Thread: Why malloc?

Code:

struct STRING
{
  int size;
  char *theString;
};

typedef struct STRING String;


String * newString( char const * const init )
{
  String *newString = NULL;
  
  newString = (String *)malloc( sizeof( String ) );
  
  if ( NULL != newString )
  {
    newString->theString = (char *)malloc( strlen( init ) + 1 );
    
    if ( NULL != newString->theString )
    {
      strcpy( newString->theString, init );
      newString->size = strlen( newString->theString );
    }
    
    // not enough memory, clean up
    else
    {
      free( newString );
      // set ptr to NULL so we don't illegally reference it
      newString = NULL;
    }
  }
  
  // don't use a struct declaration...
  // struct assignment doesn't do deep copying
  return newString;
}

In this code why do you use malloc here (line 18):
newString->theString = (char *)malloc( strlen( init ) + 1 );
instead of just directly copying the initial string
strcpy(newString -> theString, init);

I was told that when you malloc you put the object onto the heap so that don't disappear after the function ends.
So if you didn't malloc but instead copied directly what would happen?
I tried this and the terminal said: Segmentation fault (core dumped)
I don't get what happened.

This is sample code written by my teacher.
He said you have to cast the return of malloc because malloc returns a void pointer.
You're probably right, but maybe we just haven't got to that point.
I'm just learning malloc and want to know why I get a segmentation fault if I don't use malloc on line 18.

Originally Posted by seal308

I'm just learning malloc and want to know why I get a segmentation fault if I don't use malloc on line 18.

The memory you allocate on line 12 is for an instance of the STRING struct. This gives you enough space for (1) an integer, and (2) a character pointer.

But that character pointer does not, by default, point to accessible memory. So you then need to allocate memory for that pointer. Hence line 18.

When you free memory, you need to do so in reverse order - first free the char pointer (within the struct), then the struct pointer.

Originally Posted by seal308

This is sample code written by my teacher.
He said you have to cast the return of malloc because malloc returns a void pointer.
You're probably right, but maybe we just haven't got to that point.

Your teacher is giving you bad advice in this respect - read the link Elkvis posted.

Ok thx.
So then for any pointer in a struct I need to allocate memory for?

So if my struct contained a int* pointer.
Would the int it's pointing at need to be allocated as well?
so malloc(sizeOf(int))

Also I'll look into the casting issue and ask my teacher about it.

Originally Posted by seal308

Ok thx.
So then for any pointer in a struct I need to allocate memory for?

So if my struct contained a int* pointer.
Would the int it's pointing at need to be allocated as well?
so malloc(sizeOf(int))

Yes, unless it's intended to point to something that already exists.