This is doc/gcc.info, produced by makeinfo version 4.5 from doc/gcc.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gcc: (gcc). The GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the use of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gcc.info, Node: Function Attributes, Next: Attribute Syntax, Prev: Mixed Declarations, Up: C Extensions Declaring Attributes of Functions ================================= In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully. The keyword `__attribute__' allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. The following attributes are currently defined for functions on all targets: `noreturn', `noinline', `always_inline', `pure', `const', `format', `format_arg', `no_instrument_function', `section', `constructor', `destructor', `used', `unused', `deprecated', `weak', `malloc', and `alias'. Several other attributes are defined for functions on particular target systems. Other attributes, including `section' are supported for variables declarations (*note Variable Attributes::) and for types (*note Type Attributes::). 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 `__noreturn__' instead of `noreturn'. *Note Attribute Syntax::, for details of the exact syntax for using attributes. `noreturn' A few standard library functions, such as `abort' and `exit', cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them `noreturn' to tell the compiler this fact. For example, void fatal () __attribute__ ((noreturn)); void fatal (...) { ... /* Print error message. */ ... exit (1); } The `noreturn' keyword tells the compiler to assume that `fatal' cannot return. It can then optimize without regard to what would happen if `fatal' ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables. Do not assume that registers saved by the calling function are restored before calling the `noreturn' function. It does not make sense for a `noreturn' function to have a return type other than `void'. The attribute `noreturn' is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows: typedef void voidfn (); volatile voidfn fatal; `noinline' This function attribute prevents a function from being considered for inlining. `always_inline' Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified. `pure' Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute `pure'. For example, int square (int) __attribute__ ((pure)); says that the hypothetical function `square' is safe to call fewer times than the program says. Some of common examples of pure functions are `strlen' or `memcmp'. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as `feof' in a multithreading environment). The attribute `pure' is not implemented in GCC versions earlier than 2.96. `const' Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the `pure' attribute above, since function is not allowed to read global memory. Note that a function that has pointer arguments and examines the data pointed to must _not_ be declared `const'. Likewise, a function that calls a non-`const' function usually must not be `const'. It does not make sense for a `const' function to return `void'. The attribute `const' is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows: typedef int intfn (); extern const intfn square; This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value. `format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)' The `format' attribute specifies that a function takes `printf', `scanf', `strftime' or `strfmon' style arguments which should be type-checked against a format string. For example, the declaration: extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3))); causes the compiler to check the arguments in calls to `my_printf' for consistency with the `printf' style format string argument `my_format'. The parameter ARCHETYPE determines how the format string is interpreted, and should be `printf', `scanf', `strftime' or `strfmon'. (You can also use `__printf__', `__scanf__', `__strftime__' or `__strfmon__'.) The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1), while FIRST-TO-CHECK is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as `vprintf'), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For `strftime' formats, the third parameter is required to be zero. In the example above, the format string (`my_format') is the second argument of the function `my_print', and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The `format' attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless `-ffreestanding' is used) checks formats for the standard library functions `printf', `fprintf', `sprintf', `scanf', `fscanf', `sscanf', `strftime', `vprintf', `vfprintf' and `vsprintf' whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. In C99 mode, the functions `snprintf', `vsnprintf', `vscanf', `vfscanf' and `vsscanf' are also checked. Except in strictly conforming C standard modes, the X/Open function `strfmon' is also checked as are `printf_unlocked' and `fprintf_unlocked'. *Note Options Controlling C Dialect: C Dialect Options. `format_arg (STRING-INDEX)' The `format_arg' attribute specifies that a function takes a format string for a `printf', `scanf', `strftime' or `strfmon' style function and modifies it (for example, to translate it into another language), so the result can be passed to a `printf', `scanf', `strftime' or `strfmon' style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). For example, the declaration: extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2))); causes the compiler to check the arguments in calls to a `printf', `scanf', `strftime' or `strfmon' type function, whose format string argument is a call to the `my_dgettext' function, for consistency with the format string argument `my_format'. If the `format_arg' attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when `-Wformat-nonliteral' is used, but the calls could not be checked without the attribute. The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1). The `format-arg' attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to `printf', `scanf', `strftime' or `strfmon' type function whose operands are a call to one of your own function. The compiler always treats `gettext', `dgettext', and `dcgettext' in this manner except when strict ISO C support is requested by `-ansi' or an appropriate `-std' option, or `-ffreestanding' is used. *Note Options Controlling C Dialect: C Dialect Options. `no_instrument_function' If `-finstrument-functions' is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented. `section ("SECTION-NAME")' Normally, the compiler places the code it generates in the `text' section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The `section' attribute specifies that a function lives in a particular section. For example, the declaration: extern void foobar (void) __attribute__ ((section ("bar"))); puts the function `foobar' in the `bar' section. 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. `constructor' `destructor' The `constructor' attribute causes the function to be called automatically before execution enters `main ()'. Similarly, the `destructor' attribute causes the function to be called automatically after `main ()' has completed or `exit ()' has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective-C. `unused' This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++. `used' This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly. `deprecated' The `deprecated' attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions 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 function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses: int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn; results in a warning on line 3 but not line 2. The `deprecated' attribute can also be used for variables and types (*note Variable Attributes::, *note Type Attributes::.) `weak' The `weak' attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker. `malloc' The `malloc' attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization. `alias ("TARGET")' The `alias' attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance, void __f () { /* do something */; } void f () __attribute__ ((weak, alias ("__f"))); declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used. Not all target machines support this attribute. `regparm (NUMBER)' On the Intel 386, the `regparm' attribute causes the compiler to pass up to NUMBER integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack. `stdcall' On the Intel 386, the `stdcall' attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. The PowerPC compiler for Windows NT currently ignores the `stdcall' attribute. `cdecl' On the Intel 386, the `cdecl' attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd' switch. The PowerPC compiler for Windows NT currently ignores the `cdecl' attribute. `longcall' On the RS/6000 and PowerPC, the `longcall' attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called. `long_call/short_call' This attribute allows to specify how to call a particular function on ARM. Both attributes override the `-mlong-calls' (*note ARM Options::) command line switch and `#pragma long_calls' settings. The `long_call' attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The `short_call' attribute always places the offset to the function from the call site into the `BL' instruction directly. `dllimport' On the PowerPC running Windows NT, the `dllimport' attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining `__imp_' and the function name. `dllexport' On the PowerPC running Windows NT, the `dllexport' attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the `dllimport' attribute. The pointer name is formed by combining `__imp_' and the function name. `exception (EXCEPT-FUNC [, EXCEPT-ARG])' On the PowerPC running Windows NT, the `exception' attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier EXCEPT-FUNC is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier EXCEPT-ARG is placed in the fourth entry of the structured exception table. `function_vector' Use this attribute on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector. You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly. `interrupt' Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. Note, interrupt handlers for the H8/300, H8/300H and SH processors can be specified via the `interrupt_handler' attribute. Note, on the AVR interrupts will be enabled inside the function. Note, for the ARM you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this: void f () __attribute__ ((interrupt ("IRQ"))); Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF. `interrupt_handler' Use this attribute on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. `sp_switch' Use this attribute on the SH to indicate an `interrupt_handler' function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack. void *alt_stack; void f () __attribute__ ((interrupt_handler, sp_switch ("alt_stack"))); `trap_exit' Use this attribute on the SH for an `interrupt_handle' to return using `trapa' instead of `rte'. This attribute expects an integer argument specifying the trap number to be used. `eightbit_data' Use this attribute on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data. You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly. `tiny_data' Use this attribute on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data. `signal' Use this attribute on the AVR to indicate that the specified function is an signal handler. The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present. Interrupts will be disabled inside function. `naked' Use this attribute on the ARM or AVR ports to indicate that the specified function do not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences. `model (MODEL-NAME)' Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. 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), and are callable with the `bl' instruction. Medium model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3' instructions to load their addresses), and are callable with the `bl' instruction. Large model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3' instructions to load their addresses), and may not be reachable with the `bl' instruction (the compiler will generate the much slower `seth/add3/jl' instruction sequence). You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration. Some people object to the `__attribute__' feature, suggesting that ISO C's `#pragma' should be used instead. At the time `__attribute__' was designed, there were two reasons for not doing this. 1. It is impossible to generate `#pragma' commands from a macro. 2. There is no telling what the same `#pragma' might mean in another compiler. These two reasons applied to almost any application that might have been proposed for `#pragma'. It was basically a mistake to use `#pragma' for _anything_. The ISO C99 standard includes `_Pragma', which now allows pragmas to be generated from macros. In addition, a `#pragma GCC' namespace is now in use for GCC-specific pragmas. However, it has been found convenient to use `__attribute__' to achieve a natural attachment of attributes to their corresponding declarations, whereas `#pragma GCC' is of use for constructs that do not naturally form part of the grammar. *Note Miscellaneous Preprocessing Directives: (cpp)Other Directives.  File: gcc.info, Node: Attribute Syntax, Next: Function Prototypes, Prev: Function Attributes, Up: C Extensions 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. *Note Function Attributes::, for details of the semantics of attributes applying to functions. *Note Variable Attributes::, for details of the semantics of attributes applying to variables. *Note 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: * An identifier. For example, `mode' attributes use this form. * An identifier followed by a comma and a non-empty comma-separated list of expressions. For example, `format' attributes use this form. * 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. 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. 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. 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 (*note 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.  File: gcc.info, Node: Function Prototypes, Next: C++ Comments, Prev: Attribute Syntax, Up: C Extensions Prototypes and Old-Style Function Definitions ============================================= GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example: /* Use prototypes unless the compiler is old-fashioned. */ #ifdef __STDC__ #define P(x) x #else #define P(x) () #endif /* Prototype function declaration. */ int isroot P((uid_t)); /* Old-style function definition. */ int isroot (x) /* ??? lossage here ??? */ uid_t x; { return x == 0; } Suppose the type `uid_t' happens to be `short'. ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an `int', which does not match the prototype argument type of `short'. This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the `uid_t' type is `short', `int', or `long'. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following: int isroot (uid_t); int isroot (uid_t x) { return x == 0; } GNU C++ does not support old-style function definitions, so this extension is irrelevant.  File: gcc.info, Node: C++ Comments, Next: Dollar Signs, Prev: Function Prototypes, Up: C Extensions C++ Style Comments ================== In GNU C, you may use C++ style comments, which start with `//' and continue until the end of the line. Many other C implementations allow such comments, and they are likely to be in a future C standard. However, C++ style comments are not recognized if you specify `-ansi', a `-std' option specifying a version of ISO C before C99, or `-traditional', since they are incompatible with traditional constructs like `dividend//*comment*/divisor'.  File: gcc.info, Node: Dollar Signs, Next: Character Escapes, Prev: C++ Comments, Up: C Extensions Dollar Signs in Identifier Names ================================ In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.  File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: C Extensions The Character in Constants ================================ You can use the sequence `\e' in a string or character constant to stand for the ASCII character .  File: gcc.info, Node: Alignment, Next: Inline, Prev: Type Attributes, Up: C Extensions Inquiring on Alignment of Types or Variables ============================================ The keyword `__alignof__' allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like `sizeof'. For example, if the target machine requires a `double' value to be aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This is true on many RISC machines. On more traditional machine designs, `__alignof__ (double)' is 4 or even 2. Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, `__alignof__' reports the _recommended_ alignment of a type. If the operand of `__alignof__' is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's `__attribute__' extension (*note Variable Attributes::). For example, after this declaration: struct foo { int x; char y; } foo1; the value of `__alignof__ (foo1.y)' is 1, even though its actual alignment is probably 2 or 4, the same as `__alignof__ (int)'. It is an error to ask for the alignment of an incomplete type.  File: gcc.info, Node: Variable Attributes, Next: Type Attributes, Prev: Character Escapes, Up: C Extensions 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. Ten attributes are currently defined for variables: `aligned', `mode', `nocommon', `packed', `section', `transparent_union', `unused', `deprecated', `vector_size', and `weak'. Some other attributes are defined for variables on particular target systems. Other attributes are available for functions (*note Function Attributes::) and for types (*note Type Attributes::). Other front ends might define more attributes (*note Extensions to the C++ Language: C++ Extensions.). 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'. *Note 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. `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. `nocommon' This attribute specifies requests GCC not to place a variable "common" but instead to allocate space for it directly. If you specify the `-fno-common' flag, GCC will do this for all variables. Specifying the `nocommon' attribute for a variable provides an initialization of zeros. A variable may only be initialized in one source file. `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 Windows NT, 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 Windows NT. `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 *Note 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. `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 warnings 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 (*note Function Attributes::, *note Type Attributes::.) `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 *Note Function Attributes::. `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). To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.