Next: , Previous: Character Escapes, Up: C Extensions

6.36 Specifying Attributes of Variables

The keyword __attribute__ allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Some attributes are currently defined generically for variables. Other attributes are defined for variables on particular target systems. Other attributes are available for functions (see Function Attributes) and for types (see Type Attributes). Other front ends might define more attributes (see Extensions to the C++ Language).

You may also specify attributes with ‘__’ preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __aligned__ instead of aligned.

See Attribute Syntax, for details of the exact syntax for using attributes.

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 that 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 which 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 will generate 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__ will still only provide you with 8 byte alignment. See your linker documentation for further information.

The aligned attribute can also be used for functions (see 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 will be 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.

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 (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, will be printed in the warning if present.

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

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.

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")));
            /* 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 will change 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.

On Microsoft Windows, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL. For example, this small program defines shared data by putting it in a named section shared and marking the section shareable:
          int foo __attribute__((section ("shared"), shared)) = 0;
            /* Read and write foo.  All running
               copies see the same value.  */
            return 0;

You may only use the shared attribute along with section attribute with a fully initialized global definition because of the way linkers work. See section attribute for more information.

The shared attribute is only available on Microsoft Windows.

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.

This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC will not produce a warning for this variable.
This attribute, attached to a variable, means that the variable must be emitted even if it appears that the variable is not referenced.
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 will be 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.

The selectany attribute causes an initialized global variable to have link-once semantics. When multiple definitions of the variable are encountered by the linker, the first is selected and the remainder are discarded. Following usage by the Microsoft compiler, the linker is told not to warn about size or content differences of the multiple definitions.

Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.

The selectany attribute is only available on Microsoft Windows targets. You can use __declspec (selectany) as a synonym for __attribute__ ((selectany)) for compatibility with other compilers.

The weak attribute is described in Function Attributes.
The dllimport attribute is described in Function Attributes.
The dllexport attribute is described in Function Attributes.

6.36.1 AVR Variable Attributes

The progmem attribute is used on the AVR to place data in the program memory address space (flash). This is accomplished by putting respective variables into a section whose name starts with .progmem.

AVR is a Harvard architecture processor and data and reas only data normally resides in the data memory address space (RAM).

6.36.2 Blackfin Variable Attributes

Three attributes are currently defined for the Blackfin.

Use these attributes on the Blackfin to place the variable into L1 Data SRAM. Variables with l1_data attribute will be put into the specific section named Those with l1_data_A attribute will be put into the specific section named Those with l1_data_B attribute will be put into the specific section named
Use this attribute on the Blackfin to place the variable into L2 SRAM. Variables with l2 attribute will be put into the specific section named

6.36.3 M32R/D Variable Attributes

One attribute is currently defined for the M32R/D.

model (model-name)
Use this attribute on the M32R/D to set the addressability of an object. The identifier model-name is one of small, medium, or large, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction).

Medium and large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses).

6.36.4 MeP Variable Attributes

The MeP target has a number of addressing modes and busses. The near space spans the standard memory space's first 16 megabytes (24 bits). The far space spans the entire 32-bit memory space. The based space is a 128 byte region in the memory space which is addressed relative to the $tp register. The tiny space is a 65536 byte region relative to the $gp register. In addition to these memory regions, the MeP target has a separate 16-bit control bus which is specified with cb attributes.

Any variable with the based attribute will be assigned to the .based section, and will be accessed with relative to the $tp register.
Likewise, the tiny attribute assigned variables to the .tiny section, relative to the $gp register.
Variables with the near attribute are assumed to have addresses that fit in a 24-bit addressing mode. This is the default for large variables (-mtiny=4 is the default) but this attribute can override -mtiny= for small variables, or override -ml.
Variables with the far attribute are addressed using a full 32-bit address. Since this covers the entire memory space, this allows modules to make no assumptions about where variables might be stored.
io (addr)
Variables with the io attribute are used to address memory-mapped peripherals. If an address is specified, the variable is assigned that address, else it is not assigned an address (it is assumed some other module will assign an address). Example:
          int timer_count __attribute__((io(0x123)));

cb (addr)
Variables with the cb attribute are used to access the control bus, using special instructions. addr indicates the control bus address. Example:
          int cpu_clock __attribute__((cb(0x123)));

6.36.5 i386 Variable Attributes

Two attributes are currently defined for i386 configurations: ms_struct and gcc_struct

If packed is used on a structure, or if bit-fields are used it may be that the Microsoft ABI packs them differently than GCC would normally pack them. Particularly when moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.

Currently -m[no-]ms-bitfields is provided for the Microsoft Windows X86 compilers to match the native Microsoft compiler.

The Microsoft structure layout algorithm is fairly simple with the exception of the bitfield packing:

The padding and alignment of members of structures and whether a bit field can straddle a storage-unit boundary

  1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
  2. Every data object has an alignment-requirement. The alignment-requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment-requirement is the largest alignment-requirement of its members. Every object is allocated an offset so that:

    offset % alignment-requirement == 0

  3. Adjacent bit fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit fields.

Handling of zero-length bitfields:

MSVC interprets zero-length bitfields in the following ways:

  1. If a zero-length bitfield is inserted between two bitfields that would normally be coalesced, the bitfields will not be coalesced.

    For example:

                      unsigned long bf_1 : 12;
                      unsigned long : 0;
                      unsigned long bf_2 : 12;
                    } t1;

    The size of t1 would be 8 bytes with the zero-length bitfield. If the zero-length bitfield were removed, t1's size would be 4 bytes.

  2. If a zero-length bitfield is inserted after a bitfield, foo, and the alignment of the zero-length bitfield is greater than the member that follows it, bar, bar will be aligned as the type of the zero-length bitfield.

    For example:

                      char foo : 4;
                      short : 0;
                      char bar;
                    } t2;
                      char foo : 4;
                      short : 0;
                      double bar;
                    } t3;

    For t2, bar will be placed at offset 2, rather than offset 1. Accordingly, the size of t2 will be 4. For t3, the zero-length bitfield will not affect the alignment of bar or, as a result, the size of the structure.

    Taking this into account, it is important to note the following:

    1. If a zero-length bitfield follows a normal bitfield, the type of the zero-length bitfield may affect the alignment of the structure as whole. For example, t2 has a size of 4 bytes, since the zero-length bitfield follows a normal bitfield, and is of type short.
    2. Even if a zero-length bitfield is not followed by a normal bitfield, it may still affect the alignment of the structure:
                             char foo : 6;
                             long : 0;
                           } t4;

      Here, t4 will take up 4 bytes.

  3. Zero-length bitfields following non-bitfield members are ignored:
                      char foo;
                      long : 0;
                      char bar;
                    } t5;

    Here, t5 will take up 2 bytes.

6.36.6 PowerPC Variable Attributes

Three attributes currently are defined for PowerPC configurations: altivec, ms_struct and gcc_struct.

For full documentation of the struct attributes please see the documentation in i386 Variable Attributes.

For documentation of altivec attribute please see the documentation in PowerPC Type Attributes.

6.36.7 SPU Variable Attributes

The SPU supports the spu_vector attribute for variables. For documentation of this attribute please see the documentation in SPU Type Attributes.

6.36.8 Xstormy16 Variable Attributes

One attribute is currently defined for xstormy16 configurations: below100.

If a variable has the below100 attribute (BELOW100 is allowed also), GCC will place the variable in the first 0x100 bytes of memory and use special opcodes to access it. Such variables will be placed in either the .bss_below100 section or the .data_below100 section.