dynamic memory allocation and returning pointers

This is a discussion on dynamic memory allocation and returning pointers within the C Programming forums, part of the General Programming Boards category; Bare with me here. I am still lost on doing 'word' problems -- understanding syntax of code and what the ...

  1. #1
    Registered User sballew's Avatar
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    dynamic memory allocation and returning pointers

    Bare with me here. I am still lost on doing 'word' problems -- understanding syntax of code and what the problem asks for is not my forte - YET.

    I need to do the following: write a function that uses dyn mem allocation to create a copy of a string. The call of such function would be : p = strdup(str);

    Function is to :
    (1) allocate space for a string of the same length as str
    (2) copy the contents of str into the new string
    (3) return a pointer to it
    (4) return a null pointer if the memory allocation fails

    Is any of the following OK?
    Whatever is wrong or unnecessary, can you step me through the logic of it, so I can correct my
    error(s)...and thus learn this correctly.

    Hey and what should I set up in the main() to actually check this code for myself???

    Code:
    char *strdup(const char *str)
    {
    
         char *result;
         
         result = malloc(strlen(str)+1);
         if (result == NULL)  {
            printf("Error: malloc failed ");
            return NULL;
         }
    
         strcpy (result, str);
         return result;
    
    }
    Sue B.

    dazed and confused


  2. #2
    ATH0 quzah's Avatar
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    Code:
    int main( void )
    {
       char *s = strdup( "This is a test." );
       printf("%s", s );
       return 0;
    }
    Quzah.

  3. #3
    Registered User sballew's Avatar
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    compilation error

    hey Quzah:

    getting a compilation error. How do I fix??

    error:

    4 /accounts/student2/gcc copystring.c
    copystring.c: In function `strdup':
    copystring.c:21: warning: assignment makes pointer from integer without a cast


    Code:
    #include <stdio.h>
    #include <string.h>
    
    
    char *strdup(const char *str);
    
    
    main()
    {
    
       char *s = strdup( "This is a test." );
       printf("%s", s );
       return 0;
    }
    
    char *strdup(const char *str)
    {
    
         char *result;
    
         result = malloc(strlen(str)+1);            <--- line 21
         if (result == NULL)  {
            printf("Error: malloc failed ");
            return NULL;
         }
    
         strcpy (result, str);
         return result;
    
    }
    Sue B.

    dazed and confused


  4. #4
    and the hat of wrongness Salem's Avatar
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    You need to include stdlib.h to include the prototype for malloc
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  5. #5
    ATH0 quzah's Avatar
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    Dammit, Salem. You beat me to it. I was replying to this when I saw it update in another window.

    Quzah.

  6. #6
    Registered User sballew's Avatar
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    i got it to work when I added

    (char *) before malloc

    I didn't add the stdlib either

    ???
    Sue B.

    dazed and confused


  7. #7
    Registered User sballew's Avatar
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    ok added stdlib.h

    and took out (char *) before malloc

    it works that way too
    Sue B.

    dazed and confused


  8. #8
    ATH0 quzah's Avatar
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    The reason it gives the error, without you including the correct header is this:

    Any time you use a function that hasn't been declared or prototyped, the compiler assumes it returns an integer. Thus, when you tried to assign it's return value to a character pointer, it gave you an error saying, "Hey, you can't just convert an integer into a char* without casting!"

    So, if you add a cast, it doesn't give that error, but it still isn't correct, because you haven't told it how malloc works (by adding its header). It ends up working because of the way it includes the libraries when it does the linking.

    Anyway, that's the reason you got the message.

    Quzah.

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