Thread: copy = concatenate ?

  1. #1
    Registered User
    Join Date
    Oct 2006

    Question copy = concatenate ?

    As a newbye, I've justed started writing some code.
    Instead of writing 2 different functions for COPY and CONCATENATE string, I'm trying one that will do both:
    a) if destination is empty then it will be a simple copy from original string to destination,
    b) if destination is not empty, then a concatenation applies.

    So, my code is as follows:

    #include <stdio.h>

    // a CONCATENATE function is similar to a COPY function, with the only
    // difference that the copy to destination starts on first empty index

    char *f_strcpy(char *v_string)
      int length = 0; // determines what is actual length of destination string
      int i;
      char dest[100];
      // check for length, ignoring termination character
      for(i=0; dest[i]!='\0'; i++)
      length = i; // 
      // starts concatenation (or simple copy if destination is empty)
        for(i=0; v_string[i]!='\0'; i++)
          dest[length+i]=v_string[i];  // leaves cycle as soon as ='\0'
      dest[length+i] = '\0';     // assigns terminator after leaving cycle - otherwise
      return dest;
    Now, if i run this code, it returns some strange characters.

    Any ideas?

  2. #2
    Registered User SKeane's Avatar
    Join Date
    Sep 2006
    You are returning a pointer to variable (dest) that is local to your function. If you insist on using a fixed length character array, make it static, otherwise use malloc to create a dynamically allocated character array which you can safely return.

    You are also scanning though dest when it isn't initialised.

  3. #3
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Rishon LeZion, Israel
    1. dest is not initialized - undefined behavior of the loop checking for the length
    2. returning pointer to the local variable - memory is destroyed on function exit, pointer is dan
    3. You don't check the destination buffer length (possible memory overrun).
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  4. #4
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    You are using dest uninitialized. Aren't you supposed to be counting v_string's length instead?
    You can't return local variables. You'll need to use something like malloc.

    Curses, foiled again!

    Your problem is the concept of your function, really. What exactly are you trying to copy / fill? If "it's" empty... What's "it"? You imply that you have some kind of pointer that you need to be adding to. If you already have a string you want to add to, why aren't you passing both the one you want to add to and the one you are adding from?

    You really need two arguments. If the first is NULL, duplicate the second. If it's not, then stick the two together.

    Last edited by quzah; 11-03-2006 at 05:04 PM.
    Hope is the first step on the road to disappointment.

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