[Mono-dev] Number of elements in a fixed buffer

[Zoltan Varga]

Hi,

The class has an internal valuetype which represents the fixed buffer, and
that valuetype has the FixedBufferAttribute.

Runtime code generally doesn't need to care about fixed buffers, why is
this needed ?

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

.class public sequential ansi sealed beforefieldinit double_array4

       extends [mscorlib]System.ValueType

{

  .class sequential ansi sealed nested public beforefieldinit
'<f1>__FixedBuffer0'

         extends [mscorlib]System.ValueType

  {

    .pack 0

    .size 32

    .custom instance void
[mscorlib]System.Runtime.CompilerServices.UnsafeValueTypeAttribute::.ctor()
= ( 01 00 00 00 )

    .custom instance void
[mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor()
= ( 01 00 00 00 )

    .field public float64 FixedElementField

  } // end of class '<f1>__FixedBuffer0'

  .field public valuetype double_array4/'<f1>__FixedBuffer0' f1

  .custom instance void
[mscorlib]System.Runtime.CompilerServices.FixedBufferAttribute::.ctor(class
[mscorlib]System.Type,


                  int32) = ( 01 00 5A 53 79 73 74 65 6D 2E 44 6F 75 62 6C
65   // ..ZSystem.Double


                              2C 20 6D 73 63 6F 72 6C 69 62 2C 20 56 65 72
73   // , mscorlib, Vers


                              69 6F 6E 3D 34 2E 30 2E 30 2E 30 2C 20 43 75
6C   // ion=4.0.0.0, Cul


                              74 75 72 65 3D 6E 65 75 74 72 61 6C 2C 20 50
75   // ture=neutral, Pu


                              62 6C 69 63 4B 65 79 54 6F 6B 65 6E 3D 62 37
37   // blicKeyToken=b77


                              61 35 63 35 36 31 39 33 34 65 30 38 39 04 00
00   // a5c561934e089...


                              00 00 00 )

} // end of class double_array4

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

On Mon, Aug 10, 2015 at 10:53 AM, Bill Seurer <seurer at linux.vnet.ibm.com>
wrote:

> The only mention of FixedBufferAttribute I see is in the C# code in mcs.
>
> I looked through all the mono C code and I see several places where
> MonoCustomAttrInfo is used but no where is it doing anything with fixed
> buffers.  Is there some documentation or more examples of what is in the
> MonoCustomAttrInfo data for something like this?
>
> I experimented a bit and used mono_custom_attrs_from_class() to pull the
> MonoCustomAttrInfo for the class.  It looks like there are two attributes.
>
> {num_attrs = 2, cached = 0, image = 0x10566ed0, attrs = 0x1061c630}
>
> So looking at the two attributes I see
>
> attrs[0]:  {ctor = 0x1061cb10, data_size = 4, data = 0x3fffb7840e71 "\001"}
> attrs[1]:  {ctor = 0x1061c990, data_size = 4, data = 0x3fffb7840e71 "\001"}
>
>
> The data fields are identical and are 01 00 00 00 or maybe the other way
> around depending on what the field represents (this is a LE machine).
>
> The ctors are
>
> (gdb) print cinfo->attrs[0].ctor->klass->name
> $14 = 0x3fffb5b225b6 "UnsafeValueTypeAttribute"
> (gdb) print cinfo->attrs[1].ctor->klass->name
> $15 = 0x3fffb5b22281 "CompilerGeneratedAttribute"
>
> What do those represent?
>
>
> On 08/06/2015 12:23 PM, Zoltan Varga wrote:
>
>> Hi,
>>
>>    The type has a FixedBufferAttribute custom attribute which contains
>> the length of the array. There are some functions in reflection.c
>> like mono_custom_attrs_from_class () which can return information about
>> it.
>>
>>                 Zoltan
>>
>> On Thu, Aug 6, 2015 at 12:32 PM, Bill Seurer <seurer at linux.vnet.ibm.com
>> <mailto:seurer at linux.vnet.ibm.com>> wrote:
>>
>>     In some code in mono/mini I need to figure out how many elements
>>     there are in a fixed buffer field in a struct, something like this:
>>
>>              public unsafe struct double_array4 {
>>                      public fixed double f1[4];
>>              }
>>
>>     So I'd need to know "4".
>>
>>     I can get the MonoClass of the field from the MonoFieldType and if I
>>     print out the name I get
>>
>>     Test_double.double_array4.<f1>__FixedBuffer0
>>
>>     so it knows it is a fixed buffer.  If I look at the fields of the
>>     struct in the above example there is just one and it's a double.
>>
>>     So how can I figure out the number of elements?
>>
>>     Thanks!
>>
>
>
> --
>
> -Bill Seurer
>
>
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