[Spice-devel] [PATCH spice-protocol] vd_agent: Do not define structures without fields

[Uri Lublin]

On 05/31/2018 05:58 PM, Frediano Ziglio wrote:
>>
>> According to C standard the size of a structure without fields is
>> not defined (see
>> https://stackoverflow.com/questions/21851930/size-of-empty-structure-in-c-and-c).
>> As use almost use GCC we used these structures having 0-size. This is
> 
> As we almost only use GCC ...

.. currently these structures are of size 0.

This can become a problem ...  if 2 sides ...

> 
>> usually not a problem however can became a problem if 2 side talking
> 
> side -> sides
> 
>> a given protocol uses these structures.
> 
> uses -> use
> 
>> To avoid such possible mismatch (for instance having one side C and the
>> other C++ which define these structure as having 1 as size) add an
> 
> define -> defines, structure -> structures
> 
>> empty (0-size array) field.
>> Although a 0-size array is not considered also that portable we use
>> extensively this feature with different compiler never encountering
> 
> compiler -> compilers
> 
>> issues.
>>
>> Signed-off-by: Frediano Ziglio <fziglio at redhat.com>

Ack.

I tested it with a following simple program, f.c,
(building it with gcc and g++):
---
#include <stdio.h>
#include <spice/vd_agent.h>

int main()
{
	printf("sizeof(VDAgentClipboardRelease)=%lu\n", 
sizeof(VDAgentClipboardRelease));
	return 0;
}
---

Uri.

> 
> I should have checked before sending.
> 
>> ---
>>   spice/vd_agent.h | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/spice/vd_agent.h b/spice/vd_agent.h
>> index 7109ede..dda7044 100644
>> --- a/spice/vd_agent.h
>> +++ b/spice/vd_agent.h
>> @@ -234,6 +234,7 @@ typedef struct SPICE_ATTR_PACKED VDAgentClipboardRelease
>> {
>>       uint8_t selection;
>>       uint8_t __reserved[sizeof(uint32_t) - 1 * sizeof(uint8_t)];
>>   #endif
>> +    uint8_t dummy_empty_field[0]; /* C/C++ compatibility */
>>   } VDAgentClipboardRelease;
>>   
>>   typedef struct SPICE_ATTR_PACKED VDAgentMaxClipboard {
> 
> Frediano
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