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6.35.1 Common Type Attributes

The following type attributes are supported on most targets.

aligned
aligned (alignment)

The aligned attribute specifies a minimum alignment (in bytes) for variables of the specified type. When specified, alignment must be a power of 2. Specifying no alignment argument implies the maximum alignment for the target, which is often, but by no means always, 8 or 16 bytes. For example, the declarations: 

struct __attribute__ ((aligned (8))) S { short f[3]; };
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 is allocated and aligned at least on a 8-byte boundary. On a SPARC, having all variables of type struct S aligned to 8-byte boundaries allows the compiler to use the ldd and std (doubleword load and store) instructions when copying one variable of type struct S to another, thus improving run-time efficiency.

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

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 that is ever used for any data type on the target machine you are compiling for. 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 that have types that you have aligned this way.

In the example above, if the size of each short is 2 bytes, then the size of the entire struct S type is 6 bytes. The smallest power of two that 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 although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler’s ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program also does pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations is often more efficient for efficiently-aligned types than for other types.

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.

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.

warn_if_not_aligned (alignment)

This attribute specifies a threshold for the structure field, measured in bytes. If the structure field is aligned below the threshold, a warning will be issued. For example, the declaration: 

typedef unsigned long long __u64
   __attribute__((aligned (4), warn_if_not_aligned (8)));

struct foo
{
  int i1;
  int i2;
  __u64 x;
};

causes the compiler to issue an warning on struct foo, like ‘warning: alignment 4 of 'struct foo' is less than 8’. It is used to define struct foo in such a way that struct foo has the same layout and the structure field x has the same alignment when __u64 is aligned at either 4 or 8 bytes. Align struct foo to 8 bytes: 

struct __attribute__ ((aligned (8))) foo
{
  int i1;
  int i2;
  __u64 x;
};

silences the warning. The compiler also issues a warning, like ‘warning: 'x' offset 12 in 'struct foo' isn't aligned to 8’, when the structure field has the misaligned offset: 

struct __attribute__ ((aligned (8))) foo
{
  int i1;
  int i2;
  int i3;
  __u64 x;
};

This warning can be disabled by -Wno-if-not-aligned.

alloc_size (position)
alloc_size (position-1, position-2)

The alloc_size type attribute may be applied to the definition of a type of a function that returns a pointer and takes at least one argument of an integer type. It indicates that the returned pointer points to an object whose size is given by the function argument at position-1, or by the product of the arguments at position-1 and position-2. Meaningful sizes are positive values less than PTRDIFF_MAX. Other sizes are disagnosed when detected. GCC uses this information to improve the results of __builtin_object_size.

For instance, the following declarations 

typedef __attribute__ ((alloc_size (1, 2))) void*
  calloc_type (size_t, size_t);
typedef __attribute__ ((alloc_size (1))) void*
  malloc_type (size_t);

specify that calloc_type is a type of a function that, like the standard C function calloc, returns an object whose size is given by the product of arguments 1 and 2, and that malloc_type, like the standard C function malloc, returns an object whose size is given by argument 1 to the function.

copy
copy (expression)

The copy attribute applies the set of attributes with which the type of the expression has been declared to the declaration of the type to which the attribute is applied. The attribute is designed for libraries that define aliases that are expected to specify the same set of attributes as the aliased symbols. The copy attribute can be used with types, variables, or functions. However, the kind of symbol to which the attribute is applied (either varible or function) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol’s linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied. See Common Function Attributes. See Common Variable Attributes.

For example, suppose struct A below is defined in some third party library header to have the alignment requirement N and to force a warning whenever a variable of the type is not so aligned due to attribute packed. Specifying the copy attribute on the definition on the unrelated struct B has the effect of copying all relevant attributes from the type referenced by the pointer expression to struct B. 

struct __attribute__ ((aligned (N), warn_if_not_aligned (N)))
A { /* … */ };
struct __attribute__ ((copy ( (struct A *)0)) B { /* … */ };
deprecated
deprecated (msg)

The deprecated attribute results in a warning if the type is used anywhere in the source file. This is useful when identifying types that are expected to be removed in a future version of a program. If possible, the warning also includes the location of the declaration of the deprecated type, to enable users to easily find further information about why the type is deprecated, or what they should do instead. Note that the warnings only occur for uses and then only if the type is being applied to an identifier that itself is not being declared as deprecated. 

typedef int T1 __attribute__ ((deprecated));
T1 x;
typedef T1 T2;
T2 y;
typedef T1 T3 __attribute__ ((deprecated));
T3 z __attribute__ ((deprecated));

results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6. The optional msg argument, which must be a string, is printed in the warning if present. Control characters in the string will be replaced with escape sequences, and if the -fmessage-length option is set to 0 (its default value) then any newline characters will be ignored.

The deprecated attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)

The message attached to the attribute is affected by the setting of the -fmessage-length option.

unavailable
unavailable (msg)

The unavailable attribute behaves in the same manner as the deprecated one, but emits an error rather than a warning. It is used to indicate that a (perhaps previously deprecated) type is no longer usable.

The unavailable attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)

designated_init

This attribute may only be applied to structure types. It indicates that any initialization of an object of this type must use designated initializers rather than positional initializers. The intent of this attribute is to allow the programmer to indicate that a structure’s layout may change, and that therefore relying on positional initialization will result in future breakage.

GCC emits warnings based on this attribute by default; use -Wno-designated-init to suppress them.

may_alias

Accesses through pointers to types with this attribute are not subject to type-based alias analysis, but are instead assumed to be able to alias any other type of objects. In the context of section 6.5 paragraph 7 of the C99 standard, an lvalue expression dereferencing such a pointer is treated like having a character type. See -fstrict-aliasing for more information on aliasing issues. This extension exists to support some vector APIs, in which pointers to one vector type are permitted to alias pointers to a different vector type.

Note that an object of a type with this attribute does not have any special semantics.

Example of use: 

typedef short __attribute__ ((__may_alias__)) short_a;

int
main (void)
{
  int a = 0x12345678;
  short_a *b = (short_a *) &a;

  b[1] = 0;

  if (a == 0x12345678)
    abort();

  exit(0);
}

If you replaced short_a with short in the variable declaration, the above program would abort when compiled with -fstrict-aliasing, which is on by default at -O2 or above.

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

This attribute, attached to a struct, union, or C++ class type definition, specifies that each of its members (other than zero-width bit-fields) is placed to minimize the memory required. This is equivalent to specifying the packed attribute on each of the members.

When attached to an enum definition, the packed attribute indicates that the smallest integral type should be used. Specifying the -fshort-enums flag on the command line is equivalent to specifying the packed attribute on all enum definitions.

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 needs 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 the packed attribute on the definition of an enum, struct, union, or class, not on a typedef that does not also define the enumerated type, structure, union, or class.

scalar_storage_order ("endianness")

When attached to a union or a struct, this attribute sets the storage order, aka endianness, of the scalar fields of the type, as well as the array fields whose component is scalar. The supported endiannesses are big-endian and little-endian. The attribute has no effects on fields which are themselves a union, a struct or an array whose component is a union or a struct, and it is possible for these fields to have a different scalar storage order than the enclosing type.

Note that neither pointer nor vector fields are considered scalar fields in this context, so the attribute has no effects on these fields.

This attribute is supported only for targets that use a uniform default scalar storage order (fortunately, most of them), i.e. targets that store the scalars either all in big-endian or all in little-endian.

Additional restrictions are enforced for types with the reverse scalar storage order with regard to the scalar storage order of the target:

These restrictions exist because the storage order attribute is lost when the address of a scalar or the address of an array with scalar component is taken, so storing indirectly through this address generally does not work. The second case is nevertheless allowed to be able to perform a block copy from or to the array.

Moreover, the use of type punning or aliasing to toggle the storage order is not supported; that is to say, if a given scalar object can be accessed through distinct types that assign a different storage order to it, then the behavior is undefined.

transparent_union

This attribute, attached to a union type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way.

First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required. Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like const on the referenced type must be respected, just as with normal pointer conversions.

Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.

Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons. For example, suppose the wait function must accept either a value of type int * to comply with POSIX, or a value of type union wait * to comply with the 4.1BSD interface. If wait’s parameter were void *, wait would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful. Instead, <sys/wait.h> might define the interface as follows: 

typedef union __attribute__ ((__transparent_union__))
  {
    int *__ip;
    union wait *__up;
  } wait_status_ptr_t;

pid_t wait (wait_status_ptr_t);

This interface allows either int * or union wait * arguments to be passed, using the int * calling convention. The program can call wait with arguments of either type: 

int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }

With this interface, wait’s implementation might look like this: 

pid_t wait (wait_status_ptr_t p)
{
  return waitpid (-1, p.__ip, 0);
}
unused

When attached to a type (including a union or a struct), this attribute means that variables of that type are meant to appear possibly unused. GCC does not produce a warning for any variables of that type, even if the variable appears to do nothing. This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions.

vector_size (bytes)

This attribute specifies the vector size for the type, measured in bytes. The type to which it applies is known as the base type. The bytes argument must be a positive power-of-two multiple of the base type size. For example, the following declarations: 

typedef __attribute__ ((vector_size (32))) int int_vec32_t ;
typedef __attribute__ ((vector_size (32))) int* int_vec32_ptr_t;
typedef __attribute__ ((vector_size (32))) int int_vec32_arr3_t[3];

define int_vec32_t to be a 32-byte vector type composed of int sized units. With int having a size of 4 bytes, the type defines a vector of eight units, four bytes each. The mode of variables of type int_vec32_t is V8SI. int_vec32_ptr_t is then defined to be a pointer to such a vector type, and int_vec32_arr3_t to be an array of three such vectors. See Vector Extensions, for details of manipulating objects of vector types.

This attribute is only applicable to integral and floating scalar types. In function declarations the attribute applies to the function return type.

For example, the following: 

__attribute__ ((vector_size (16))) float get_flt_vec16 (void);

declares get_flt_vec16 to be a function returning a 16-byte vector with the base type float.

visibility

In C++, attribute visibility (see Function Attributes) can also be applied to class, struct, union and enum types. Unlike other type attributes, the attribute must appear between the initial keyword and the name of the type; it cannot appear after the body of the type.

Note that the type visibility is applied to vague linkage entities associated with the class (vtable, typeinfo node, etc.). In particular, if a class is thrown as an exception in one shared object and caught in another, the class must have default visibility. Otherwise the two shared objects are unable to use the same typeinfo node and exception handling will break.

objc_root_class (Objective-C and Objective-C++ only)

This attribute marks a class as being a root class, and thus allows the compiler to elide any warnings about a missing superclass and to make additional checks for mandatory methods as needed.

To specify multiple attributes, separate them by commas within the double parentheses: for example, ‘__attribute__ ((aligned (16), packed))’.

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