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;

}

---

Code:

---

int main( void )
{
  char *s = strdup( "This is a test." );
  printf("%s", s );
  return 0;
}

---

Quzah.

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;

}

---

You need to include stdlib.h to include the prototype for malloc

Dammit, Salem. You beat me to it. I was replying to this when I saw it update in another window.

Quzah.

i got it to work when I added

(char *) before malloc

I didn't add the stdlib either

???

ok added stdlib.h

and took out (char *) before malloc

it works that way too

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.