[Sussex] C programming help - again!

[Jim Nicholson]

Captain Redbeard wrote:
> Hi everybody!
> 
> Consider the following program:
> 
> 
> 
> #include <stdlib.h>
> #include <stdio.h>
> 
> int main()
> {
>   struct TestStruct
>   {
>     int X;
>     int Y;
> 
>     char *CharString;
>   } *TestStructPtr;
> 
>   TestStructPtr = malloc (sizeof (struct TestStruct));
>   TestStructPtr->CharStr = malloc ((sizeof (char)) * 16);
This allocates 16 bytes (the sizeof(char) is superfluous as the C 
standard guarantees it will always be 1) and sets 
TestStructPtr->CharString to point to it

> 
>   TestStruct->X = 3;
>   TestStruct->Y = 4;
>   TestStruct->CharString = "Hello World!\n\n\n");
This sets TestStructPtr->CharString to point to the string literal 
"Hello World!\n\n\n".  The block of memory malloced above is no longer 
pointed to, this is you memory leak.
> 
>   printf ("TestStructPtr = (%i, %i).\n\n", TestStructPtr->X, 
> TestStructPtr->Y);
>   printf ("CharString = %s.", TestStructPtr->CharString);
> 
>   free (TestStructPtr);
This frees the memory allocated to the structure.
> 
>   return 0;
> }

[snip]
> .
> .
> .
>   printf ("TestStructPtr = (%i, %i).\n\n", TestStructPtr->X, 
> TestStructPtr->Y);
>   printf ("CharString = %s.", TestStructPtr->CharString);
> 
>   free (TestStruct->CharString);
>   free (TestStructPtr);
> 
> *** glibc detected *** free(): invalid pointer: 0x080485b4 ***
Here you are trying to free the memory used by the string literal "Hello 
World!\n\n\n" which was not allocated by malloc, hence the invalid 
pointer error.

To fix this problem, use:

...
   TestStructPtr = malloc (sizeof (struct TestStruct));
   TestStructPtr->CharString = malloc (16);

   TestStructPtr->X = 3;
   TestStructPtr->Y = 4;
   strcpy(TestStructPtr->CharString, "Hello World!\n\n\n");

   printf ("TestStructPtr = (%i, %i).\n\n", TestStructPtr->X,
TestStructPtr->Y);
   printf ("CharString = %s.", TestStructPtr->CharString);

   free (TestStructPtr->CharString);
   free (TestStructPtr);
...

This copies the "Hello World!\n\n\" string literal into the malloced 
memory pointed to by TestStructPtr->CharString rather than making 
TestStructPtr point to the string literal, so you are freeing up the 
malloced memory.

Jim