[vox-tech] malloc() is ... old school?

[Tim Riley]

"Micah J. Cowan" wrote:

> On Thu, Mar 20, 2003 at 04:52:15PM -0800, Tim Riley wrote:
> >
> >
> > "Micah J. Cowan" wrote:
> >
> > > On Thu, Mar 20, 2003 at 10:15:43AM -0800, Tim Riley wrote:
> > > >
> <snip>
> >
> > The vulnerability with malloc() occurs when working with pointers.
> > It's common to test if a pointer has been set
> > by placing it inside an "if" statement before referencing it.
> > If you always use calloc(), all of your pointers will be
> > initialized with zero.
>
> All bits zero != NULL. This is a common misbelief, which happens to be
> true on a number of platforms. But it is not guaranteed to be true,
> and is not on several platforms, though I conced that they are not the
> mainstream. See:
>
>   Sec. 5 of the C FAQ: ~http://www.eskimo.com/~scs/C-faq/s5.html
>
>   Any number of comp.lang.c posts:
>     http://groups.google.com/groups?as_q=NULL%20calloc%28%29&as_ugroup=comp.lang.c
>
>   Especially this one by the author of the FreeBSD C Library (last paragraph):
>     http://groups.google.com/groups?q=NULL+calloc()+group:comp.lang.c+author:Chris+author:Torek&selm=18764%40dog.ee.lbl.gov
>
>

"man calloc" on a Linux machine says bluntly:
       calloc() allocates memory for an array of  nmemb  elements
       of  size bytes each and returns a pointer to the allocated
       memory.  The memory is set to zero.

       malloc() allocates size bytes and returns a pointer to the
       allocated memory.  The memory is not cleared.


> > For the following example
> > it's clear that a core dump might occur; however, if the program were
> > 1000 lines long and the variables set in different locations, tracing
> > could be a bear.
> >
> > typedef struct
> > {
> >     char *name;
> >     char *address;
> >     char *city;
> >     char *state;
> >     char *zip_code;
> > } PERSON;
> >
> > int main( int argc, char **argv )
> > {
> >     PERSON *person = malloc( sizeof( PERSON ) );
> >
> >     person->name = "fred";
>
> A core dump could occur right here, with or without calloc(),
> considering you didn't check malloc()'s return.

Allocation error checking was intentionally left out for simplicity.

>
>
> >     if ( person->name && person->zip_code )
> >     {
> >         printf( "For person = %s, got zip code = %s\n",
> >                 person->name, person->zip_code );
> >     }
> > }
> >
> > If calloc() had been used, no one would have noticed the delay and no
> > core would be dumped.
>
> Provided that one is using a system on which NULL happens to be
> all-bits-zero.

What ANSI C implementation are you referring to here?

> Which we all are on this list, but can we guarantee
> that the code you write won't be ported to such a system?
>

Isn't this a Linux users group? What system are you referring to?
Does it not have gcc? Is it open source so we can fix it?

>
> And I submit that without the core dump, the bug of having forgotten
> to set *person's fields just got much, much harder to track down.
>

Core dumps are useful for tracking down bugs.

>
> -Micah
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