Compiling gcc code in Visual Studio causes error C3646: '__attribute__': unknown override specifier

Question

I am getting the following error:

error C3646: \'__attribute__\': unknown override specifier

Code:

LEMUR_PREALIGN ch         


        

Answer 1

The __attribute__ command is a compiler specific command to gcc. And it is used on line 52 of this file with the ((align)) command, which:

Specifies a minimum alignment (in bytes) for variables of the specified type

Visual studio does in fact have a similar alignment command: align. But there are two problems:

1. __declspec(align(#)) does not support the defaulted: __attribute__ ((aligned)) behavior which will:

Align a type to the maximum useful alignment for the target machine you are compiling for

2. __declspec(align(#)) is a prefix. __attribute__((aligned(#))) is a suffix. This means that your actual code would need to differ on the placement:

struct S { short f[3]; } __attribute__ ((aligned)); // gcc alignment definition __declspec(align(16)) strict S { short f[3]; }; // MSVC alignment

The point here is you'd probably be better off #ifdefing by compiler any line that uses __attribute__ ((aligned)) and cooking your own __declspec(align(#)).

For more info see: GCC vs MSVC class packing and alignment

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After a bit more study into lemur_platform.h it looks like the code has already done all the above work for you! You'll notice that #define LEMUR_POSTALIGN __attribute__ ((aligned)) is wrapped in an #ifndef WIN32. So what you need to do is define WIN32 in your Visual Studio project!