This is Info file gcc.info, produced by Makeinfo version 1.68 from the input file ../../../src/gcc-2.95.3/gcc/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 and the internals of the GNU compiler. 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 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Funding for Free Software" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License" and "Funding for Free Software", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Variable Length, Next: Macro Varargs, Prev: Zero Length, Up: C Extensions Arrays of Variable Length ========================= Variable-length automatic arrays are allowed in GNU C. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example: FILE * concat_fopen (char *s1, char *s2, char *mode) { char str[strlen (s1) + strlen (s2) + 1]; strcpy (str, s1); strcat (str, s2); return fopen (str, mode); } Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it. You can use the function `alloca' to get an effect much like variable-length arrays. The function `alloca' is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant. There are other differences between these two methods. Space allocated with `alloca' exists until the containing *function* returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and `alloca' in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with `alloca'.) You can also use variable-length arrays as arguments to functions: struct entry tester (int len, char data[len][len]) { ... } The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with `sizeof'. If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension. struct entry tester (int len; char data[len][len], int len) { ... } The `int len' before the semicolon is a "parameter forward declaration", and it serves the purpose of making the name `len' known when the declaration of `data' is parsed. You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type.  File: gcc.info, Node: Macro Varargs, Next: Subscripting, Prev: Variable Length, Up: C Extensions Macros with Variable Numbers of Arguments ========================================= In GNU C, a macro can accept a variable number of arguments, much as a function can. The syntax for defining the macro looks much like that used for a function. Here is an example: #define eprintf(format, args...) \ fprintf (stderr, format , ## args) Here `args' is a "rest argument": it takes in zero or more arguments, as many as the call contains. All of them plus the commas between them form the value of `args', which is substituted into the macro body where `args' is used. Thus, we have this expansion: eprintf ("%s:%d: ", input_file_name, line_number) ==> fprintf (stderr, "%s:%d: " , input_file_name, line_number) Note that the comma after the string constant comes from the definition of `eprintf', whereas the last comma comes from the value of `args'. The reason for using `##' is to handle the case when `args' matches no arguments at all. In this case, `args' has an empty value. In this case, the second comma in the definition becomes an embarrassment: if it got through to the expansion of the macro, we would get something like this: fprintf (stderr, "success!\n" , ) which is invalid C syntax. `##' gets rid of the comma, so we get the following instead: fprintf (stderr, "success!\n") This is a special feature of the GNU C preprocessor: `##' before a rest argument that is empty discards the preceding sequence of non-whitespace characters from the macro definition. (If another macro argument precedes, none of it is discarded.) It might be better to discard the last preprocessor token instead of the last preceding sequence of non-whitespace characters; in fact, we may someday change this feature to do so. We advise you to write the macro definition so that the preceding sequence of non-whitespace characters is just a single token, so that the meaning will not change if we change the definition of this feature.  File: gcc.info, Node: Subscripting, Next: Pointer Arith, Prev: Macro Varargs, Up: C Extensions Non-Lvalue Arrays May Have Subscripts ===================================== Subscripting is allowed on arrays that are not lvalues, even though the unary `&' operator is not. For example, this is valid in GNU C though not valid in other C dialects: struct foo {int a[4];}; struct foo f(); bar (int index) { return f().a[index]; }  File: gcc.info, Node: Pointer Arith, Next: Initializers, Prev: Subscripting, Up: C Extensions Arithmetic on `void'- and Function-Pointers =========================================== In GNU C, addition and subtraction operations are supported on pointers to `void' and on pointers to functions. This is done by treating the size of a `void' or of a function as 1. A consequence of this is that `sizeof' is also allowed on `void' and on function types, and returns 1. The option `-Wpointer-arith' requests a warning if these extensions are used.  File: gcc.info, Node: Initializers, Next: Constructors, Prev: Pointer Arith, Up: C Extensions Non-Constant Initializers ========================= As in standard C++, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements: foo (float f, float g) { float beat_freqs[2] = { f-g, f+g }; ... }  File: gcc.info, Node: Constructors, Next: Labeled Elements, Prev: Initializers, Up: C Extensions Constructor Expressions ======================= GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. Usually, the specified type is a structure. Assume that `struct foo' and `structure' are declared as shown: struct foo {int a; char b[2];} structure; Here is an example of constructing a `struct foo' with a constructor: structure = ((struct foo) {x + y, 'a', 0}); This is equivalent to writing the following: { struct foo temp = {x + y, 'a', 0}; structure = temp; } You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here: char **foo = (char *[]) { "x", "y", "z" }; Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a `switch' statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array constructor: output = ((int[]) { 2, x, 28 }) [input]; Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.  File: gcc.info, Node: Labeled Elements, Next: Cast to Union, Prev: Constructors, Up: C Extensions Labeled Elements in Initializers ================================ Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized. In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++. To specify an array index, write `[INDEX]' or `[INDEX] =' before the element value. For example, int a[6] = { [4] 29, [2] = 15 }; is equivalent to int a[6] = { 0, 0, 15, 0, 29, 0 }; The index values must be constant expressions, even if the array being initialized is automatic. To initialize a range of elements to the same value, write `[FIRST ... LAST] = VALUE'. For example, int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; Note that the length of the array is the highest value specified plus one. In a structure initializer, specify the name of a field to initialize with `FIELDNAME:' before the element value. For example, given the following structure, struct point { int x, y; }; the following initialization struct point p = { y: yvalue, x: xvalue }; is equivalent to struct point p = { xvalue, yvalue }; Another syntax which has the same meaning is `.FIELDNAME ='., as shown here: struct point p = { .y = yvalue, .x = xvalue }; You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example, union foo { int i; double d; }; union foo f = { d: 4 }; will convert 4 to a `double' to store it in the union using the second element. By contrast, casting 4 to type `union foo' would store it into the union as the integer `i', since it is an integer. (*Note Cast to Union::.) You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example, int a[6] = { [1] = v1, v2, [4] = v4 }; is equivalent to int a[6] = { 0, v1, v2, 0, v4, 0 }; Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an `enum' type. For example: int whitespace[256] = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };  File: gcc.info, Node: Case Ranges, Next: Function Attributes, Prev: Cast to Union, Up: C Extensions Case Ranges =========== You can specify a range of consecutive values in a single `case' label, like this: case LOW ... HIGH: This has the same effect as the proper number of individual `case' labels, one for each integer value from LOW to HIGH, inclusive. This feature is especially useful for ranges of ASCII character codes: case 'A' ... 'Z': *Be careful:* Write spaces around the `...', for otherwise it may be parsed wrong when you use it with integer values. For example, write this: case 1 ... 5: rather than this: case 1...5:  File: gcc.info, Node: Cast to Union, Next: Case Ranges, Prev: Labeled Elements, Up: C Extensions Cast to a Union Type ==================== A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with `union TAG' or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (*Note Constructors::.) The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables: union foo { int i; double d; }; int x; double y; both `x' and `y' can be cast to type `union' foo. Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union: union foo u; ... u = (union foo) x == u.i = x u = (union foo) y == u.d = y You can also use the union cast as a function argument: void hack (union foo); ... hack ((union foo) x);  File: gcc.info, Node: Function Attributes, Next: Function Prototypes, Prev: Case Ranges, 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. Nine attributes, `noreturn', `const', `format', `no_instrument_function', `section', `constructor', `destructor', `unused' and `weak' are currently defined for functions. 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'. `noreturn' A few standard library functions, such as `abort' and `exit', cannot return. GNU CC 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 GNU C 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; `const' Many functions do not examine any values except their arguments, and have no effects except the return value. 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 `const'. For example, int square (int) __attribute__ ((const)); says that the hypothetical function `square' is safe to call fewer times than the program says. The attribute `const' is not implemented in GNU C 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. 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'. `format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)' The `format' attribute specifies that a function takes `printf', `scanf', or `strftime' 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 either `printf', `scanf', or `strftime'. 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. 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 GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI 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'. `format_arg (STRING-INDEX)' The `format_arg' attribute specifies that a function takes `printf' or `scanf' style arguments, modifies it (for example, to translate it into another language), and passes it to a `printf' or `scanf' style function. 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 `my_dgettext' whose result is passed to a `printf', `scanf', or `strftime' type function for consistency with the `printf' style format string argument `my_format'. 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 GNU CC can check the calls to `printf', `scanf', or `strftime' function whose operands are a call to one of your own function. The compiler always treats `gettext', `dgettext', and `dcgettext' in this manner. `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. GNU CC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++. `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. `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. `no_check_memory_usage' If `-fcheck-memory-usage' is given, calls to support routines will be generated before most memory accesses, to permit support code to record usage and detect uses of uninitialized or unallocated storage. Since the compiler cannot handle them properly, `asm' statements are not allowed. Declaring a function with this attribute disables the memory checking code for that function, permitting the use of `asm' statements without requiring separate compilation with different options, and allowing you to write support routines of your own if you wish, without getting infinite recursion if they get compiled with this option. `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. `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 option 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 option to work correctly. `interrupt_handler' Use this option on the H8/300 and H8/300H 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. `eightbit_data' Use this option 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 option to work correctly. `tiny_data' Use this option 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. `interrupt' Use this option on the M32R/D 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. `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 ANSI C's `#pragma' should be used instead. There are 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 apply to almost any application that might be proposed for `#pragma'. It is basically a mistake to use `#pragma' for *anything*.  File: gcc.info, Node: Function Prototypes, Next: C++ Comments, Prev: Function Attributes, Up: C Extensions Prototypes and Old-Style Function Definitions ============================================= GNU C extends ANSI 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'. ANSI 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 ANSI 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' 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. When the operand of `__alignof__' is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a structure and inherits alignment from that structure. For example, after this declaration: struct foo { int x; char y; } foo1; the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as `__alignof__ (int)', even though the data type of `foo1.y' does not itself demand any alignment. A related feature which lets you specify the alignment of an object is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.  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. Eight attributes are currently defined for variables: `aligned', `mode', `nocommon', `packed', `section', `transparent_union', `unused', and `weak'. Other attributes are available for functions (*note Function 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 `__aligned__' instead of `aligned'. `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. It is not possible to specify the alignment of functions; the alignment of functions is determined by the machine's requirements and cannot be changed. You cannot specify alignment for a typedef name because such a name is just an alias, not a distinct type. 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 GNU CC not to place a variable "common" but instead to allocate space for it directly. If you specify the `-fno-common' flag, GNU CC 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. GNU CC 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. `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. GNU CC will not produce a warning for this variable. `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))'.