values.

Writing through an uninitialized pointer is undefined behavior. — Adam Rosenfield
– Adam Rosenfield, Commented Nov 12, 2012 at 20:11
@Lundin, I'd suggest char t[sizeof(s)]; instead. Especially, taking into account it's c-tagged. — Michael Krelin - hacker
– Michael Krelin - hacker, Commented Nov 12, 2012 at 20:16
@MichaelKrelin-hacker That's fine too, of course. And static_assert is part of the C language, since C11. — Lundin
– Lundin, Commented Nov 12, 2012 at 20:25

Justin Ethier · Accepted Answer · 2012-11-12 20:11:00Z

3

You never point tt to anything. You need to point it to t:

tt=t;

answered Nov 12, 2012 at 20:11

Justin Ethier

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Comments

ouah · Accepted Answer · 2012-11-12 20:12:04Z

3

You forgot to inialize tt to t
Your array is too small.
You forgot to null terminate your array.

answered Nov 12, 2012 at 20:12

ouah

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2 Comments

Michael Krelin - hacker Over a year ago

Who cares about 2, given 1? :)

ouah Over a year ago

@MichaelKrelin-hacker yes, I'm trying to be exhaustive;)

Luca Stein · Accepted Answer · 2012-11-12 20:23:52Z

1

Tho it can be fun to experiment with arbitrary locations in memory, if you want a defined behaviour the target of access has to be defined.

tt has to be pointed towards some defined area in memory space before you do operations on it.

*tt++ = *ss++;

s is 30 bytes. t, if that is the one you want to use for tt is 25.

edited Nov 12, 2012 at 20:23

answered Nov 12, 2012 at 20:15

Luca Stein

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2 Comments

Luca Stein Over a year ago

@MichaelKrelin-hacker; you are very correct, I was a bit to dyslectic in counting chars there. Thanks :)

Michael Krelin - hacker Over a year ago

That's because instead of counting I've pasted it into console echo -n ....|wc -c (and added one for terminator) ;-)

paulsm4 · Accepted Answer · 2012-11-12 20:17:12Z

0

Several problems:

1) "tt" was never initialized, and

2) "s[]" might be READ ONLY (depending on compiler/platform)!!!!!

Suggestion:

#include <stdio.h>

#define MAX_STRING 80

int 
main()  
{  
  char s[MAX_STRING] = "churchgate: no church no gate";
  char t[MAX_STRING];  
  char *ss = s, *tt = t;
  while (*ss)  
    *tt++ = *ss++;
  *tt = '\0'; 
  printf("%s\n", t);  
  return 0;  
}

edited Nov 12, 2012 at 20:17

answered Nov 12, 2012 at 20:14

paulsm4

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Comments

glglgl · Accepted Answer · 2012-11-12 20:33:44Z

0

Several possibilities, e. g.:

#include <stdio.h>    

int main()  
{  
  char s[] = "churchgate: no church no gate";  
  char t[25];  
  char *ss, *tt;  
  for (ss=s, tt=t; *ss != '\0' && tt-t < sizeof(t)-1; ss++, tt++)
    *tt = *ss;
  }
  *tt = '\0';

  // or just use strncpy.
  // strncpy doesn't copy the \0 if the size is reached,
  // so account for that.
  strncpy(t, s, sizeof(t)-1);
  t[sizeof(t)-1] = '\0';

  printf("%s\n", t);
  return 0;  
}

You know your major problems from the other answers:

tt uninitialized
no bounds checking for t
no 0-termination after copying

answered Nov 12, 2012 at 20:33

glglgl

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