2010年12月9日 星期四
2010年6月25日 星期五
Bounce Buffer
Bounce Buffer是一個資料傳遞的暫存空間，在計算機系統中，有時週邊裝置的定址能力與CPU不相同，此時週邊裝置將無法存取超出其定址能力範圍的記憶體位址，為解決這個問題，因而有了Bounce Buffer機制的出現。Device可以透過DMA將資料搬運到可定址的Bounce Buffer中，然後CPU再將Bounce Buffer內的內容搬到系統期待的位址。
2010年5月26日 星期三
x86的Descriptor Table Control Register
gdtr : Global Descriptor Table Control Register. It points to address and size of GDT in main memory.
ldtr : Local Descriptor Table Control Register. It points to address and size of the currently used LDT.
ldtr : Local Descriptor Table Control Register. It points to address and size of the currently used LDT.
x86的六個Segmentation Register
CS : The code segment register, which points to a segment containing program instructions.
SS : The stack segment register, which points to a segment containing the current program stack.
DS : The data segment register, which points to a segment containing global and static data.
ES :
FS :
GS :
SS : The stack segment register, which points to a segment containing the current program stack.
DS : The data segment register, which points to a segment containing global and static data.
ES :
FS :
GS :
2010年5月25日 星期二
Attribute Syntax
http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Attribute-Syntax.html#Attribute-Syntax
This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective-C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.
There are some problems with the semantics of attributes in C++. For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading. Similarly, typeid does not distinguish between types with different attributes. Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators.
See Function Attributes, for details of the semantics of attributes applying to functions. See Variable Attributes, for details of the semantics of attributes applying to variables. See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types.
An attribute specifier is of the form __attribute__ ((attribute-list)). An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:
* Empty. Empty attributes are ignored.
* A word (which may be an identifier such as unused, or a reserved word such as const).
* A word, followed by, in parentheses, parameters for the attribute. These parameters take one of the following forms:
o An identifier. For example, mode attributes use this form.
o An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, format attributes use this form.
o A possibly empty comma-separated list of expressions. For example, format_arg attributes use this form with the list being a single integer constant expression, and alias attributes use this form with the list being a single string constant.
An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.
In GNU C, an attribute specifier list may appear after the colon following a label, other than a case or default label. The only attribute it makes sense to use after a label is unused. This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with -Wall. It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional. GNU C++ does not permit such placement of attribute lists, as it is permissible for a declaration, which could begin with an attribute list, to be labelled in C++. Declarations cannot be labelled in C90 or C99, so the ambiguity does not arise there.
An attribute specifier list may appear as part of a struct, union or enum specifier. It may go either immediately after the struct, union or enum keyword, or after the closing brace. It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used—that is, in usages such as struct __attribute__((foo)) bar with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.
Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.
Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.
At present, the first parameter in a function prototype must have some type specifier which is not an attribute specifier; this resolves an ambiguity in the interpretation of void f(int (__attribute__((foo)) x)), but is subject to change. At present, if the parentheses of a function declarator contain only attributes then those attributes are ignored, rather than yielding an error or warning or implying a single parameter of type int, but this is subject to change.
An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
d2 (void)
the noreturn attribute applies to all the functions declared; the format attribute only applies to d1.
An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator. In simple cases there is no difference, but, for example, in
void (****f)(void) __attribute__((noreturn));
at present the noreturn attribute applies to f, which causes a warning since f is not a function, but in future it may apply to the function ****f. The precise semantics of what attributes in such cases will apply to are not yet specified. Where an assembler name for an object or function is specified (see Asm Labels), at present the attribute must follow the asm specification; in future, attributes before the asm specification may apply to the adjacent declarator, and those after it to the declared object or function.
An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).
Attribute specifiers may be mixed with type qualifiers appearing inside the [] of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted. Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored.
An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the * of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax. It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type Type (such as int) and D1 is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.
If D1 has the form ( attribute-specifier-list D ), and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list attribute-specifier-list Type” for ident.
If D1 has the form * type-qualifier-and-attribute-specifier-list D, and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list type-qualifier-and-attribute-specifier-list Type” for ident.
For example,
void (__attribute__((noreturn)) ****f) (void);
specifies the type “pointer to pointer to pointer to pointer to non-returning function returning void”. As another example,
char *__attribute__((aligned(8))) *f;
specifies the type “pointer to 8-byte-aligned pointer to char”. Note again that this does not work with most attributes; for example, the usage of `aligned' and `noreturn' attributes given above is not yet supported.
For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.
This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective-C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.
There are some problems with the semantics of attributes in C++. For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading. Similarly, typeid does not distinguish between types with different attributes. Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators.
See Function Attributes, for details of the semantics of attributes applying to functions. See Variable Attributes, for details of the semantics of attributes applying to variables. See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types.
An attribute specifier is of the form __attribute__ ((attribute-list)). An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:
* Empty. Empty attributes are ignored.
* A word (which may be an identifier such as unused, or a reserved word such as const).
* A word, followed by, in parentheses, parameters for the attribute. These parameters take one of the following forms:
o An identifier. For example, mode attributes use this form.
o An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, format attributes use this form.
o A possibly empty comma-separated list of expressions. For example, format_arg attributes use this form with the list being a single integer constant expression, and alias attributes use this form with the list being a single string constant.
An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.
In GNU C, an attribute specifier list may appear after the colon following a label, other than a case or default label. The only attribute it makes sense to use after a label is unused. This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with -Wall. It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional. GNU C++ does not permit such placement of attribute lists, as it is permissible for a declaration, which could begin with an attribute list, to be labelled in C++. Declarations cannot be labelled in C90 or C99, so the ambiguity does not arise there.
An attribute specifier list may appear as part of a struct, union or enum specifier. It may go either immediately after the struct, union or enum keyword, or after the closing brace. It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used—that is, in usages such as struct __attribute__((foo)) bar with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.
Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.
Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.
At present, the first parameter in a function prototype must have some type specifier which is not an attribute specifier; this resolves an ambiguity in the interpretation of void f(int (__attribute__((foo)) x)), but is subject to change. At present, if the parentheses of a function declarator contain only attributes then those attributes are ignored, rather than yielding an error or warning or implying a single parameter of type int, but this is subject to change.
An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
d2 (void)
the noreturn attribute applies to all the functions declared; the format attribute only applies to d1.
An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator. In simple cases there is no difference, but, for example, in
void (****f)(void) __attribute__((noreturn));
at present the noreturn attribute applies to f, which causes a warning since f is not a function, but in future it may apply to the function ****f. The precise semantics of what attributes in such cases will apply to are not yet specified. Where an assembler name for an object or function is specified (see Asm Labels), at present the attribute must follow the asm specification; in future, attributes before the asm specification may apply to the adjacent declarator, and those after it to the declared object or function.
An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).
Attribute specifiers may be mixed with type qualifiers appearing inside the [] of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted. Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored.
An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the * of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax. It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type Type (such as int) and D1 is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.
If D1 has the form ( attribute-specifier-list D ), and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list attribute-specifier-list Type” for ident.
If D1 has the form * type-qualifier-and-attribute-specifier-list D, and the declaration T D specifies the type “derived-declarator-type-list Type” for ident, then T D1 specifies the type “derived-declarator-type-list type-qualifier-and-attribute-specifier-list Type” for ident.
For example,
void (__attribute__((noreturn)) ****f) (void);
specifies the type “pointer to pointer to pointer to pointer to non-returning function returning void”. As another example,
char *__attribute__((aligned(8))) *f;
specifies the type “pointer to 8-byte-aligned pointer to char”. Note again that this does not work with most attributes; for example, the usage of `aligned' and `noreturn' attributes given above is not yet supported.
For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.
Specifying Attributes of Variables
http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Variable-Attributes.html#Variable-Attributes
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 maximum useful alignment for the target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which 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 or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.
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.
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.
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
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 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.
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));
};
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"))) = 0;
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 an initialized definition of a global variable, as shown in the example. GCC issues a warning and otherwise ignores the section attribute in uninitialized variable declarations.
You may only use the section attribute with a fully initialized global definition because of the way linkers work. 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”. 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.
shared
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;
int
main()
{
/* 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.
transparent_union
This attribute, attached to a function parameter which is a union, means that the corresponding argument may have the type of any union member, but the argument is passed as if its type were that of the first union member. For more details see See Type Attributes. You can also use this attribute on a typedef for a union data type; then it applies to all function parameters with that type.
unused
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.
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.
weak
The weak attribute is described in See Function Attributes.
dllimport
The dllimport attribute is described in See Function Attributes.
dlexport
The dllexport attribute is described in See Function Attributes.
5.31.1 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).
5.31.2 i386 Variable Attributes
Two attributes are currently defined for i386 configurations: ms_struct and gcc_struct
ms_struct
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.
5.31.3 Xstormy16 Variable Attributes
One attribute is currently defined for xstormy16 configurations: below100
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.
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 maximum useful alignment for the target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which 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 or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.
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.
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.
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
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 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.
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));
};
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"))) = 0;
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 an initialized definition of a global variable, as shown in the example. GCC issues a warning and otherwise ignores the section attribute in uninitialized variable declarations.
You may only use the section attribute with a fully initialized global definition because of the way linkers work. 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”. 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.
shared
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;
int
main()
{
/* 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.
transparent_union
This attribute, attached to a function parameter which is a union, means that the corresponding argument may have the type of any union member, but the argument is passed as if its type were that of the first union member. For more details see See Type Attributes. You can also use this attribute on a typedef for a union data type; then it applies to all function parameters with that type.
unused
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.
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.
weak
The weak attribute is described in See Function Attributes.
dllimport
The dllimport attribute is described in See Function Attributes.
dlexport
The dllexport attribute is described in See Function Attributes.
5.31.1 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).
5.31.2 i386 Variable Attributes
Two attributes are currently defined for i386 configurations: ms_struct and gcc_struct
ms_struct
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.
5.31.3 Xstormy16 Variable Attributes
One attribute is currently defined for xstormy16 configurations: below100
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.
Specifying Attributes of Types
http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Type-Attributes.html#Type-Attributes
The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Six attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated and may_alias. Other attributes are defined for functions (see Function Attributes) and for variables (see Variable Attributes).
You may also specify any one of these attributes with `__' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use __aligned__ instead of aligned.
You may specify the aligned and transparent_union attributes either in a typedef declaration or just past the closing curly brace of a complete enum, struct or union type definition and the packed attribute only past the closing brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
See Attribute Syntax, for details of the exact syntax for using attributes.
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 insure (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. 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 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. 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 which 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 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 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 will also be doing 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 will often be more efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.
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.
packed
This attribute, attached to 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. Specifying the -fshort-enums flag on the 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 would need to be packed too.
struct my_unpacked_struct
{
char c;
int i;
};
struct my_packed_struct __attribute__ ((__packed__))
{
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.
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, might define the interface as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
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 will 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.
deprecated
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 deprecated attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)
may_alias
Accesses to objects with types with this attribute are not subjected to type-based alias analysis, but are instead assumed to be able to alias any other type of objects, just like the char type. See -fstrict-aliasing for more information on aliasing issues.
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 in recent GCC versions.
5.32.1 ARM Type Attributes
On those ARM targets that support dllimport (such as Symbian OS), you can use the notshared attribute to indicate that the virtual table and other similar data for a class should not be exported from a DLL. For example:
class __declspec(notshared) C {
public:
__declspec(dllimport) C();
virtual void f();
}
__declspec(dllexport)
C::C() {}
In this code, C::C is exported from the current DLL, but the virtual table for C is not exported. (You can use __attribute__ instead of __declspec if you prefer, but most Symbian OS code uses __declspec.)
5.32.2 i386 Type Attributes
Two attributes are currently defined for i386 configurations: ms_struct and gcc_struct
ms_struct
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.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Six attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated and may_alias. Other attributes are defined for functions (see Function Attributes) and for variables (see Variable Attributes).
You may also specify any one of these attributes with `__' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use __aligned__ instead of aligned.
You may specify the aligned and transparent_union attributes either in a typedef declaration or just past the closing curly brace of a complete enum, struct or union type definition and the packed attribute only past the closing brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
See Attribute Syntax, for details of the exact syntax for using attributes.
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 insure (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. 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 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. 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 which 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 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 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 will also be doing 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 will often be more efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.
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.
packed
This attribute, attached to 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. Specifying the -fshort-enums flag on the 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 would need to be packed too.
struct my_unpacked_struct
{
char c;
int i;
};
struct my_packed_struct __attribute__ ((__packed__))
{
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.
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,
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
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 will 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.
deprecated
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 deprecated attribute can also be used for functions and variables (see Function Attributes, see Variable Attributes.)
may_alias
Accesses to objects with types with this attribute are not subjected to type-based alias analysis, but are instead assumed to be able to alias any other type of objects, just like the char type. See -fstrict-aliasing for more information on aliasing issues.
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 in recent GCC versions.
5.32.1 ARM Type Attributes
On those ARM targets that support dllimport (such as Symbian OS), you can use the notshared attribute to indicate that the virtual table and other similar data for a class should not be exported from a DLL. For example:
class __declspec(notshared) C {
public:
__declspec(dllimport) C();
virtual void f();
}
__declspec(dllexport)
C::C() {}
In this code, C::C is exported from the current DLL, but the virtual table for C is not exported. (You can use __attribute__ instead of __declspec if you prefer, but most Symbian OS code uses __declspec.)
5.32.2 i386 Type Attributes
Two attributes are currently defined for i386 configurations: ms_struct and gcc_struct
ms_struct
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.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
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