Re: OT: Possible memory leak in an exercise of a C handbook

[Franco Martelli]

On 16/12/24 at 20:49, Jeffrey Walton wrote:
Here's the problem:
void dealloc()
{
     for ( const DIGIT *p = first; p->next != NULL; p = p->next )
         if ( p->prev != NULL )
             free( p->prev );
     free( last );
}

You seem to be checking backwards (p->prev) but walking the list
forwards (p = p->next) at the same time.

Try something like this. It allows you to walk the list forward.

     void dealloc()
     {
         for ( const DIGIT *next, *p = head; p->next != NULL; )
             if ( p != NULL )
                 next = p->next, free( p ), p = next;
         free( last );
     }

The use of 'next' stashes away the pointer so you can free 'p' and
still access the next pointer.

Thanks, your code works, Valgrind says 0 errors and: "All heap blocks 
were freed -- no leaks are possible".
GCC gives me a warning, so I've to remove the "const" modifier in the 
"for loop":

$ gcc -Wall e09-01.c
e09-01.c: In function ‘dealloc’:
e09-01.c:57:47: warning: passing argument 1 of ‘free’ discards ‘const’ 
qualifier from pointer target type [-Wdiscarded-qualifiers]
   57 |                         next = p->next, free( p ), p = next;
      |                                               ^
In file included from e09-01.c:5:
/usr/include/stdlib.h:568:25: note: expected ‘void *’ but argument is of 
type ‘const DIGIT *’ {aka ‘const struct digit *’}
  568 | extern void free (void *__ptr) __THROW;

after done this, all works nicely. Thanks again.

--
Franco Martelli