TIGCC (like all GNU C compilers) provides several language features not found in ANSI standard C. To test for the availability of these features in conditional compilation, check for a predefined macro
__GNUC__
, which is always defined under GNU C compilers.
Statement Expressions: | Putting statements and declarations inside expressions (useful for creating smart "safe" macros). |
Local Labels: | Labels local to a statement-expression. |
Labels as Values: | Getting pointers to labels, and computed gotos. |
Nested Functions: | As in Algol and Pascal, lexical scoping of functions. |
Constructing Calls: | Dispatching a call to another function. |
Naming Types: | Giving a name to the type of some expression. |
Typeof operator: | typeof : referring to the type of an expression. |
Generalized Lvalues: | Using '?:' , ',' and casts in lvalues. |
Conditionals: | Omitting the middle operand of a '?:' expression. |
Long Long: | Double-longword integers - long long int . |
Complex Numbers: | Data types for complex numbers. |
Hex Floats: | Hexadecimal floating-point constants. |
Zero Length: | Zero-length arrays. |
Variable Length: | Arrays whose length is computed at run time. |
Macro Varargs: | Macros with variable number of arguments. |
Subscripting: | Any array can be subscripted, even if not an lvalue. |
Pointer Arithmetics: | Arithmetic on void -pointers and function pointers. |
Initializers: | Non-constant initializers. |
Cast Constructors: | Constructor expressions give structures, unions or arrays as values. |
Labeled Elements: | Labeling elements of initializers. |
Cast to Union: | Casting to union type from any member of the union. |
Case Ranges: | 'case 1 ... 9' and such. |
Function Attributes: | Declaring that functions have no side effects, or that they can never return. |
Function Prototypes: | Prototype declarations and old-style definitions. |
C++ Comments: | C++ comments are recognized. |
Dollar Signs: | Dollar sign is allowed in identifiers. |
Character Escapes: | '\e' stands for the character ESC. |
Alignment: | Inquiring about the alignment of a type or variable. |
Variable Attributes: | Specifying attributes of variables. |
Type Attributes: | Specifying attributes of types. |
Inline: | Defining inline functions (as fast as macros). |
Extended Asm: | Assembler instructions with C expressions as operands (with them you can define "built-in" functions). |
Asm Labels: | Specifying the assembler name to use for a C symbol. |
Explicit Reg Vars: | Defining variables residing in specified registers. |
Alternate Keywords: | __const__ , __asm__ , etc., for header files. |
Incomplete Enums: | enum foo; , with details to follow. |
Function Names: | Printable strings which are the name of the current function. |
Return Address: | Getting the return or frame address of a function. |
Other Builtins: | Other built-in functions. |
({ 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 ()
.
void
, and thus
effectively no value).
#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.
typeof
(see section Referring to a Type with 'typeof') or type naming (see section Naming an Expression's Type).
goto
statement, but only from within the
statement expression it belongs to.
__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.
label:
, within the statements of the statement expression.
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; \ })
'&&'
. 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,
goto *exp;
. For example,
goto *ptr;Any expression of type
'void *'
is allowed.
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.
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.
static const int array[] = {&&foo - &&foo, &&bar - &&foo, &&hack - &&foo}; goto *(&&foo + array[i]);This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.
square
, and call it twice:
int square_sum (int a, int b) { int square (int 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
:
int 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.
int 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.
goto
and any intermediate functions as well. Here is an example:
int 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).
int 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]; } ... }
'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_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 fnc. 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_apply
.
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 eventual conflicts with variable names that occur within the expressions that are substituted for
a
and b
. Eventually GNU
team 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
.
The syntax of using of this keyword looks like sizeof
, but the
construct acts semantically like a type name defined with typedef
.
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.
typeof (int *)Here the type described is that of pointers to
int
.
typeof
is __typeof__
.
See section Alternate Keywords.
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
. It will be ilustrated with
some examples:
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
.
(a, b) += 5 a, (b += 5)Similarly, the address of the compound expression can be taken. These two expressions are equivalent:
&(a, b) a, &bA 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.
int *
pointer with the address of
f
, you can simply write (int *)&f
.
x ? : yhas the value of
x
if that is nonzero; otherwise, the value of
y
.
This example is perfectly equivalent to
x ? x : yIn 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.
long int
. Simply write long long int for a signed integer, or
unsigned long long int for a double-long 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.
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 (as usual) to avoid such problems is to
use prototypes.
__complex__
.
For example,
__complex__ float x;
declares x
as a
variable whose real part and imaginary part are both of type
float
. __complex__ short int y;
declares y
to
have real and imaginary parts of type short int
; this is not
likely to be very useful (expect if you like Gaussian integers), but it shows
that the set of complex types is complete.
'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.
__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 C 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 C 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
(not applicable on TIGCC yet: there is still no source-level debuggers
for it). Maybe a future version of debuggers will know how to recognize
such pairs and treat them as a single variable with a complex type.
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
.
typedef struct { int length; char contents[0]; } line; ... line *thisline = (line *) malloc (sizeof (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.
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.
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
.
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.
stderr
stream does not exist):
#define eprintf(format, args...) \ fprintf (stderr, format , ##args)Note that an extra space is added after format in the macro definition: this is important, and will be elaborated below.
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.
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). This also explains the usage of an extra space in the macro definition.
'&'
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 on pointers to functions. This is done by treating the
size of a void
or of a function as 1.
sizeof
is also allowed on void
and on function types, and returns 1.
int foo (int f, int g) { int beat_freqs[2] = {f-g, f+g}; ... }
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
cast 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 a 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 (note that casting to an union type is also a GNU extension).
[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.
[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.
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).
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};
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.
case 'A' ... 'Z':NOTE: Always 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:
union tag
or with a typedef name. A cast to union is actually
a cast constructor though, not a simple cast, and hence does not yield an lvalue like
normal casts (see section Constructor Expressions).
union foo {int i; double d;}; int x; double y;both
x
and y
can be cast to type union foo
.
union foo u; ... u = (union foo)x <==> u.i = x u = (union foo)y <==> u.d = yYou can also use the union cast as a function argument:
void hack (union foo); ... hack ((union foo)x);
__attribute__
allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. Eleven attributes,
noreturn, const, format,
format-arg,
no_instrument_function, section,
constructor,
destructor, unused,
weak and malloc 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
(NOTE: as function malloc is defined as a macro in TIGCCLIB,
always use malloc
attribute as __malloc__
).
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.
noreturn
function.
Also, it does not make sense for a noreturn
function to have a return
type other than void
.
An alternative way to declare that a function does
not return is as follows:
typedef void voidfn (); volatile voidfn fatal;This was a method used in older versions of GNU C compilers.
const
. For example,
int square (int) __attribute__ ((const));says that the hypothetical function
square
is safe to call
fewer times than the program says.
An alternative way to declare that a function has no side
effects is as follows:
typedef int intfn (); extern const intfn square;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
attribute specifies that a function takes
printf 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.
printf
, scanf
, or
strftime
(note that functions scanf
and
strftime
are not yet implemented in TIGCCLIB). 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.
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.
format
attribute allows you to identify your own functions
which take format strings as arguments, so that GNU C can check the
calls to these functions for errors.
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.
format-arg
attribute allows you to identify your own
functions which modify format strings, so that GNU C can check the
calls to printf style functions whose
operands are a call to one of your own function.
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.
constructor
attribute
should cause the function to be called automatically before execution enters
_main ()
. Similarly, the destructor
attribute should cause
the function to be called automatically after _main ()
has completed or
exit has been called. Functions with these attributes
might be useful for initializing data that will be used implicitly during the execution of
the program.
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 (I don't know whether it is
applicable with TIGCC).
malloc
attribute is used to tell the compiler that a function
may be treated as if it were the malloc function.
The compiler assumes
that calls to malloc result in a pointers that
cannot alias anything. This will often improve optimization.
__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.
#pragma
. It is basically a mistake to use
#pragma
for anything.
// 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 char
. 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 char
.
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 char
, 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; }
dividend//*comment*/divisor
.
'\e'
in a string or character constant to
stand for the ASCII character ESC (0x1b
).
__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. This is the case with TIGCC, which works with MC68000, so
__alignof__(anything)
is always 2 in TIGCC, except for
char
variables.
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.
__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.
__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
.
int x __attribute__ ((aligned (16))) = 0;causes the compiler to allocate the global variable
x
on a
16-byte boundary.
int
pair, you could write:
struct foo {int x[2] __attribute__ ((aligned (8)));};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.
short array[3] __attribute__ ((aligned));although this is useless on TIGCC.
'byte'
or '__byte__'
to
indicate the mode corresponding to a one-byte integer, 'word'
or
'__word__'
for the mode of a one-word integer, and 'pointer'
or '__pointer__'
for the mode used to represent pointers.
'-fno-common'
flag, GNU C will do this for all
variables. Specifying the nocommon
attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
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.
x
is packed, so that it
immediately follows a
:
struct foo { char a; int x[2] __attribute__ ((packed)); };NOTE: I am not sure whether it is applicable to TIGCC.
data
and bss
. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections.
The section
attribute specifies that a variable (or function) lives in a particular
section. Usage of this attribute is limited with TIGCC, because its linker supports
only few types of sections, but it may be improved in the future.
For example, this small hypothetic 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; void _main (void) { // 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 C issues
a warning and otherwise ignores the section
attribute in
uninitialized variable declarations.
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' compiler options or the
nocommon
attribute (note that TIGCC does not allow uninitialized
global definitions in nostub mode anyway).
typedef
for a union data type; then it
applies to all function parameters with that type.
weak
attribute is described in See section Declaring Attributes of Functions.
__attribute__ ((aligned (16), packed))'
.
__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).
__aligned__
instead of aligned
.
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.
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.
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.
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));although this will not have any effect with TIGCC.
enum
, struct
, or
union
type definition, specified that the minimum required memory
be used to represent the type.
struct
and union
types is
equivalent to specifying the packed
attribute on each of the
structure or union members. Specifying the '-fshort-enums'
commpiler option is equivalent to specifying the packed
attribute on all enum
definitions.
enum
definition, not in a typedef
declaration, unless that
declaration also contains the definition of the enum
.
union
type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
const
on
the referenced type must be respected, just as with normal pointer
conversions.
wait
function must accept either a value of type int *
to
comply with one platform, or a value of type union wait *
to comply with
another platform. 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, hypothetic <wait.h<
header file 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); }where
waitpid
is some system function.
union
or a struct
),
this attribute means that variables of that type are meant to appear
possibly unused. GNU C will not produce a warning for any variables of
that type, even if the variable appears to do nothing.
'__attribute__ ((aligned (16), packed))'
.
inline
, you can direct GNU C 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' compiler options, no function is really inline.
inline
keyword in its
declaration, like this:
inline int inc (int *a) { (*a)++; }Alternatively, you can write
__inline__
instead of inline
(see section Alternate Keywords).
You can also make all "simple enough" functions inline with the compiler option
'-finline-functions'.
inline
could not be substituted,
and will give the reason for the failure.
inline
keyword
does not affect the linkage of the function.
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 C does not actually output assembler code for the
function, unless you specify the compiler 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.
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.
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 proposed 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
(although TIGCC linker does not support function libraries yet).
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.
asm
keyword, which allows the
access to the inline assembler. The simplest form of it is
asm ("string");where string contains one or more assembly instrutcions, which are separated by semi-colons. In TIGCC, all register names must be preceeded by a percent sign, to avoid confusion with an eventual C variable named as one of the CPU registers, for example:
asm ("move.l 0xC8,%a0; move.l (%a0,1656); jsr (%a0)");Notice that hex constants must be written in according to C syntax (like
'0xC8'
),
in opposite to the notation '$C8'
which is common in various assemblers.
Note that something like
asm ("move.l 0xC8,a0");will be interpreted quite differently:
'a0'
will be regarded as a C language
variable, not a register! Read the documentation about the GNU Assembler
for more information about the exact syntax and directives which it accepts; exactly the same
features are accepted in the inline assembler.
asm ("move.l 0xC8,%a0 move.l (%a0,1656) jsr (%a0)");But this is not all. The GNU built-in assembler is much more powerful. 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.
fsinx
instruction (note that this example is not applicable with TI calculators,
because the MC68000 processor which is built into the TI-89 and TI-92+ does not have this
instruction, but the example is ilustrative anyway):
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). In this example,
the compiler will convert this instruction into a sequence of assembly instructions
which will ensure that the evaluated expression angle is really stored in
some of 68881's floating point registers, and which will store the result of the fsinx
(which is in some another floating point register) into the expression result
(note that result need not to be a variable; it can be any lvalue expression, for
example a dereferenced pointer).
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 C
will use the register as the output of the asm
, and then store
that register into the output.
asm ("move.l %1,%%d0; rol #1,%%d0; move.l %%d0,%0" : "=g" (output) : "g" (input));Note that when extended asm constructions are used, the percent sign before the register names must be doubled. It is important to say that in above example input and output may be any valid expressions; in simple case when both of them are just variables, simple asm construction like
asm ("move.l input,%d0; rol #1,%d0; move.l %d0,output");would be quite enough. Extended asm constructions allows encapsulating them in macros which look like functions, like in the following example, which defines macro
'rotate'
which acts like a void function:
#define rotate(input, output) \ ({ asm ("move.l %1,%%d0; rol #1,%%d0; move.l %%d0,%0" \ : "=g" (output) : "g" (input)); })All examples given above have a serious drawback: they clobbers the register
'd0'
.
If the compiler keep something important in it (which is very likely), this may cause a lot
of troubles. Of course,
you can save it on the stack at the begining and restore it at the end, but there exist much better
solution which will save clobbered registers only when necessary. To describe clobbered registers,
write a third colon after the input operands, followed by the names of
the clobbered registers (given as strings), separated by commas:
#define rotate(input, output) \ ({ asm ("move.l %1,%%d0; rol #1,%%d0; move.l %%d0,%0" \ : "=g" (output) : "g" (input) : "d0"); })Whenever 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. Alternatively, you can force the compiler to use any data register instead of
'd0'
, using the following trick:
#define rotate(input, output) \ ({ register long __temp; \ asm ("move.l %1,%2; rol #1,%2; move.l %2,%0" \ : "=g" (output) : "g" (input), "d" (__temp)); })Here,
"d"
constraint ensures that '__temp'
will be stored in
the data register (note that rol
is applicable only to data registers).
In fact, there is no any need for a temporary register if we forced the output to be in
the data register, which can be implemented as in following example:
#define rotate(input, output) \ ({ asm ("move.l %1,%0; rol #1,%0" : "=d" (output) : "g" (input)); })If your assembler instruction can alter the condition code register, add
'cc'
to the list of clobbered registers. GNU C 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.
'memory'
to the list of clobbered registers. This
will cause GNU C to not keep memory values cached in registers across
the assembler instruction.
'rotate'
macro which accept just one argument (and which rotates it one byte left)
is quite unreliable:
#define rotate(inout) ({ asm ("rol #1,%0" : "=d" (inout)); })To solve such difficulties, extended asm also supports input-output or read-write operands. Use the constraint character
'+'
to indicate such an
operand and list it with the output operands.
'combine'
instruction will be used 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.
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 C 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 C can't tell that.
volatile
for the asm
construct, as described below, to
prevent GNU C from deleting the asm
statement as unused.
asm
template, separated either with newlines or with semicolons
(GNU assembler supports semicolons).
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 a0 and a1:
asm ("move.l %0,a0; move.l %1,a1; jsr _foo" : /* no outputs */ : "g" (from), "g" (to) : "a0", "a1");Unless an output operand has the
'&'
constraint modifier, GNU C
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.
asm
construct, as follows:
asm ("clr.l %0; test.l %1; beq 0f; moveq #1,%0; 0:" : "g" (result) : "g" (input));This assumes your assembler supports local labels, which is true with the GNU Assembler (it uses suffix
'f'
for forward and 'b'
for
backward local labels; see section about Local Symbol Names).
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.
asm
instructions is to encapsulate them in macros that look like
functions. Here is an example how to define a function-looking macro with
non-void return type:
#define rotate(x) \ ({ unsigned long __result, __arg = (x); \ asm ("move.l %1,%0; rol #1,%0": "=d" (__result): "g" (__arg)); \ __result; })This macro acts nearly exactly like the function which takes one
unsigned long
argument, and which returns an unsigned long
result.
Here the variable __arg
is used to make sure that the instruction
operates on a proper unsigned long
value, and to accept only those
arguments x
which can convert automatically to a unsigned long
.
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.
asm
has output operands, GNU C 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. If you are not happy with such
behaviour, 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 C 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 C from moving the instruction around within a core region.
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.
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
.
__asm__
instead of asm
.
See section Alternate Keywords.
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'
.
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 (int x, int 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 C does not as yet have the ability to store static variables in registers.
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
).
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).
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
it might have put something else in that register (if you are prepared to
recompile it with the same global register variable, you can solve this problem).
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 (do not worry about it, because you use TIGCC for exactly one
processor, MC 68000). Specific registers are most often useful with explicit
assembler instructions (see section Assembler Instructions with C
Expression Operands).
asm
statement
and assume it will always refer to this variable.
asm
, typeof
and
inline
cannot be used since they won't work in a program compiled with
'-ansi' option, while
the keywords const
, volatile
, signed
, typeof
and inline
won't work in a program compiled with
'-traditional' option.
__asm__
instead of asm
, __const__
instead of const
, and
__inline__
instead of inline
.
#ifndef __GNUC__ #define __asm__ asm #endifThe compiler option '-pedantic' and other options cause 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
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.
enum
more consistent with the way struct
and union
are handled.
__FUNCTION__
holds the name of the function
as it appears in the source. The identifier __PRETTY_FUNCTION__
holds 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; don't care about this, because TIGCC is not a C++ compiler).
__FILE__
and __LINE__
, nor
variables. This means that they catenate with other string literals, and
that they can be used to initialize char arrays. For example
char here[] = "Function " __FUNCTION__ " in " __FILE__;On the other hand,
'#ifdef __FUNCTION__'
does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier __FUNCTION__
.
__func__
, defined by the
draft standard for C-99.
The identifier __func__
is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration
static const char __func__[] = "function-name";appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.
__func__
is a variable, not a string literal.
In particular, __func__
does not catenate with other string
literals.
__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.
__builtin_frame_address
will return the value of the frame
pointer register (a6).
'-fno-builtin'
command option.
But this feature mainly has no sense in TIGCC, because nearly all standard C library functions
are redefined to indirect TIOS calls in TIGCCLIB library.
__builtin_constant_p
to
determine if a value is known to be constant at compile-time and hence
that GNU C 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 C 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 C 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.