Re: [Qemu-devel] [PATCH 4/5] tests/i44fx-test: No need for zeroing memory before memset

[Laszlo Ersek <lersek@redhat.com>]

On 10/09/15 08:46, Markus Armbruster wrote:
> Laszlo Ersek <lersek@redhat.com> writes:
> 
>> On 10/08/15 21:35, Thomas Huth wrote:
>>> Change a g_malloc0 into g_malloc since the following
>>> memset fills the whole buffer anyway.
>>>
>>> Cc: Laszlo Ersek <lersek@redhat.com>
>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>> ---
>>>  tests/i440fx-test.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/tests/i440fx-test.c b/tests/i440fx-test.c
>>> index d0bc8de..7fa1709 100644
>>> --- a/tests/i440fx-test.c
>>> +++ b/tests/i440fx-test.c
>>> @@ -191,7 +191,7 @@ static void write_area(uint32_t start, uint32_t end, uint8_t value)
>>>      uint32_t size = end - start + 1;
>>>      uint8_t *data;
>>>  
>>> -    data = g_malloc0(size);
>>> +    data = g_malloc(size);
>>>      memset(data, value, size);
>>>      memwrite(start, data, size);
>>>  
>>>
>>
>> Technically you are right of course, but I remember some historical mess
>> around this, in this file.
>>
>> Plus I vaguely recall g_new[0]() being the most recent preference.
>>
>> https://developer.gnome.org/glib/stable/glib-Memory-Allocation.html#g-new
>>
>> See e.g. commit 97f3ad3551. Markus?
> 
> TL;DR: the patch is fine.
> 
> The argument of g_malloc() isn't always the size of a type.  But when it
> is, you should be using g_new() instead.  The commit message explains:
> 
>     g_new(T, n) is neater than g_malloc(sizeof(T) * n).  It's also safer,
>     for two reasons.  One, it catches multiplication overflowing size_t.
>     Two, it returns T * rather than void *, which lets the compiler catch
>     more type errors.
>     
> The multiplication argument applies only when n != 1.
> 
> The type checking argument applies regardless of n.  That's why the
> commit also transfroms patterns like g_malloc(sizeof(T)).
> 
> When the argument isn't written as sizeof a type, but could be, we enter
> "matter of taste" territory.  For instance, some may perefer the
> idiomatic T *p = g_malloc(sizeof(*p)) to the more tightly typed
> p = g_new(T, 1).  I therefore leave them alone.  Commit messsage again:
> 
>     This commit only touches allocations with size arguments of the form
>     sizeof(T).
> 
> A separate issue is a zeroing memset() after allocation.  Please use a
> zeroing allocator instead, because that's more concise.  Precedence:
> commit 0bd0adb.
> 
> Now let's apply this to the patch.
> 
> The memset() isn't zeroing, so "use a zeroing allocator instead" doesn't
> apply.  Before the patch, the code uses one additionally, which is
> wasteful.  The patch stops the waste.
> 
> The size argument is not the size of any type.  You could still write
> 
>     data = g_new(uint8_t, size);
> 
> but the extra verbosity pretty clearly outweighs what we could gain from
> type checking here.
> 

Okay, thanks!

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Cheers
Laszlo