Re: [Qemu-devel] [PATCH] fix qemu_malloc() error check for size==0

[malc]

On Mon, 18 May 2009, Eduardo Habkost wrote:

> On Tue, May 19, 2009 at 01:56:55AM +0400, malc wrote:
> > On Mon, 18 May 2009, Eduardo Habkost wrote:
> > 
> > > This patch is similar to a previous qemu_realloc() fix
> > > (commit 322691a5c9f1c8531554148d47c078b5be590805), but for qemu_malloc().
> > > 
> > > malloc(0) may correctly return NULL if size==0. We don't want to abort 
> > > qemu on
> > > this case.
> > 
> > Only it wouldn't (on Linux):
> > 
> > $ cat malloc.c
> > #include <stdlib.h>
> > 
> > int main (void)
> > {
> >     printf ("%p\n", malloc (0));
> >     return 0;
> > }
> > $ gcc malloc.c
> > $ ./a.out 
> > 0x10011008
> > 
> > Standard (in 7.20.3) says that malloc's behaviour in case of size being
> > zero is implementation defined.
> > 
> > Try `git show 63c75dcd669d011f438421980b4379827da4bb1c'.
> > 
> > The best(only?) thing to do is to check size passed to qemu_malloc[z]
> > and abort the program if this situation is encountered.
> 
> Why? malloc(0) is as valid as realloc(p, 0). It will either return NULL
> or a pointer, and on any case the value can be safely passed to free()
> later.

I believe you haven't examined the commit that i referenced. Thing is
existing code used to, i'd venture a guess accidentaly, rely on the
behaviour that current GLIBC provides and consequently failed to
operate on AIX where malloc(0) returns NULL, IOW making qemu_malloc[z]
return whatever the underlying system returns is just hiding the bugs,
the code becomes unportable.

-- 
mailto:address@hidden