GNU C provides several language features not found in ANSI standard C.
(The `-pedantic' option directs GNU CC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GNU CC.
These extensions are available in C and Objective C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct. (If you use some other kind of statement
last within the braces, the construct has type void, and thus
effectively no value.)
This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here let's assume int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see section Referring to a Type with typeof) or type naming (see section Naming an Expression's Type).
Each statement expression is a scope in which local labels can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary goto statement, but only from within the
statement expression it belongs to.
A local label declaration looks like this:
__label__ label;
or
__label__ label1, label2, ...;
Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.
The label declaration defines the label name, but does not define
the label itself. You must do this in the usual way, with
label:, within the statements of the statement expression.
The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a goto
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
You can get the address of a label defined in the current function
(or a containing function) with the unary operator `&&'. The
value has type void *. This value is a constant and can be used
wherever a constant of that type is valid. For example:
void *ptr; ... ptr = &&foo;
To use these values, you need to be able to jump to one. This is done
with the computed goto statement(1), goto *exp;. For example,
goto *ptr;
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
Then you can select a label with indexing, like this:
goto *array[i];
Note that this does not check whether the subscript is in bounds--array indexing in C never does that.
Such an array of label values serves a purpose much like that of the
switch statement. The switch statement is cleaner, so
use that rather than an array unless the problem does not fit a
switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.
You can use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.
A nested function is a function defined inside another function.
(Nested functions are not supported for GNU C++.) The nested function's
name is local to the block where it is defined. For example, here we
define a nested function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called lexical scoping. For example, here we show a nested
function which uses an inherited variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
}
Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.
It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
Here, the function intermediate receives the address of
store as an argument. If intermediate calls store,
the arguments given to store are used to store into array.
But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
GNU CC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as `http://master.debian.org/~karlheg/Usenix88-lexic.pdf'.
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (see section Locally Declared Labels). Such a jump returns instantly to the
containing function, exiting the nested function which did the
goto and any intermediate functions as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
...
for (i = 0; i < size; i++)
... access (array, i) ...
...
return 0;
/* Control comes here from access
if it detects an error. */
failure:
return -1;
}
A nested function always has internal linkage. Declaring one with
extern is erroneous. If you need to declare the nested function
before its definition, use auto (which is otherwise meaningless
for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
...
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
...
}
Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.
You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).
__builtin_apply_args ()
void * to data
describing how to perform a call with the same arguments as were passed
to the current function.
The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack. Then it returns the
address of that block.
__builtin_apply (function, arguments, size)
void (*)())
with a copy of the parameters described by arguments (type
void *) and size (type int).
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size
of the stack argument data, in bytes.
This function returns a pointer of type void * to data describing
how to return whatever value was returned by function. The data
is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
__builtin_return (result)
__builtin_apply.
You can give a name to the type of an expression using a typedef
declaration with an initializer. Here is how to define name as a
type name for the type of exp:
typedef name = exp;
This is useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe "maximum" macro that operates on any arithmetic type:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
typeof
Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x is an array of functions; the type described
is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ANSI C
programs, write __typeof__ instead of typeof.
See section Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.
y with the type of what x points to.
typeof (*x) y;
y as an array of such values.
typeof (*x) y[4];
y as an array of pointers to characters:
typeof (typeof (char *)[4]) y;It is equivalent to the following traditional C declaration:
char *y[4];To see the meaning of the declaration using
typeof, and why it
might be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *) #define array(T, N) typeof(T [N])Now the declaration can be rewritten this way:
array (pointer (char), 4) y;Thus,
array (pointer (char), 4) is the type of arrays of 4
pointers to char.
Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.
Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code.
For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:
(a, b) += 5 a, (b += 5)
Similarly, the address of the compound expression can be taken. These two expressions are equivalent:
&(a, b) a, &b
A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:
(a ? b : c) = 5 (a ? b = 5 : (c = 5))
A cast is a valid lvalue if its operand is an lvalue. A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression. After this is stored, the value is
converted back to the specified type to become the value of the
assignment. Thus, if a has type char *, the following two
expressions are equivalent:
(int)a = 5 (int)(a = (char *)(int)5)
An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:
(int)a += 5 (int)(a = (char *)(int) ((int)a + 5))
You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently. Suppose that &(int)f were
permitted, where f has type float. Then the following
statement would try to store an integer bit-pattern where a floating
point number belongs:
*&(int)f = 1;
This is quite different from what (int)f = 1 would do--that
would convert 1 to floating point and store it. Rather than cause this
inconsistency, we think it is better to prohibit use of `&' on a cast.
If you really do want an int * pointer with the address of
f, you can simply write (int *)&f.
The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.
Therefore, the expression
x ? : y
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
GNU C supports data types for integers that are twice as long as
int. Simply write long long int for a signed integer, or
unsigned long long int for an unsigned integer. To make an
integer constant of type long long int, add the suffix LL
to the integer. To make an integer constant of type unsigned long
long int, add the suffix ULL to the integer.
You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GNU CC.
There may be pitfalls when you use long long types for function
arguments, unless you declare function prototypes. If a function
expects type int for its argument, and you pass a value of type
long long int, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects long long int and you pass
int. The best way to avoid such problems is to use prototypes.
GNU C supports complex data types. You can declare both complex integer
types and complex floating types, using the keyword __complex__.
For example, `__complex__ double x;' declares x as a
variable whose real part and imaginary part are both of type
double. `__complex__ short int y;' declares y to
have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is
complete.
To write a constant with a complex data type, use the suffix `i' or
`j' (either one; they are equivalent). For example, 2.5fi
has type __complex__ float and 3i has type
__complex__ int. Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant.
To extract the real part of a complex-valued expression exp, write
__real__ exp. Likewise, use __imag__ to
extract the imaginary part.
The operator `~' performs complex conjugation when used on a value with a complex type.
GNU CC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa). None of the
supported debugging info formats has a way to represent noncontiguous
allocation like this, so GNU CC describes a noncontiguous complex
variable as if it were two separate variables of noncomplex type.
If the variable's actual name is foo, the two fictitious
variables are named foo$real and foo$imag. You can
examine and set these two fictitious variables with your debugger.
A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type.
GNU CC recognizes floating-point numbers written not only in the usual
decimal notation, such as 1.55e1, but also numbers such as
0x1.fp3 written in hexadecimal format. In that format the
0x hex introducer and the p or P exponent field are
mandatory. The exponent is a decimal number that indicates the power of
2 by which the significand part will be multiplied. Thus 0x1.f is
1 15/16, p3 multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1.
Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation. Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., 0x1.f. This
could mean 1.0f or 1.9375 since f is also the
extension for floating-point constants of type float.
Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
{
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
}
In standard C, you would have to give contents a length of 1, which
means either you waste space or complicate the argument to malloc.
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.
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.
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];
}
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.
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 };
...
}
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.
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. (See section Cast to a Union Type.)
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 };
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:
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. (See section Constructor Expressions.)
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);
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 (see section Specifying Attributes of Variables) and for types (see section Specifying Attributes of Types).
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
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
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)
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)
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
section ("section-name")
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
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
weak
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")
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
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)
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
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
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
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
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
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])
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
interrupt_handler
eightbit_data
tiny_data
interrupt
model (model-name)
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.
#pragma commands from a macro.
#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.
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.
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.
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.
You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.
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.
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 (see section Declaring Attributes of Functions) and
for types (see section Specifying Attributes of Types).
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)
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)
nocommon
nocommon attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
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")
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
typedef for a union data type; then it
applies to all function parameters with that type.
unused
weak
weak attribute is described in See section Declaring Attributes of Functions.
model (model-name)
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))'.
The keyword __attribute__ allows you to specify special
attributes of struct and union types when you define such
types. This keyword is followed by an attribute specification inside
double parentheses. Three attributes are currently defined for types:
aligned, packed, and transparent_union. Other
attributes are defined for functions (see section Declaring Attributes of Functions) and
for variables (see section Specifying Attributes of Variables).
You may also specify any one of these attributes with `__'
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned.
You may specify the aligned and transparent_union
attributes either in a typedef declaration or just past the
closing curly brace of a complete enum, struct or union type
definition and the packed attribute only past the closing
brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
aligned (alignment)
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable whose
type is struct S or more_aligned_int will be allocated and
aligned at least on a 8-byte boundary. On a Sparc, having all
variables of type struct S aligned to 8-byte boundaries allows
the compiler to use the ldd and std (doubleword load and
store) instructions when copying one variable of type struct S to
another, thus improving run-time efficiency.
Note that the alignment of any given struct or union type
is required by the ANSI C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct or union in question. This means that you can
effectively adjust the alignment of a struct or union
type by attaching an aligned attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.
In the example above, if the size of each short is 2 bytes, then
the size of the entire struct S type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of
variables having the relevant (efficiently aligned) type. If you
declare or use arrays of variables of an efficiently-aligned type, then
it is likely that your program will also be doing pointer arithmetic (or
subscripting, which amounts to the same thing) on pointers to the
relevant type, and the code that the compiler generates for these
pointer arithmetic operations will often be more efficient for
efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
packed
enum, struct, or
union type definition, specified that the minimum required memory
be used to represent the type.
Specifying this attribute for struct and union types is
equivalent to specifying the packed attribute on each of the
structure or union members. Specifying the `-fshort-enums'
flag on the line is equivalent to specifying the packed
attribute on all enum definitions.
You may only specify this attribute after a closing curly brace on an
enum definition, not in a typedef declaration, unless that
declaration also contains the definition of the enum.
transparent_union
union type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling
conventions of first member of the transparent union, not the calling
conventions of the union itself. All members of the union must have the
same machine representation; this is necessary for this argument passing
to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait function must accept either a value of type int * to
comply with Posix, or a value of type union wait * to comply with
the 4.1BSD interface. If wait's parameter were void *,
wait would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h> might define the interface
as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
This interface allows either int * or union wait *
arguments to be passed, using the int * calling convention.
The program can call wait with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
unused
union or a struct),
this attribute means that variables of that type are meant to appear
possibly unused. GNU CC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.
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. See section 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. Among these usages are: use of varargs, use of
alloca, use of variable sized data types (see section Arrays of Variable Length),
use of computed goto (see section Labels as Values), use of nonlocal goto,
and nested functions (see section Nested Functions). Using `-Winline'
will warn when a function marked inline could not be substituted,
and will give the reason for the failure.
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';
see section Options Controlling C++ Dialect.)
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.
In an assembler instruction using asm, you can specify the
operands of the instruction using C expressions. This means you need not
guess 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 (see section Operand 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 the operands within each group. 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 but there are input operands, you must place 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 even whether it is valid
assembler input. The extended asm feature is most often used for
machine instructions the compiler itself does not know exist. If
the output expression cannot be directly addressed (for example, it is a
bit field), your constraint must allow a register. In that case, GNU CC
will use the register as the output of the asm, and then store
that register into the output.
The ordinary 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 supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands.
When the constraints for the read-write operand (or the operand in which
only some of the bits are to be changed) allows a register, you may, as
an alternative, 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 reliably:
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");
It is an error for a clobber description to overlap an input or output
operand (for example, an operand describing a register class with one
member, mentioned in the clobber list). Most notably, it is invalid to
describe that an input operand is modified, but unused as output. It has
to be specified as an input and output operand anyway. Note that if there
are only unused output operands, you will then also need to specify
volatile for the asm construct, as described below.
If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some 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 most 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 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 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. See section Constraint Modifier Characters.
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 the instruction has no side effects except to change the output
operands. This does not mean 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 get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \
__old; })
If you write an asm instruction with no outputs, GNU CC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops. If the side-effects of your instruction are
not purely external, but will affect variables in your program in ways
other than reading the inputs and clobbering the specified registers or
memory, you should write the volatile keyword to prevent future
versions of GNU CC from moving the instruction around within a core
region.
An asm instruction without any operands or clobbers (and "old
style" asm) will not be deleted or moved significantly,
regardless, unless it is unreachable, the same wasy as if you had
written a volatile keyword.
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. See section Alternate Keywords.
There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:
asm ("foo" : "=t" (a) : "f" (b));
This asm says that input B is not popped by the asm, and that
the asm pushes a result onto the reg-stack, ie, the stack is one
deeper after the asm than it was before. But, it is possible that
reload will think that it can use the same reg for both the input and
the output, if input B dies in this insn.
If any input operand uses the f constraint, all output reg
constraints must use the & earlyclobber.
The asm above would be written as
asm ("foo" : "=&t" (a) : "f" (b));
=f is not allowed: the operand
constraints must select a class with a single reg.
Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
This asm takes two inputs, which are popped by the fyl2xp1 opcode,
and replaces them with one output. The user must code the st(1)
clobber for reg-stack.c to know that fyl2xp1 pops both inputs.
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
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.
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.
asm feature (see section Assembler Instructions with C Expression Operands), 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.)
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.
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 (see section Assembler Instructions with C Expression Operands). 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.
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; excessive use of this feature leaves the compiler too few available registers to compile certain functions.
This option does not guarantee that GNU CC will generate code that has
this variable in the register you specify at all times. You may not
code an explicit reference to this register in an asm statement
and assume it will always refer to this variable.
Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.
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.
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++.
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)
These names are not macros: they are predefined string variables.
For example, `#ifdef __FUNCTION__' does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier __FUNCTION__.
These functions may be used to get information about the callers of a function.
__builtin_return_address (level)
0 yields the return address
of the current function, a value of 1 yields the return address
of the caller of the current function, and so forth.
The level argument must be a constant integer.
On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return 0.
This function should only be used with a non-zero argument for debugging
purposes.
__builtin_frame_address (level)
__builtin_return_address, but it
returns the address of the function frame rather than the return address
of the function. Calling __builtin_frame_address with a value of
0 yields the frame address of the current function, a value of
1 yields the frame address of the caller of the current function,
and so forth.
The frame is the area on the stack which holds local variables and saved
registers. The frame address is normally the address of the first word
pushed on to the stack by the function. However, the exact definition
depends upon the processor and the calling convention. If the processor
has a dedicated frame pointer register, and the function has a frame,
then __builtin_frame_address will return the value of the frame
pointer register.
The caveats that apply to __builtin_return_address apply to this
function as well.
GNU CC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
The remaining functions are provided for optimization purposes.
GNU CC includes builtin versions of many of the functions in the
standard C library. These will always be treated as having the same
meaning as the C library function even if you specify the
`-fno-builtin' (see section Options Controlling C Dialect) option. These functions
correspond to the C library functions alloca, ffs,
abs, fabsf, fabs, fabsl, labs,
memcpy, memcmp, strcmp, strcpy,
strlen, sqrtf, sqrt, sqrtl, sinf,
sin, sinl, cosf, cos, and cosl.
You can use the builtin function __builtin_constant_p to
determine if a value is known to be constant at compile-time and hence
that GNU CC can perform constant-folding on expressions involving that
value. The argument of the function is the value to test. The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant. A
return of 0 does not indicate that the value is not a constant,
but merely that GNU CC cannot prove it is a constant with the specified
value of the `-O' option.
You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:
#define Scale_Value(X) \ (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X))
You may use this builtin function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the builtin, GNU CC will never return 1 when you call the inline function with a string constant or constructor expression (see section Constructor Expressions) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O' option.
In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superceded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fthis-is-variable
-fexternal-templates
-falt-external-templates