Return value of malloc(0)

[Erik Trulsson]

On Thu, Jun 29, 2006 at 11:44:23AM -0400, Pat Lashley wrote:
> >The C Standard says the following about malloc(0):
> >
> >  If the size of the space requested is zero, the behavior is
> >  implementation-defined: either a null pointer is returned, or the
> >  behavior is as if the size were some nonzero value, except that the
> >  returned pointer shall not be used to access an object.
> >
> >So our default behaviour to crash if a pointer returned by malloc(0) is
> >dereferenced is legal and a good one because it catches errors like the
> >above one.
> 
> No, our implementation is NOT legal.  We always return the SAME value.  To 
> be legal, we should not return that value again unless it has been 
> free()-ed.
> 
>        first = malloc(0) ;
>        second = malloc(0) ;
> 
>        if ( first == second )  ERROR( "C standards violation" ) ;


Almost.  The test should be

	if ( first != NULL && first == second)  ERROR( "C standards violation" ) ;

It is after all legal for malloc(0) to return NULL.


Otherwise you are correct.  Having malloc(0) always returning the same
(non-NULL) value is not legal according to the C standard.

C99 says:

7.20.3 Memory management functions 
[...]  Each such allocation shall yield a pointer to an object disjoint from
any other object. [...] If the size of the space requested is zero, the
behavior is implementation-defined: either a null pointer is returned, or
the behavior is as if the size were some nonzero value, except that the
returned pointer shall not be used to access an object.



> 
> 
> Firefox, or the extension, has a bug in the code. It should not be 
> attempting to de-reference the result of a 'malloc(0)' call. They probably 
> depend on having it return NULL, which is checked elsewhere. (The fix is 
> for them to test for the size == zero case and just set the pointer to NULL 
> instead of calling malloc(0). But that's their problem, not ours.)
> 
> 
> 
> -Pat 
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-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at student.uu.se