Using the GNU Compiler Collection (GCC): Common Variable Attributes

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#### 6.32.1 Common Variable Attributes

The following attributes are supported on most targets.

`aligned (alignment)`

  This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration:

      int x __attribute__ ((aligned (16))) = 0;

  causes the compiler to allocate the global variable `x` on a 16-byte boundary. On a 68040, this could be used in conjunction with an `asm` expression to access the `move16` instruction which requires 16-byte aligned operands.

  You can also specify the alignment of structure fields. For example, to create a double-word aligned `int` pair, you could write:

      struct foo { int x[2] __attribute__ ((aligned (8))); };

  This is an alternative to creating a union with a `double` member, which forces the union to be double-word aligned.

  As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the default alignment for the target architecture you are compiling for. The default alignment is sufficient for all scalar types, but may not be enough for all vector types on a target that supports vector operations. The default alignment is fixed for a particular target ABI.

  GCC also provides a target specific macro `__BIGGEST_ALIGNMENT__`, which is the largest alignment ever used for any data type on the target machine you are compiling for. For example, you could write:

      short array[3] __attribute__ ((aligned (__BIGGEST_ALIGNMENT__)));

  The compiler automatically sets the alignment for the declared variable or field to `__BIGGEST_ALIGNMENT__`. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way. Note that the value of `__BIGGEST_ALIGNMENT__` may change depending on command-line options.

  When used on a struct, or struct member, the `aligned` attribute can only increase the alignment; in order to decrease it, the `packed` attribute must be specified as well. When used as part of a typedef, the `aligned` attribute can both increase and decrease alignment, and specifying the `packed` attribute generates a warning.

  Note that the effectiveness of `aligned` attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8-byte alignment, then specifying `aligned(16)` in an `__attribute__` still only provides you with 8-byte alignment. See your linker documentation for further information.

  The `aligned` attribute can also be used for functions (see Common Function Attributes.)

`cleanup (cleanup_function)`

  The `cleanup` attribute runs a function when the variable goes out of scope. This attribute can only be applied to auto function scope variables; it may not be applied to parameters or variables with static storage duration. The function must take one parameter, a pointer to a type compatible with the variable. The return value of the function (if any) is ignored.

  If `-fexceptions` is enabled, then cleanup_function is run during the stack unwinding that happens during the processing of the exception. Note that the `cleanup` attribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally.

`common`
`nocommon`

  The `common` attribute requests GCC to place a variable in “common” storage. The `nocommon` attribute requests the opposite—to allocate space for it directly.

  These attributes override the default chosen by the `-fno-common` and `-fcommon` flags respectively.

`deprecated`
`deprecated (msg)`

  The `deprecated` attribute results in a warning if the variable is used anywhere in the source file. This is useful when identifying variables that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated variable, to enable users to easily find further information about why the variable is deprecated, or what they should do instead. Note that the warning only occurs for uses:

      extern int old_var __attribute__ ((deprecated));
      extern int old_var;
      int new_fn () { return old_var; }

  results in a warning on line 3 but not line 2. The optional msg argument, which must be a string, is printed in the warning if present.

  The `deprecated` attribute can also be used for functions and types (see Common Function Attributes, see Common Type Attributes).

`mode (mode)`

  This attribute specifies the data type for the declaration—whichever type corresponds to the mode mode. This in effect lets you request an integer or floating-point type according to its width.

  See Machine Modes in GNU Compiler Collection (GCC) Internals, for a list of the possible keywords for mode. You may also specify a mode of `byte` or `__byte__` to indicate the mode corresponding to a one-byte integer, `word` or `__word__` for the mode of a one-word integer, and `pointer` or `__pointer__` for the mode used to represent pointers.

`packed`

  The `packed` attribute specifies that a variable or structure field should have the smallest possible alignment—one byte for a variable, and one bit for a field, unless you specify a larger value with the `aligned` attribute.

  Here is a structure in which the field `x` is packed, so that it immediately follows `a`:

      struct foo
      {
        char a;
        int x[2] __attribute__ ((packed));
      };

  Note: The 4.1, 4.2 and 4.3 series of GCC ignore the `packed` attribute on bit-fields of type `char`. This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. See the documentation of `-Wpacked-bitfield-compat` for more information.

`section ("section-name")`

  Normally, the compiler places the objects it generates in sections like `data` and `bss`. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The `section` attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names:

      struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
      struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
      char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
      int init_data __attribute__ ((section ("INITDATA")));

      main()
      {
        /* Initialize stack pointer */
        init_sp (stack + sizeof (stack));

        /* Initialize initialized data */
        memcpy (&init_data, &data, &edata - &data);

        /* Turn on the serial ports */
        init_duart (&a);
        init_duart (&b);
      }

  Use the `section` attribute with global variables and not local variables, as shown in the example.

  You may use the `section` attribute with initialized or uninitialized global variables but the linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the `common` (or `bss`) section and can be multiply “defined”. Using the `section` attribute changes what section the variable goes into and may cause the linker to issue an error if an uninitialized variable has multiple definitions. You can force a variable to be initialized with the `-fno-common` flag or the `nocommon` attribute.

  Some file formats do not support arbitrary sections so the `section` attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

`tls_model ("tls_model")`

  The `tls_model` attribute sets thread-local storage model (see Thread-Local) of a particular `__thread` variable, overriding `-ftls-model=` command-line switch on a per-variable basis. The tls_model argument should be one of `global-dynamic`, `local-dynamic`, `initial-exec` or `local-exec`.

  Not all targets support this attribute.

`unused`

  This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC does not produce a warning for this variable.

`used`

  This attribute, attached to a variable with static storage, means that the variable must be emitted even if it appears that the variable is not referenced.

  When applied to a static data member of a C++ class template, the attribute also means that the member is instantiated if the class itself is instantiated.

`vector_size (bytes)`

  This attribute specifies the vector size for the variable, measured in bytes. For example, the declaration:

      int foo __attribute__ ((vector_size (16)));

  causes the compiler to set the mode for `foo`, to be 16 bytes, divided into `int` sized units. Assuming a 32-bit int (a vector of 4 units of 4 bytes), the corresponding mode of `foo` is V4SI.

  This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.

  Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:

      struct S { int a; };
      struct S  __attribute__ ((vector_size (16))) foo;

  is invalid even if the size of the structure is the same as the size of the `int`.

`visibility ("visibility_type")`

  This attribute affects the linkage of the declaration to which it is attached. The `visibility` attribute is described in Common Function Attributes.

`weak`

  The `weak` attribute is described in Common Function Attributes.

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