This is Info file gcc.info, produced by Makeinfo-1.49 from the input
file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
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File: gcc.info,  Node: Function Attributes,  Next: Function Prototypes,  Prev: Case Ranges,  Up: Extensions

Declaring Attributes of Functions

   In GNU C, you declare certain things about functions called in your
program which help the compiler optimize function calls.

   A few standard library functions, such as `abort' and `exit', cannot
return.  GNU CC knows this automatically.  Some programs define their
own functions that never return.  You can declare them `volatile' to
tell the compiler this fact.  For example,

     extern void volatile fatal ();
     fatal (...)
       ... /* Print error message. */ ...
       exit (1);

   The `volatile' keyword tells the compiler to assume that `fatal'
cannot return.  This makes slightly better code, but more importantly
it helps avoid spurious warnings of uninitialized variables.

   It does not make sense for a `volatile' function to have a return
type other than `void'.

   Many functions do not examine any values except their arguments, and
have no effects except the return value.  Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be.  These functions should be declared
`const'.  For example,

     extern int const square ();

says that the hypothetical function `square' is safe to call fewer
times than the program says.

   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'.

   We recommend placing the keyword `const' after the function's return
type.  It makes no difference in the example above, but when the return
type is a pointer, it is the only way to make the function itself
const.  For example,

     const char *mincp (int);

says that `mincp' returns `const char *'--a pointer to a const object. 
To declare `mincp' const, you must write this:

     char * const mincp (int);

   Some people object to this feature, suggesting that ANSI C's
`#pragma' should be used instead.  There are two reasons for not doing

  1. It is impossible to generate `#pragma' commands from a macro.

  2. The `#pragma' command is just as likely as these keywords to mean
     something else 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

   The keyword `__attribute__' allows you to specify special attributes
when making a declaration.  This keyword is followed by an attribute
specification inside double parentheses.  One attribute, `format', is
currently defined for functions.  Others are implemented for variables
and structure fields (*note Function Attributes::.).

     The `format' attribute specifies that a function takes `printf' or
     `scanf' style arguments which should be type-checked against a
     format string.  For example, the declaration:

          extern int
          my_printf (void *my_object, const char *my_format, ...)
                __attribute__ ((format (printf, 2, 3)));

     causes the compiler to check the arguments in calls to `my_printf'
     for consistency with the `printf' style format string argument

     The parameter ARCHETYPE determines how the format string is
     interpreted, and should be either `printf' or `scanf'.  The
     parameter STRING-INDEX specifies which argument is the format
     string argument (starting from 1), while FIRST-TO-CHECK is the
     number of the first argument to check against the format string. 
     For functions where the arguments are not available to be checked
     (such as `vprintf'), specify the third parameter as zero.  In this
     case the compiler only checks the format string for consistency.

     In the example above, the format string (`my_format') is the second
     argument of the function `my_print', and the arguments to check
     start with the third argument, so the correct parameters for the
     format attribute are 2 and 3.

     The `format' attribute allows you to identify your own functions
     which take format strings as arguments, so that GNU CC can check
     the calls to these functions for errors.  The compiler always
     checks formats for the ANSI library functions `printf', `fprintf',
     `sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and
     `vsprintf' whenever such warnings are requested (using
     `-Wformat'), so there is no need to modify the header file

File: gcc.info,  Node: Function Prototypes,  Next: Dollar Signs,  Prev: Function Attributes,  Up: Extensions

Prototypes and Old-Style Function Definitions

   GNU C extends ANSI C to allow a function prototype to override a
later old-style non-prototype definition.  Consider the following

     /* Use prototypes unless the compiler is old-fashioned.  */
     #if __STDC__
     #define P(x) (x)
     #define P(x) ()
     /* Prototype function declaration.  */
     int isroot P((uid_t));
     /* Old-style function definition.  */
     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

     int isroot (uid_t);
     isroot (uid_t x)
       return x == 0;

File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: Function Prototypes,  Up: Extensions

Dollar Signs in Identifier Names

   In GNU C, you may use dollar signs in identifier names.  This is
because many traditional C implementations allow such identifiers.

   On some machines, dollar signs are allowed in identifiers if you
specify `-traditional'.  On a few systems they are allowed by default,
even if you do not use `-traditional'.  But they are never allowed if
you specify `-ansi'.

   There are certain ANSI C programs (obscure, to be sure) that would
compile incorrectly if dollar signs were permitted in identifiers.  For

     #define foo(a) #a
     #define lose(b) foo (b)
     #define test$
     lose (test)

File: gcc.info,  Node: Character Escapes,  Next: Variable Attributes,  Prev: Dollar Signs,  Up: Extensions

The Character ESC in Constants

   You can use the sequence `\e' in a string or character constant to
stand for the ASCII character ESC.

File: gcc.info,  Node: Alignment,  Next: Inline,  Prev: Variable Attributes,  Up: Extensions

Inquiring on Alignment of Types or Variables

   The keyword `__alignof__' allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type.  Its
syntax is just like `sizeof'.

   For example, if the target machine requires a `double' value to be
aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This
is true on many RISC machines.  On more traditional machine designs,
`__alignof__ (double)' is 4 or even 2.

   Some machines never actually require alignment; they allow reference
to any data type even at an odd addresses.  For these machines,
`__alignof__' reports the *recommended* alignment of a type.

   When the operand of `__alignof__' is an lvalue rather than a type,
the value is the largest alignment that the lvalue is known to have. 
It may have this alignment as a result of its data type, or because it
is part of a structure and inherits alignment from that structure. For
example, after this declaration:

     struct foo { int x; char y; } foo1;

the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as
`__alignof__ (int)', even though the data type of `foo1.y' does not
itself demand any alignment.

   A related feature which lets you specify the alignment of an object
is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.

File: gcc.info,  Node: Variable Attributes,  Next: Alignment,  Prev: Character Escapes,  Up: Extensions

Specifying Attributes of Variables

   The keyword `__attribute__' allows you to specify special attributes
of variables or structure fields.  This keyword is followed by an
attribute specification inside double parentheses.  Four attributes are
currently defined: `aligned', `format', `mode' and `packed'.  `format'
is used for functions, and thus not documented here; see *Note Function

`aligned (ALIGNMENT)'
     This attribute specifies the alignment of the variable or structure
     field, measured in bytes.  For example, the declaration:

          int x __attribute__ ((aligned (16))) = 0;

     causes the compiler to allocate the global variable `x' on a
     16-byte boundary.  On a 68040, this could be used in conjunction
     with an `asm' expression to access the `move16' instruction which
     requires 16-byte aligned operands.

     You can also specify the alignment of structure fields.  For
     example, to create a double-word aligned `int' pair, you could

          struct foo { int x[2] __attribute__ ((aligned (8))); };

     This is an alternative to creating a union with a `double' member
     that forces the union to be double-word aligned.

     It is not possible to specify the alignment of functions; the
     alignment of functions is determined by the machine's requirements
     and cannot be changed.  You cannot specify alignment for a typedef
     name because such a name is just an alias, not a distinct type.

     The linker of your operating system imposes a maximum alignment. 
     If the linker aligns each object file on a four byte boundary,
     then it is beyond the compiler's power to cause anything to be
     aligned to a larger boundary than that.  For example, if  the
     linker happens to put this object file at address 136 (eight more
     than a multiple of 64), then the compiler cannot guarantee an
     alignment of more than 8 just by aligning variables in the object

`mode (MODE)'
     This attribute specifies the data type for the
     declaration--whichever type corresponds to the mode MODE.  This in
     effect lets you request an integer or floating point type
     according to its width.

     The `packed' attribute specifies that a variable or structure field
     should have the smallest possible alignment--one byte for a
     variable, and one bit for a field, unless you specify a larger
     value with the `aligned' attribute.

File: gcc.info,  Node: Inline,  Next: Extended Asm,  Prev: Alignment,  Up: Extensions

An Inline Function is As Fast As a Macro

   By declaring a function `inline', you can direct GNU CC to integrate
that function's code into the code for its callers.  This makes
execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included.  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)

   (If you are writing a header file to be included in ANSI C programs,
write `__inline__' instead of `inline'.  *Note Alternate Keywords::.)

   You can also make all "simple enough" functions inline with the
option `-finline-functions'.  Note that certain usages in a function
definition can make it unsuitable for inline substitution.

   When a function is both inline and `static', if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GNU CC does not actually output assembler code for the
function, unless you specify the option `-fkeep-inline-functions'. Some
calls cannot be integrated for various reasons (in particular, calls
that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition).  If there is a
nonintegrated call, then the function is compiled to assembler code as
usual.  The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.

   When an inline function is not `static', then the compiler must
assume that there may be calls from other source files; since a global
symbol can be defined only once in any program, the function must not
be defined in the other source files, so the calls therein cannot be
integrated. Therefore, a non-`static' inline function is always
compiled on its own in the usual fashion.

   If you specify both `inline' and `extern' in the function
definition, then the definition is used only for inlining.  In no case
is the function compiled on its own, not even if you refer to its
address explicitly.  Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.

   This combination of `inline' and `extern' has almost the effect of a
macro.  The way to use it is to put a function definition in a header
file with these keywords, and put another copy of the definition
(lacking `inline' and `extern') in a library file. The definition in
the header file will cause most calls to the function to be inlined. 
If any uses of the function remain, they will refer to the single copy
in the library.

   GNU C does not inline any functions when not optimizing.  It is not
clear whether it is better to inline or not, in this case, but we found
that a correct implementation when not optimizing was difficult.  So we
did the easy thing, and turned it off.

File: gcc.info,  Node: Extended Asm,  Next: Asm Labels,  Prev: Inline,  Up: Extensions

Assembler Instructions with C Expression Operands

   In an assembler instruction using `asm', you can now specify the
operands of the instruction using C expressions.  This means no more
guessing which registers or memory locations will contain the data you
want to use.

   You must specify an assembler instruction template much like what
appears in a machine description, plus an operand constraint string for
each operand.

   For example, here is how to use the 68881's `fsinx' instruction:

     asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here `angle' is the C expression for the input operand while `result'
is that of the output operand.  Each has `"f"' as its operand
constraint, saying that a floating point register is required.  The `='
in `=f' indicates that the operand is an output; all output operands'
constraints must use `='.  The constraints use the same language used
in the machine description (*note Constraints::.).

Each operand is described by an operand-constraint string followed by
the C expression in parentheses.  A colon separates the assembler
template from the first output operand, and another separates the last
output operand from the first input, if any.  Commas separate output
operands and separate inputs.  The total number of operands is limited
to ten or to the maximum number of operands in any instruction pattern
in the machine description, whichever is greater.

   If there are no output operands, and there are input operands, then
there must be two consecutive colons surrounding the place where the
output operands would go.

   Output operand expressions must be lvalues; the compiler can check
this. The input operands need not be lvalues.  The compiler cannot
check whether the operands have data types that are reasonable for the
instruction being executed.  It does not parse the assembler
instruction template and does not know what it means, or whether it is
valid assembler input.  The extended `asm' feature is most often used
for machine instructions that the compiler itself does not know exist.

   The output operands must be write-only; GNU CC will assume that the
values in these operands before the instruction are dead and need not be
generated.  Extended asm does not support input-output or read-write
operands.  For this reason, the constraint character `+', which
indicates such an operand, may not be used.

   When the assembler instruction has a read-write operand, or an
operand in which only some of the bits are to be changed, you must
logically split its function into two separate operands, one input
operand and one write-only output operand.  The connection between them
is expressed by constraints which say they need to be in the same
location when the instruction executes.  You can use the same C
expression for both operands, or different expressions.  For example,
here we write the (fictitious) `combine' instruction with `bar' as its
read-only source operand and `foo' as its read-write destination:

     asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint `"0"' for operand 1 says that it must occupy the same
location as operand 0.  A digit in constraint is allowed only in an
input operand, and it must refer to an output operand.

   Only a digit in the constraint can guarantee that one operand will
be in the same place as another.  The mere fact that `foo' is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code.  The following would not

     asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

   Various optimizations or reloading could cause operands 0 and 1 to
be in different registers; GNU CC knows no reason not to do so.  For
example, the compiler might find a copy of the value of `foo' in one
register and use it for operand 1, but generate the output operand 0 in
a different register (copying it afterward to `foo''s own address).  Of
course, since the register for operand 1 is not even mentioned in the
assembler code, the result will not work, but GNU CC can't tell that.

   Some instructions clobber specific hard registers.  To describe
this, write a third colon after the input operands, followed by the
names of the clobbered hard registers (given as strings).  Here is a
realistic example for the Vax:

     asm volatile ("movc3 %0,%1,%2"
                   : /* no outputs */
                   : "g" (from), "g" (to), "g" (count)
                   : "r0", "r1", "r2", "r3", "r4", "r5");

   If you refer to a particular hardware register from the assembler
code, then you will probably have to list the register after the third
colon to tell the compiler that the register's value is modified.  In
many assemblers, the register names begin with `%'; to produce one `%'
in the assembler code, you must write `%%' in the input.

   If your assembler instruction can alter the condition code register,
add `cc' to the list of clobbered registers.  GNU CC on some machines
represents the condition codes as a specific hardware register; `cc'
serves to name this register.  On other machines, the condition code is
handled differently, and specifying `cc' has no effect.  But it is
valid no matter what the machine.

   If your assembler instruction modifies memory in an unpredicable
fashion, add `memory' to the list of clobbered registers. This will
cause GNU CC to not keep memory values cached in registers across the
assembler instruction.

   You can put multiple assembler instructions together in a single
`asm' template, separated either with newlines (written as `\n') or with
semicolons if the assembler allows such semicolons.  The GNU assembler
allows semicolons and all Unix assemblers seem to do so.  The input
operands are guaranteed not to use any of the clobbered registers, and
neither will the output operands' addresses, so you can read and write
the clobbered registers as many times as you like.  Here is an example
of multiple instructions in a template; it assumes that the subroutine
`_foo' accepts arguments in registers 9 and 10:

     asm ("movl %0,r9;movl %1,r10;call _foo"
          : /* no outputs */
          : "g" (from), "g" (to)
          : "r9", "r10");

   Unless an output operand has the `&' constraint modifier, GNU CC may
allocate it in the same register as an unrelated input operand, on the
assumption that the inputs are consumed before the outputs are produced.
This assumption may be false if the assembler code actually consists of
more than one instruction.  In such a case, use `&' for each output
operand that may not overlap an input. *Note Modifiers::.

   If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the `asm'
construct, as follows:

     asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
          : "g" (result)
          : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler
and most Unix assemblers do.

   Usually the most convenient way to use these `asm' instructions is to
encapsulate them in macros that look like functions.  For example,

     #define sin(x)       \
     ({ double __value, __arg = (x);   \
        asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
        __value; })

Here the variable `__arg' is used to make sure that the instruction
operates on a proper `double' value, and to accept only those arguments
`x' which can convert automatically to a `double'.

   Another way to make sure the instruction operates on the correct
data type is to use a cast in the `asm'.  This is different from using a
variable `__arg' in that it converts more different types.  For
example, if the desired type were `int', casting the argument to `int'
would accept a pointer with no complaint, while assigning the argument
to an `int' variable named `__arg' would warn about using a pointer
unless the caller explicitly casts it.

   If an `asm' has output operands, GNU CC assumes for optimization
purposes that the instruction has no side effects except to change the
output operands.  This does not mean that instructions with a side
effect cannot be used, but you must be careful, because the compiler
may eliminate them if the output operands aren't used, or move them out
of loops, or replace two with one if they constitute a common
subexpression.  Also, if your instruction does have a side effect on a
variable that otherwise appears not to change, the old value of the
variable may be reused later if it happens to be found in a register.

   You can prevent an `asm' instruction from being deleted, moved
significantly, or combined, by writing the keyword `volatile' after the
`asm'.  For example:

     #define set_priority(x)  \
     asm volatile ("set_priority %0": /* no outputs */ : "g" (x))

An instruction without output operands will not be deleted or moved
significantly, regardless, unless it is unreachable.

   Note that even a volatile `asm' instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions.  You can't expect a sequence of volatile `asm'
instructions to remain perfectly consecutive.  If you want consecutive
output, use a single `asm'.

   It is a natural idea to look for a way to give access to the
condition code left by the assembler instruction.  However, when we
attempted to implement this, we found no way to make it work reliably. 
The problem is that output operands might need reloading, which would
result in additional following "store" instructions.  On most machines,
these instructions would alter the condition code before there was time
to test it.  This problem doesn't arise for ordinary "test" and
"compare" instructions because they don't have any output operands.

   If you are writing a header file that should be includable in ANSI C
programs, write `__asm__' instead of `asm'.  *Note Alternate Keywords::.

File: gcc.info,  Node: Asm Labels,  Next: Explicit Reg Vars,  Prev: Extended Asm,  Up: Extensions

Controlling Names Used in Assembler Code

   You can specify the name to be used in the assembler code for a C
function or variable by writing the `asm' (or `__asm__') keyword after
the declarator as follows:

     int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable `foo' in the
assembler code should be `myfoo' rather than the usual `_foo'.

   On systems where an underscore is normally prepended to the name of
a C function or variable, this feature allows you to define names for
the linker that do not start with an underscore.

   You cannot use `asm' in this way in a function *definition*; but you
can get the same effect by writing a declaration for the function
before its definition and putting `asm' there, like this:

     extern func () asm ("FUNC");
     func (x, y)
          int x, y;

   It is up to you to make sure that the assembler names you choose do
not conflict with any other assembler symbols.  Also, you must not use a
register name; that would produce completely invalid assembler code. 
GNU CC does not as yet have the ability to store static variables in
registers. Perhaps that will be added.

File: gcc.info,  Node: Explicit Reg Vars,  Next: Alternate Keywords,  Prev: Asm Labels,  Up: Extensions

Variables in Specified Registers

   GNU C allows you to put a few global variables into specified
hardware registers.  You can also specify the register in which an
ordinary register variable should be allocated.

   * Global register variables reserve registers throughout the program.
     This may be useful in programs such as programming language
     interpreters which have a couple of global variables that are
     accessed very often.

   * Local register variables in specific registers do not reserve the
     registers.  The compiler's data flow analysis is capable of
     determining where the specified registers contain live values, and
     where they are available for other uses.

     These local variables are sometimes convenient for use with the
     extended `asm' feature (*note Extended Asm::.), if you want to
     write one output of the assembler instruction directly into a
     particular register. (This will work provided the register you
     specify fits the constraints specified for that operand in the

* Menu:

* Global Reg Vars::
* Local Reg Vars::

File: gcc.info,  Node: Global Reg Vars,  Next: Local Reg Vars,  Up: Explicit Reg Vars

Defining Global Register Variables

   You can define a global register variable in GNU C like this:

     register int *foo asm ("a5");

Here `a5' is the name of the register which should be used.  Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.

   Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type.  The register `a5'
would be a good choice on a 68000 for a variable of pointer type.  On
machines with register windows, be sure to choose a "global" register
that is not affected magically by the function call mechanism.

   In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals. 
For example, some 68000 operating systems call this register `%a5'.

   Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice.  No solution is

   Defining a global register variable in a certain register reserves
that register entirely for this use, at least within the current
compilation. The register will not be allocated for any other purpose
in the functions in the current compilation.  The register will not be
saved and restored by these functions.  Stores into this register are
never deleted even if they would appear to be dead, but references may
be deleted or moved or simplified.

   It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things
(unless you recompile them specially for the task at hand).

   It is not safe for one function that uses a global register variable
to call another such function `foo' by way of a third function `lose'
that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared).  This is
because `lose' might save the register and put some other value there.
For example, you can't expect a global register variable to be
available in the comparison-function that you pass to `qsort', since
`qsort' might have put something else in that register.  (If you are
prepared to recompile `qsort' with the same global register variable,
you can solve this problem.)

   If you want to recompile `qsort' or other source files which do not
actually use your global register variable, so that they will not use
that register for any other purpose, then it suffices to specify the
compiler option `-ffixed-REG'.  You need not actually add a global
register declaration to their source code.

   A function which can alter the value of a global register variable
cannot safely be called from a function compiled without this variable,
because it could clobber the value the caller expects to find there on
return. Therefore, the function which is the entry point into the part
of the program that uses the global register variable must explicitly
save and restore the value which belongs to its caller.

   On most machines, `longjmp' will restore to each global register
variable the value it had at the time of the `setjmp'.  On some
machines, however, `longjmp' will not change the value of global
register variables.  To be portable, the function that called `setjmp'
should make other arrangements to save the values of the global register
variables, and to restore them in a `longjmp'.  This way, the same
thing will happen regardless of what `longjmp' does.

   All global register variable declarations must precede all function
definitions.  If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register
from being used for other purposes in the preceding functions.

   Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.

   On the Sparc, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as `getwd', as well as
the subroutines for division and remainder, modify g3 and g4.  g1 and
g2 are local temporaries.

   On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of
course, it will not do to use more than a few of those.

File: gcc.info,  Node: Local Reg Vars,  Prev: Global Reg Vars,  Up: Explicit Reg Vars

Specifying Registers for Local Variables

   You can define a local register variable with a specified register
like this:

     register int *foo asm ("a5");

Here `a5' is the name of the register which should be used.  Note that
this is the same syntax used for defining global register variables,
but for a local variable it would appear within a function.

   Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (*note Extended Asm::.).  Both of these things
generally require that you conditionalize your program according to cpu

   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

   Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control determines
the variable's value is not live.  However, these registers are made
unavailable for use in the reload pass.  I would not be surprised if
excessive use of this feature leaves the compiler too few available
registers to compile certain functions.

File: gcc.info,  Node: Alternate Keywords,  Next: Incomplete Enums,  Prev: Explicit Reg Vars,  Up: Extensions

Alternate Keywords

   The option `-traditional' disables certain keywords; `-ansi'
disables certain others.  This causes trouble when you want to use GNU C
extensions, or ANSI C features, in a general-purpose header file that
should be usable by all programs, including ANSI C programs and
traditional ones.  The keywords `asm', `typeof' and `inline' cannot be
used since they won't work in a program compiled with `-ansi', while
the keywords `const', `volatile', `signed', `typeof' and `inline' won't
work in a program compiled with `-traditional'.

   The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword.  For example, use `__asm__' instead
of `asm', `__const__' instead of `const', and `__inline__' instead of

   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

   `-pedantic' causes warnings for many GNU C extensions.  You can
prevent such warnings within one expression by writing `__extension__'
before the expression.  `__extension__' has no effect aside from this.

File: gcc.info,  Node: Incomplete Enums,  Prev: Alternate Keywords,  Up: Extensions

Incomplete `enum' Types

   You can define an `enum' tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
`struct foo' without describing the elements.  A later declaration
which does specify the possible values completes the type.

   You can't allocate variables or storage using the type while it is
incomplete.  However, you can work with pointers to that type.

   This extension may not be very useful, but it makes the handling of
`enum' more consistent with the way `struct' and `union' are handled.

File: gcc.info,  Node: Trouble,  Next: Bugs,  Prev: Extensions,  Up: Top

Known Causes of Trouble with GNU CC

   This section describes known problems that affect users of GNU CC. 
Most of these are not GNU CC bugs per se--if they were, we would fix
them. But the result for a user may be like the result of a bug.

   Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.

* Menu:

* Actual Bugs::		      Bugs we will fix later.
* Installation Problems::     Problems that manifest when you install GNU CC.
* Cross-Compiler Problems::   Common problems of cross compiling with GNU CC.
* Interoperation::      Problems using GNU CC with other compilers,
			   and with certain linkers, assemblers and debuggers.
* Incompatibilities::   GNU CC is incompatible with traditional C.
* Disappointments::     Regrettable things we can't change, but not quite bugs.
* Protoize Caveats::    Things to watch out for when using `protoize'.
* Non-bugs::		Things we think are right, but some others disagree.

File: gcc.info,  Node: Actual Bugs,  Next: Installation Problems,  Up: Trouble

Actual Bugs We Haven't Fixed Yet

   * Loop unrolling doesn't work properly for certain C++ programs. 
     This is because of difficulty in updating the debugging
     information within the loop being unrolled.  We plan to revamp the
     representation of debugging information so that this will work
     properly, but we have not done this in version 2.3 because we
     don't want to delay it any further.

File: gcc.info,  Node: Installation Problems,  Next: Cross-Compiler Problems,  Prev: Actual Bugs,  Up: Trouble

Installation Problems

   This is a list of problems (and some apparent problems which don't
really mean anything is wrong) that show up during installation of GNU

   * On certain systems, defining certain environment variables such as
     `CC' can interfere with the functioning of `make'.

   * If you encounter seemingly strange errors when trying to build the
     compiler in a directory other than the source directory, it could
     be because you have previously configured the compiler in the
     source directory.  Make sure you have done all the necessary
     preparations. *Note Other Dir::.

   * In previous versions of GNU CC, the `gcc' driver program looked for
     `as' and `ld' in various places such as files beginning with
     `/usr/local/lib/gcc-'.  GNU CC version 2 looks for them in the
     directory `/usr/local/lib/gcc-lib/TARGET/VERSION'.

     Thus, to use a version of `as' or `ld' that is not the system
     default, for example `gas' or GNU `ld', you must put them in that
     directory (or make links to them from that directory).

   * Some commands executed when making the compiler may fail (return a
     non-zero status) and be ignored by `make'.  These failures, which
     are often due to files that were not found, are expected, and can
     safely be ignored.

   * It is normal to have warnings in compiling certain files about
     unreachable code and about enumeration type clashes.  These files'
     names begin with `insn-'.

   * Sometimes `make' recompiles parts of the compiler when installing
     the compiler.  In one case, this was traced down to a bug in
     `make'.  Either ignore the problem or switch to GNU Make.

   * On some 386 systems, building the compiler never finishes because
     `enquire' hangs due to a hardware problem in the motherboard--it
     reports floating point exceptions to the kernel incorrectly.  You
     can install GNU CC except for `float.h' by patching out the
     command to run `enquire'.  You may also be able to fix the problem
     for real by getting a replacement motherboard.  This problem was
     observed in Revision E of the Micronics motherboard, and is fixed
     in Revision F.

   * On some 386 systems, GNU CC crashes trying to compile `enquire.c'.
     This happens on machines that don't have a 387 FPU chip.  On 386
     machines, the system kernel is supposed to emulate the 387 when you
     don't have one.  The crash is due to a bug in the emulator.

     One of these systems is the Unix from Interactive Systems: 386/ix.
     On this system, an alternate emulator is provided, and it does
     work. To use it, execute this command as super-user:

          ln /etc/emulator.rel1 /etc/emulator

     and then reboot the system.  (The default emulator file remains
     present under the name `emulator.dflt'.)

     If you have such a problem on the SCO system, try using

     Another system which has this problem is Esix.  We don't know
     whether it has an alternate emulator that works.

   * Sometimes on a Sun 4 you may observe a crash in the program
     `genflags' or `genoutput' while building GNU CC.  This is said to
     be due to a bug in `sh'.  You can probably get around it by running
     `genflags' or `genoutput' manually and then retrying the `make'.

   * If you use the 1.31 version of the MIPS assembler (such as was
     shipped with Ultrix 3.1), you will need to use the
     -fno-delayed-branch switch when optimizing floating point code. 
     Otherwise, the assembler will complain when the GCC compiler fills
     a branch delay slot with a floating point instruction, such as

   * Users have reported some problems with version 2.0 of the MIPS
     compiler tools that were shipped with Ultrix 4.1.  Version 2.10
     which came with Ultrix 4.2 seems to work fine.

   * Some versions of the MIPS linker will issue an assertion failure
     when linking code that uses `alloca' against shared libraries on
     RISC-OS 5.0, and DEC's OSF/1 systems.  This is a bug in the
     linker, that is supposed to be fixed in future revisions. To
     protect against this, GCC passes `-non_shared' to the linker
     unless you pass an explicit `-shared' or `-call_shared' switch.

   * On System V release 3, you may get this error message while

          ld fatal: failed to write symbol name SOMETHING
           in strings table for file WHATEVER

     This indicates that the disk is full or your ULIMIT won't allow
     the file to be as large as it needs to be.

   * On HP 9000 series 300 or 400 running HP-UX release 8.0, there is a
     bug in the assembler that must be fixed before GNU CC can be
     built.  This bug manifests itself during the first stage of
     compilation, while building `libgcc2.a':

          cc1: warning: `-g' option not supported on this version of GCC
          cc1: warning: `-g1' option not supported on this version of GCC
          ./gcc: Internal compiler error: program as got fatal signal 11

     A patched version of the assembler is available by anonymous ftp
     from `altdorf.ai.mit.edu' as the file
     `archive/cph/hpux-8.0-assembler'.  If you have HP software support,
     the patch can also be obtained directly from HP, as described in
     the following note:

          This is the patched assembler, to patch SR#1653-010439, where
          the assembler aborts on floating point constants.

          The bug is not really in the assembler, but in the shared
          library version of the function "cvtnum(3c)".  The bug on
          "cvtnum(3c)" is SR#4701-078451.  Anyway, the attached
          assembler uses the archive library version of "cvtnum(3c)"
          and thus does not exhibit the bug.

     This patch is also known as PHCO_0800.

   * Another assembler problem on the HP PA results in an error message
     like this while compiling part of `libgcc2.a':

          as: /usr/tmp/cca08196.s @line#30 [err#1060]
            Argument 1 or 3 in FARG upper
                   - lookahead = RTNVAL=GR

     This happens because HP changed the assembler syntax after system
     release 8.02.  GNU CC assumes the newer syntax; if your assembler
     wants the older syntax, comment out this line in the file


   * Some versions of the Pyramid C compiler are reported to be unable
     to compile GNU CC.  You must use an older version of GNU CC for
     bootstrapping.  One indication of this problem is if you get a
     crash when GNU CC compiles the function `muldi3' in file

     You may be able to succeed by getting GNU CC version 1, installing
     it, and using it to compile GNU CC version 2.  The bug in the
     Pyramid C compiler does not seem to affect GNU CC version 1.

   * On the Tower models 4N0 and 6N0, by default a process is not
     allowed to have more than one megabyte of memory.  GNU CC cannot
     compile itself (or many other programs) with `-O' in that much

     To solve this problem, reconfigure the kernel adding the following
     line to the configuration file:

          MAXUMEM = 4096

   * On the Altos 3068, programs compiled with GNU CC won't work unless
     you fix a kernel bug.  This happens using system versions V.2.2
     1.0gT1 and V.2.2 1.0e and perhaps later versions as well.  See the
     file `README.ALTOS'.

   * You will get several sorts of compilation and linking errors on the
     we32k if you don't follow the special instructions.  *Note WE32K

File: gcc.info,  Node: Cross-Compiler Problems,  Next: Interoperation,  Prev: Installation Problems,  Up: Trouble

Cross-Compiler Problems

   * Cross compilation can run into trouble for certain machines because
     some target machines' assemblers require floating point numbers to
     be written as *integer* constants in certain contexts.

     The compiler writes these integer constants by examining the
     floating point value as an integer and printing that integer,
     because this is simple to write and independent of the details of
     the floating point representation.  But this does not work if the
     compiler is running on a different machine with an incompatible
     floating point format, or even a different byte-ordering.

     In addition, correct constant folding of floating point values
     requires representing them in the target machine's format. (The C
     standard does not quite require this, but in practice it is the
     only way to win.)

     It is now possible to overcome these problems by defining macros
     such as `REAL_VALUE_TYPE'.  But doing so is a substantial amount of
     work for each target machine.  *Note Cross-compilation::.

   * At present, the program `mips-tfile' which adds debug support to
     object files on MIPS systems does not work in a cross compile