Attributes of Types
Attributes of Types
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Attributes of Types Attributes of Types.

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__attribute__ ((aligned)) __attribute__ ((aligned (n))) __attribute__ ((packed))

## Description

The keyword `__attribute__` allows you to specify special attributes of `enum`, `struct` and `union` types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Two attributes are currently defined for types: aligned, and packed.

You may specify type attributes in an `enum`, `struct`, or `union` type declaration or definition, or for other types in a `typedef` declaration.

For an `enum`, `struct`, or `union` type, you may specify attributes either between the `enum`, `struct` or `union` tag and the name of the type, or just past the closing curly brace of the definition. The former syntax is preferred.

## aligned (`alignment`)

This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations:

`struct S { short f[3]; } __attribute__ ((aligned (8))); typedef int more_aligned_int __attribute__ ((aligned (8)));`

force the compiler to ensure (as far as it can) that each variable whose type is `struct S` or `more_aligned_int` will be allocated and aligned at least on a 8-byte boundary.

Note that the alignment of any given `struct` or `union` type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the `struct` or `union` in question and must also be a power of two. This means that you can effectively adjust the alignment of a `struct` or `union` type by attaching an aligned attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire `struct` or `union` type.

As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given `struct` or `union` type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

`struct S { short f[3]; } __attribute__ ((aligned));`

Whenever you leave out the alignment factor in an `aligned` attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. In the example above, the size of each `short` is 2 bytes, and therefore the size of the entire `struct S` type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire `struct S` type to 8 bytes.

Note that the effectiveness of aligned attributes may be limited by inherent limitations of the OpenCL device and compiler. For some devices, the OpenCL compiler may only be able to arrange for variables to be aligned up to a certain maximum alignment. If the OpenCL compiler is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)` in an `__attribute__` will still only provide you with 8 byte alignment. See your platform-specific documentation for further information.

The `aligned` attribute can only increase the alignment; but you can decrease it by specifying `packed` as well. See below.

## packed

The `packed` attribute, attached to the `struct` or `union` type definition, specifies that each member of the structure or union is placed to minimize the memory required. When attached to an `enum` definition, it indicates that the smallest integral type should be used.

Specifying this attribute for `struct` and `union` types is equivalent to specifying the `packed` attribute on each of the structure or union members.

In the following example `struct my_packed_struct`'s members are packed closely together, but the internal layout of its `s` member is not packed. To do that, `struct my_unpacked_struct` would need to be packed, too.

`struct my_unpacked_struct { char c; int i; }; struct __attribute__ ((packed)) my_packed_struct { char c; int i; struct my_unpacked_struct s; };`

You may only specify this attribute on the definition of a `enum`, `struct`, or `union`, not on a `typedef` which does not also define the enumerated type, structure or union.

## Specification

OpenCL Specification

## Also see

__attribute__, Blocks and Control-Flow Statement Attributes Attributes, Variable Attributes

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