[Sussex] C programming help - again!

[Captain Redbeard]

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);

   TestStruct->X = 3;
   TestStruct->Y = 4;
   TestStruct->CharString = "Hello World!\n\n\n");

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

   free (TestStructPtr);

   return 0;
}



This program contains a pointer to a struct (TestStructPtr) 
that itself contains a pointer (CharString), nested pointers 
if you will.

When I compile this program with the option "-Wall" included 
I get no error or warning messages and when when run the 
program runs as expected so all seems OK.  However when I 
run the same program under Valgrind I get warned of a memory 
leak.  This makes sense as the CharString pointer has not 
been freed up.  To handle this leak I added an extra free() 
command specifically for that pointer so the program now 
looks like this:



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

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



Now the program still compiles OK but when the new line is 
reached at run-time I get the following error message:


*** glibc detected *** free(): invalid pointer: 0x080485b4 ***


Obviously that doesn't work but I've no idea why.  OK, let's 
try something else, let's create a new char pointer that 
points to the same memory address as CharString, that way we 
can free() CharString by proxy, as it were.  This is the 
next version of the code:



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

   char *TempPtr = TestStructPtr->CharString;

   free (TempPtr);
   free (TestStructPtr);
.
.
.



The program still compiles OK but when I run it I get an 
error message similar to the one above when it comes to the 
"free (TempPtr);" line.  Why does free() not work in this 
situation and how do I properly free nested pointers?



Captain Redbeard



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