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GNU Info File | 1994-11-17 | 51KB | 928 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1988, 1989, 1992, 1993, 1994 Free Software Foundation, 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," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" 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," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", 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: 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. Four attributes, `noreturn', `const', `format', and `section' are currently defined for functions. Other attributes, including `section' are supported for variables declarations (*note Variable Attributes::.). You may also specify attributes with `__' preceeding 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' or `scanf' 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' or `scanf'. 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', `vprintf', `vfprintf' and `vsprintf' whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. `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 (".init"))); puts the function `foobar' in the `.init' 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. 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: Dollar Signs, 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. */ #if __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: Dollar Signs, Next: Character Escapes, Prev: Function Prototypes, Up: C Extensions Dollar Signs in Identifier Names ================================ In GNU C, you may use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. On some machines, dollar signs are allowed in identifiers if you specify `-traditional'. On a few systems they are allowed by default, even if you do not use `-traditional'. But they are never allowed if you specify `-ansi'. There are certain ANSI C programs (obscure, to be sure) that would compile incorrectly if dollar signs were permitted in identifiers. For example: #define foo(a) #a #define lose(b) foo (b) #define test$ lose (test) File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: C Extensions The Character ESC in Constants ============================== You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC. File: gcc.info, Node: Alignment, Next: Inline, Prev: Variable 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: Alignment, 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. Four attributes are currently defined for variables: `aligned', `mode', `packed', and `section'. Other attributes are defined for functions, and thus not documented here; see *Note Function Attributes::. You may also specify attributes with `__' preceeding 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. The `aligned' attribute can only increase the alignment; but you can decrease it by specifying `packed' as well. See below. The linker of your operating system imposes a maximum alignment. If the linker aligns each object file on a four byte boundary, then it is beyond the compiler's power to cause anything to be aligned to a larger boundary than that. For example, if the linker happens to put this object file at address 136 (eight more than a multiple of 64), then the compiler cannot guarantee an alignment of more than 8 just by aligning variables in the object file. `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. `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_copy __attribute__ ((section ("INITDATACOPY"))) = 0; main() { /* Initialize stack pointer */ init_sp (stack + sizeof (stack)); /* Initialize initialized data */ memcpy (&init_data_copy, &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". 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 argument variable which is a union, means to pass the argument in the same way that the first union alternative would be passed. You can also use this attribute on a `typedef' for a union data type; then it applies to all function arguments with that type. To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'. File: gcc.info, Node: Inline, Next: Extended Asm, Prev: Alignment, Up: C Extensions An Inline Function is As Fast As a Macro ======================================== By declaring a function `inline', you can direct GNU CC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really "works" only in optimizing compilation. If you don't use `-O', no function is really inline. To declare a function inline, use the `inline' keyword in its declaration, like this: inline int inc (int *a) { (*a)++; } (If you are writing a header file to be included in ANSI C programs, write `__inline__' instead of `inline'. *Note Alternate Keywords::.) You can also make all "simple enough" functions inline with the option `-finline-functions'. Note that certain usages in a function definition can make it unsuitable for inline substitution. Note that in C and Objective C, unlike C++, the `inline' keyword does not affect the linkage of the function. GNU CC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared `inline'. (You can override this with `-fno-default-inline'; *note Options Controlling C++ Dialect: C++ Dialect Options..) When a function is both inline and `static', if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions'. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined. When an inline function is not `static', then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-`static' inline function is always compiled on its own in the usual fashion. If you specify both `inline' and `extern' in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of `inline' and `extern' has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking `inline' and `extern') in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library. GNU C does not inline any functions when not optimizing. It is not clear whether it is better to inline or not, in this case, but we found that a correct implementation when not optimizing was difficult. So we did the easy thing, and turned it off. File: gcc.info, Node: Extended Asm, Next: Asm Labels, Prev: Inline, Up: C Extensions Assembler Instructions with C Expression Operands ================================================= In an assembler instruction using `asm', you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory locations will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. For example, here is how to use the 68881's `fsinx' instruction: asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); Here `angle' is the C expression for the input operand while `result' is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (*note Constraints::.). Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand, and another separates the last output operand from the first input, if any. Commas separate output operands and separate inputs. The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater. If there are no output operands, and there are input operands, then there must be two consecutive colons surrounding the place where the output operands would go. Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means, or whether it is valid assembler input. The extended `asm' feature is most often used for machine instructions that the compiler itself does not know exist. The output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm does not support input-output or read-write operands. For this reason, the constraint character `+', which indicates such an operand, may not be used. When the assembler instruction has a read-write operand, or an operand in which only some of the bits are to be changed, you must logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) `combine' instruction with `bar' as its read-only source operand and `foo' as its read-write destination: asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand, and it must refer to an output operand. Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that `foo' is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work: asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy of the value of `foo' in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to `foo''s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can't tell that. Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the Vax: asm volatile ("movc3 %0,%1,%2" : /* no outputs */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5"); If you refer to a particular hardware register from the assembler code, then you will probably have to list the register after the third colon to tell the compiler that the register's value is modified. In many assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input. If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GNU CC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine. If your assembler instruction modifies memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GNU CC to not keep memory values cached in registers across the assembler instruction. You can put multiple assembler instructions together in a single `asm' template, separated either with newlines (written as `\n') or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and all Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes that the subroutine `_foo' accepts arguments in registers 9 and 10: asm ("movl %0,r9;movl %1,r10;call _foo" : /* no outputs */ : "g" (from), "g" (to) : "r9", "r10"); Unless an output operand has the `&' constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. *Note Modifiers::. If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the `asm' construct, as follows: asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" : "g" (result) : "g" (input)); This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do. Speaking of labels, jumps from one `asm' to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize. Usually the most convenient way to use these `asm' instructions is to encapsulate them in macros that look like functions. For example, #define sin(x) \ ({ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; }) Here the variable `__arg' is used to make sure that the instruction operates on a proper `double' value, and to accept only those arguments `x' which can convert automatically to a `double'. Another way to make sure the instruction operates on the correct data type is to use a cast in the `asm'. This is different from using a variable `__arg' in that it converts more different types. For example, if the desired type were `int', casting the argument to `int' would accept a pointer with no complaint, while assigning the argument to an `int' variable named `__arg' would warn about using a pointer unless the caller explicitly casts it. If an `asm' has output operands, GNU CC assumes for optimization purposes that the instruction has no side effects except to change the output operands. This does not mean that instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register. You can prevent an `asm' instruction from being deleted, moved significantly, or combined, by writing the keyword `volatile' after the `asm'. For example: #define set_priority(x) \ asm volatile ("set_priority %0": /* no outputs */ : "g" (x)) An instruction without output operands will not be deleted or moved significantly, regardless, unless it is unreachable. Note that even a volatile `asm' instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can't expect a sequence of volatile `asm' instructions to remain perfectly consecutive. If you want consecutive output, use a single `asm'. It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands. If you are writing a header file that should be includable in ANSI C programs, write `__asm__' instead of `asm'. *Note Alternate Keywords::. File: gcc.info, Node: Asm Labels, Next: Explicit Reg Vars, Prev: Extended Asm, Up: C Extensions Controlling Names Used in Assembler Code ======================================== You can specify the name to be used in the assembler code for a C function or variable by writing the `asm' (or `__asm__') keyword after the declarator as follows: int foo asm ("myfoo") = 2; This specifies that the name to be used for the variable `foo' in the assembler code should be `myfoo' rather than the usual `_foo'. On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore. You cannot use `asm' in this way in a function *definition*; but you can get the same effect by writing a declaration for the function before its definition and putting `asm' there, like this: extern func () asm ("FUNC"); func (x, y) int x, y; ... It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added. File: gcc.info, Node: Explicit Reg Vars, Next: Alternate Keywords, Prev: Asm Labels, Up: C Extensions Variables in Specified Registers ================================ GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated. * Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often. * Local register variables in specific registers do not reserve the registers. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. These local variables are sometimes convenient for use with the extended `asm' feature (*note Extended Asm::.), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the `asm'.) * Menu: * Global Reg Vars:: * Local Reg Vars:: File: gcc.info, Node: Global Reg Vars, Next: Local Reg Vars, Up: Explicit Reg Vars Defining Global Register Variables ---------------------------------- You can define a global register variable in GNU C like this: register int *foo asm ("a5"); Here `a5' is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it. Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register `a5' would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a "global" register that is not affected magically by the function call mechanism. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5'. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified. It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand). It is not safe for one function that uses a global register variable to call another such function `foo' by way of a third function `lose' that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because `lose' might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to `qsort', since `qsort' might have put something else in that register. (If you are prepared to recompile `qsort' with the same global register variable, you can solve this problem.) If you want to recompile `qsort' or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option `-ffixed-REG'. You need not actually add a global register declaration to their source code. A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller. On most machines, `longjmp' will restore to each global register variable the value it had at the time of the `setjmp'. On some machines, however, `longjmp' will not change the value of global register variables. To be portable, the function that called `setjmp' should make other arrangements to save the values of the global register variables, and to restore them in a `longjmp'. This way, the same thing will happen regardless of what `longjmp' does. All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions. Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register. On the Sparc, there are reports that g3 ... g7 are suitable registers, but certain library functions, such as `getwd', as well as the subroutines for division and remainder, modify g3 and g4. g1 and g2 are local temporaries. On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those. File: gcc.info, Node: Local Reg Vars, Prev: Global Reg Vars, Up: Explicit Reg Vars Specifying Registers for Local Variables ---------------------------------------- You can define a local register variable with a specified register like this: register int *foo asm ("a5"); Here `a5' is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function. Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (*note Extended Asm::.). Both of these things generally require that you conditionalize your program according to cpu type. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5'. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass. I would not be surprised if excessive use of this feature leaves the compiler too few available registers to compile certain functions. File: gcc.info, Node: Alternate Keywords, Next: Incomplete Enums, Prev: Explicit Reg Vars, Up: C Extensions Alternate Keywords ================== The option `-traditional' disables certain keywords; `-ansi' disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. The keywords `asm', `typeof' and `inline' cannot be used since they won't work in a program compiled with `-ansi', while the keywords `const', `volatile', `signed', `typeof' and `inline' won't work in a program compiled with `-traditional'. The way to solve these problems is to put `__' at the beginning and end of each problematical keyword. For example, use `__asm__' instead of `asm', `__const__' instead of `const', and `__inline__' instead of `inline'. Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this: #ifndef __GNUC__ #define __asm__ asm #endif `-pedantic' causes warnings for many GNU C extensions. You can prevent such warnings within one expression by writing `__extension__' before the expression. `__extension__' has no effect aside from this. File: gcc.info, Node: Incomplete Enums, Next: Function Names, Prev: Alternate Keywords, Up: C Extensions Incomplete `enum' Types ======================= You can define an `enum' tag without specifying its possible values. This results in an incomplete type, much like what you get if you write `struct foo' without describing the elements. A later declaration which does specify the possible values completes the type. You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type. This extension may not be very useful, but it makes the handling of `enum' more consistent with the way `struct' and `union' are handled. This extension is not supported by GNU C++. File: gcc.info, Node: Function Names, Prev: Incomplete Enums, Up: C Extensions Function Names as Strings ========================= GNU CC predefines two string variables to be the name of the current function. The variable `__FUNCTION__' is the name of the function as it appears in the source. The variable `__PRETTY_FUNCTION__' is the name of the function pretty printed in a language specific fashion. These names are always the same in a C function, but in a C++ function they may be different. For example, this program: extern "C" { extern int printf (char *, ...); } class a { public: sub (int i) { printf ("__FUNCTION__ = %s\n", __FUNCTION__); printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); } }; int main (void) { a ax; ax.sub (0); return 0; } gives this output: __FUNCTION__ = sub __PRETTY_FUNCTION__ = int a::sub (int) File: gcc.info, Node: C++ Extensions, Next: Trouble, Prev: C Extensions, Up: Top Extensions to the C++ Language ****************************** The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro `__GNUC__'. You can also use `__GNUG__' to test specifically for GNU C++ (*note Standard Predefined Macros: (cpp.info)Standard Predefined.). * Menu: * Naming Results:: Giving a name to C++ function return values. * Min and Max:: C++ Minimum and maximum operators. * Destructors and Goto:: Goto is safe to use in C++ even when destructors are needed. * C++ Interface:: You can use a single C++ header file for both declarations and definitions. * Template Instantiation:: Methods for ensuring that exactly one copy of each needed template instantiation is emitted. * C++ Signatures:: You can specify abstract types to get subtype polymorphism independent from inheritance. File: gcc.info, Node: Naming Results, Next: Min and Max, Up: C++ Extensions Named Return Values in C++ ========================== GNU C++ extends the function-definition syntax to allow you to specify a name for the result of a function outside the body of the definition, in C++ programs: TYPE FUNCTIONNAME (ARGS) return RESULTNAME; { ... BODY ... } You can use this feature to avoid an extra constructor call when a function result has a class type. For example, consider a function `m', declared as `X v = m ();', whose result is of class `X': X m () { X b; b.a = 23; return b; } Although `m' appears to have no arguments, in fact it has one implicit argument: the address of the return value. At invocation, the address of enough space to hold `v' is sent in as the implicit argument. Then `b' is constructed and its `a' field is set to the value 23. Finally, a copy constructor (a constructor of the form `X(X&)') is applied to `b', with the (implicit) return value location as the target, so that `v' is now bound to the return value. But this is wasteful. The local `b' is declared just to hold something that will be copied right out. While a compiler that combined an "elision" algorithm with interprocedural data flow analysis could conceivably eliminate all of this, it is much more practical to allow you to assist the compiler in generating efficient code by manipulating the return value explicitly, thus avoiding the local variable and copy constructor altogether. Using the extended GNU C++ function-definition syntax, you can avoid the temporary allocation and copying by naming `r' as your return value as the outset, and assigning to its `a' field directly: X m () return r; { r.a = 23; } The declaration of `r' is a standard, proper declaration, whose effects are executed *before* any of the body of `m'. Functions of this type impose no additional restrictions; in particular, you can execute `return' statements, or return implicitly by reaching the end of the function body ("falling off the edge"). Cases X m () return r (23); { return; } (or even `X m () return r (23); { }') are unambiguous, since the return value `r' has been initialized in either case. The following code may be hard to read, but also works predictably: X m () return r; { X b; return b; } The return value slot denoted by `r' is initialized at the outset, but the statement `return b;' overrides this value. The compiler deals with this by destroying `r' (calling the destructor if there is one, or doing nothing if there is not), and then reinitializing `r' with `b'. This extension is provided primarily to help people who use overloaded operators, where there is a great need to control not just the arguments, but the return values of functions. For classes where the copy constructor incurs a heavy performance penalty (especially in the common case where there is a quick default constructor), this is a major savings. The disadvantage of this extension is that you do not control when the default constructor for the return value is called: it is always called at the beginning.