Re: [Qemu-devel] [PATCH] Fix conversions from pointer to int and vice versa

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 24.02.2011 08:21, schrieb Markus Armbruster:
>> Stefan Weil <address@hidden> writes:
>> 
>>> Here the int values fds[0], sigfd, s, sock and fd are converted
>>> to void pointers which are later converted back to an int value.
>>>
>>> These conversions should always use intptr_t instead of unsigned long.
>>>
>>> They are needed for environments where sizeof(long) != sizeof(void *).
>> 
>> To be precise: when you want to cast a pointer to a signed integer type
>> and back without loss, intptr_t is the signed integer type to use.
>> 
>> But here we're dealing with the opposite case: cast int to pointer and
>> back.
>> 
>>> Signed-off-by: Stefan Weil <address@hidden>
>>> ---
>>>  cpus.c           |    8 ++++----
>>>  migration-tcp.c  |    4 ++--
>>>  migration-unix.c |    4 ++--
>>>  qemu-char.c      |    4 ++--
>>>  4 files changed, 10 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/cpus.c b/cpus.c
>>> index 0f33945..3c4e1b8 100644
>>> --- a/cpus.c
>>> +++ b/cpus.c
>>> @@ -267,7 +267,7 @@ static void qemu_event_increment(void)
>>>  
>>>  static void qemu_event_read(void *opaque)
>>>  {
>>> -    int fd = (unsigned long)opaque;
>>> +    int fd = (intptr_t)opaque;
>>>      ssize_t len;
>>>      char buffer[512];
>>>  
>> 
>> Why can't you cast straight to int?
>
> You would get warnings about a pointer being cast to an integer of
> different size

Fair enough.  Stop reading here unless you like language-lawyering ;)

>                (the behaviour is undefined if the integer is too small).

Correct (I looked it up).  The detour via intptr_t makes it
implementation-defined.

> I think you might also get a warning for the opposite direction.

Implementation-defined.

The standard defines semantics of valid void * -> intptr_t, uintptr_t ->
void *: you get your original pointer back ("will compare equal").

The standard is silent on converting integer type to pointer type and
back.  Doesn't matter.  No sane implementation screws that up.