TopWare Tools/TOOLS.iso/tools/top1244/gccinfo.zoo/gccinfo/gcc.info-5

GNU Info File | 1992-02-16 | 45.8 KB | 1,055 lines

This is Info file gcc.info, produced by Makeinfo-1.43 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 preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public
License" and this permission notice may be included in translations
approved by the Free Software Foundation instead of in the original
English.


File: gcc.info,  Node: Function Attributes,  Next: Dollar Signs,  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 ();

     void
     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
this.

  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
*anything*.


File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: Function Attributes,  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.

   Dollar signs are allowed on certain machines if you specify
`-traditional'.  On a few systems they are allowed by default, even if
`-traditional' is not used.  But they are never allowed if you specify
`-ansi'.

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

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


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


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.  The only attributes
currently defined are the `aligned' and `format' attributes.

   The `aligned' attribute specifies the alignment of the variable or
structure field.  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 68000, this could be used in conjunction with an `asm'
expression to access the `move16' instruction which requires 16-byte
aligned operands.

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

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

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

   It is not possible to specify the alignment of functions; the
alignment of functions is determined by the machine's requirements and
cannot be changed.

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

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

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

   The first parameter of the `format' attribute determines how the
format string is interpreted, and should be either `printf' or
`scanf'.  The second parameter specifies the number of the format
string argument (starting from 1).  The third parameter specifies the
number of the first argument which should be checked 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 to `my_print' and the arguments to check start with the third
argument, so the correct parameters for the format attribute are 2 and
3.

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


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.

   To declare a function inline, use the `inline' keyword in its
declaration, like this:

     inline int
     inc (int *a)
     {
       (*a)++;
     }

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

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

   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.


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 work:

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

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

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

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

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

   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
     `asm'.)

* Menu:

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


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

Defining Global Register Variables
----------------------------------

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

     register int *foo asm ("a5");

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

Specifying Registers for Local Variables
----------------------------------------

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

     register int *foo asm ("a5");

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

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

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

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

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


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

Alternate Keywords
==================

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

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

   Other C compilers won't accept these alternative keywords; if you
want to compile with another compiler, you can define the alternate
keywords as macros to replace them with the customary keywords.  It
looks like this:

     #ifndef __GNUC__
     #define __asm__ asm
     #endif

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


File: gcc.info,  Node: Incomplete Enums,  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: Bugs,  Next: VMS,  Prev: Extensions,  Up: Top

Reporting Bugs
**************

   Your bug reports play an essential role in making GNU CC reliable.

   When you encounter a problem, the first thing to do is to see if it
is already known.  *Note Trouble::.  Also look in *Note
Incompatibilities::.  If it isn't known, then you should report the
problem.

   Reporting a bug may help you by bringing a solution to your
problem, or it may not.  (If it does not, look in the service
directory; see *Note Service::.)  In any case, the principal function
of a bug report is to help the entire community by making the next
version of GNU CC work better.  Bug reports are your contribution to
the maintenance of GNU CC.

   In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.

* Menu:

* Criteria:  Bug Criteria.   Have you really found a bug?
* Reporting: Bug Reporting.  How to report a bug effectively.
* Non-bugs::                 Some things we think are not problems.
* Known: Trouble.            Known problems.
* Help: Service.             Where to ask for help.


File: gcc.info,  Node: Bug Criteria,  Next: Bug Reporting,  Prev: Bugs,  Up: Bugs

Have You Found a Bug?
=====================

   If you are not sure whether you have found a bug, here are some
guidelines:

   * If the compiler gets a fatal signal, for any input whatever, that
     is a compiler bug.  Reliable compilers never crash.

   * If the compiler produces invalid assembly code, for any input
     whatever (except an `asm' statement), that is a compiler bug,
     unless the compiler reports errors (not just warnings) which
     would ordinarily prevent the assembler from being run.

   * If the compiler produces valid assembly code that does not
     correctly execute the input source code, that is a compiler bug.

     However, you must double-check to make sure, because you may have
     run into an incompatibility between GNU C and traditional C
     (*note Incompatibilities::.).  These incompatibilities might be
     considered bugs, but they are inescapable consequences of
     valuable features.

     Or you may have a program whose behavior is undefined, which
     happened by chance to give the desired results with another C
     compiler.

     For example, in many nonoptimizing compilers, you can write `x;'
     at the end of a function instead of `return x;', with the same
     results.  But the value of the function is undefined if `return'
     is omitted; it is not a bug when GNU CC produces different
     results.

     Problems often result from expressions with two increment
     operators, as in `f (*p++, *p++)'.  Your previous compiler might
     have interpreted that expression the way you intended; GNU CC
     might interpret it another way.  Neither compiler is wrong.  The
     bug is in your code.

     After you have localized the error to a single source line, it
     should be easy to check for these things.  If your program is
     correct and well defined, you have found a compiler bug.

   * If the compiler produces an error message for valid input, that
     is a compiler bug.

     Note that the following is not valid input, and the error message
     for it is not a bug:

          int foo (char);

          int
          foo (x)
               char x;
          { ... }

     The prototype says to pass a `char', while the definition says to
     pass an `int' and treat the value as a `char'.  This is what the
     ANSI standard says, and it makes sense.

   * If the compiler does not produce an error message for invalid
     input, that is a compiler bug.  However, you should note that
     your idea of "invalid input" might be my idea of "an extension"
     or "support for traditional practice".

   * If you are an experienced user of C compilers, your suggestions
     for improvement of GNU CC are welcome in any case.


File: gcc.info,  Node: Bug Reporting,  Next: Non-bugs,  Prev: Bug Criteria,  Up: Bugs

How to Report Bugs
==================

   Send bug reports for GNU C to one of these addresses:

     [e-mail]
     {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc

   *Do not send bug reports to `help-gcc', or to the newsgroup
`gnu.gcc.help'.* Most users of GNU CC do not want to receive bug
reports.  Those that do, have asked to be on `bug-gcc'.

   The mailing list `bug-gcc' has a newsgroup which serves as a
repeater.  The mailing list and the newsgroup carry exactly the same
messages.  Often people think of posting bug reports to the newsgroup
instead of mailing them.  This appears to work, but it has one problem
which can be crucial: a newsgroup posting does not contain a mail path
back to the sender.  Thus, if I need to ask for more information, I
may be unable to reach you.  For this reason, it is better to send bug
reports to the mailing list.

   As a last resort, send bug reports on paper to:

     GNU Compiler Bugs
     Free Software Foundation
     675 Mass Ave
     Cambridge, MA 02139

   The fundamental principle of reporting bugs usefully is this:
*report all the facts*.  If you are not sure whether to state a fact
or leave it out, state it!

   Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter.  Thus, you
might assume that the name of the variable you use in an example does
not matter.  Well, probably it doesn't, but one cannot be sure.
Perhaps the bug is a stray memory reference which happens to fetch
from the location where that name is stored in memory; perhaps, if the
name were different, the contents of that location would fool the
compiler into doing the right thing despite the bug.  Play it safe and
give a specific, complete example.  That is the easiest thing for you
to do, and the most helpful.

   Keep in mind that the purpose of a bug report is to enable me to fix
the bug if it is not known.  It isn't very important what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug is not known.

   Sometimes people give a few sketchy facts and ask, "Does this ring a
bell?"  Those bug reports are useless, and I urge everyone to *refuse
to respond to them* except to chide the sender to report bugs properly.

   To enable me to fix the bug, you should include all these things:

   * The version of GNU CC.  You can get this by running it with the
     `-v' option.

     Without this, I won't know whether there is any point in looking
     for the bug in the current version of GNU CC.

   * A complete input file that will reproduce the bug.  If the bug is
     in the C preprocessor, send me a source file and any header files
     that it requires.  If the bug is in the compiler proper (`cc1'),
     run your source file through the C preprocessor by doing `gcc -E
     SOURCEFILE > OUTFILE', then include the contents of OUTFILE in
     the bug report.  (Any `-I', `-D' or `-U' options that you used in
     actual compilation should also be used when doing this.)

     A single statement is not enough of an example.  In order to
     compile it, it must be embedded in a function definition; and the
     bug might depend on the details of how this is done.

     Without a real example I can compile, all I can do about your bug
     report is wish you luck.  It would be futile to try to guess how
     to provoke the bug.  For example, bugs in register allocation and
     reloading frequently depend on every little detail of the function
     they happen in.

   * The command arguments you gave GNU CC to compile that example and
     observe the bug.  For example, did you use `-O'?  To guarantee
     you won't omit something important, list them all.

     If I were to try to guess the arguments, I would probably guess
     wrong and then I would not encounter the bug.

   * The type of machine you are using, and the operating system name
     and version number.

   * The operands you gave to the `configure' command when you
     installed the compiler.

   * A description of what behavior you observe that you believe is
     incorrect.  For example, "It gets a fatal signal," or, "There is
     an incorrect assembler instruction in the output."

     Of course, if the bug is that the compiler gets a fatal signal,
     then I will certainly notice it.  But if the bug is incorrect
     output, I might not notice unless it is glaringly wrong.  I won't
     study all the assembler code from a 50-line C program just on the
     off chance that it might be wrong.

     Even if the problem you experience is a fatal signal, you should
     still say so explicitly.  Suppose something strange is going on,
     such as, your copy of the compiler is out of synch, or you have
     encountered a bug in the C library on your system.  (This has
     happened!)  Your copy might crash and mine would not.  If you
     told me to expect a crash, then when mine fails to crash, I would
     know that the bug was not happening for me.  If you had not told
     me to expect a crash, then I would not be able to draw any
     conclusion from my observations.

     Often the observed symptom is incorrect output when your program
     is run.  Sad to say, this is not enough information for me unless
     the program is short and simple.  If you send me a large program,
     I don't have time to figure out how it would work if compiled
     correctly, much less which line of it was compiled wrong.  So you
     will have to do that.  Tell me which source line it is, and what
     incorrect result happens when that line is executed.  A person
     who understands the program can find this as easily as a bug in
     the program itself.

   * If you send me examples of output from GNU CC, please use `-g'
     when you make them.  The debugging information includes source
     line numbers which are essential for correlating the output with
     the input.

   * If you wish to suggest changes to the GNU CC source, send me
     context diffs.  If you even discuss something in the GNU CC
     source, refer to it by context, not by line number.

     The line numbers in my development sources don't match those in
     your sources.  Your line numbers would convey no useful
     information to me.

   * Additional information from a debugger might enable me to find a
     problem on a machine which I do not have available myself.
     However, you need to think when you collect this information if
     you want it to have any chance of being useful.

     For example, many people send just a backtrace, but that is never
     useful by itself.  A simple backtrace with arguments conveys
     little about GNU CC because the compiler is largely data-driven;
     the same functions are called over and over for different RTL
     insns, doing different things depending on the details of the
     insn.

     Most of the arguments listed in the backtrace are useless because
     they are pointers to RTL list structure.  The numeric values of
     the pointers, which the debugger prints in the backtrace, have no
     significance whatever; all that matters is the contents of the
     objects they point to (and most of the contents are other such
     pointers).

     In addition, most compiler passes consist of one or more loops
     that scan the RTL insn sequence.  The most vital piece of
     information about such a loop--which insn it has reached--is
     usually in a local variable, not in an argument.

     What you need to provide in addition to a backtrace are the
     values of the local variables for several stack frames up.  When
     a local variable or an argument is an RTX, first print its value
     and then use the GDB command `pr' to print the RTL expression
     that it points to.  (If GDB doesn't run on your machine, use your
     debugger to call the function `debug_rtx' with the RTX as an
     argument.)  In general, whenever a variable is a pointer, its
     value is no use without the data it points to.

     In addition, include a debugging dump from just before the pass
     in which the crash happens.  Most bugs involve a series of insns,
     not just one.

   Here are some things that are not necessary:

   * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating
     which changes to the input file will make the bug go away and
     which changes will not affect it.

     This is often time consuming and not very useful, because the way
     I will find the bug is by running a single example under the
     debugger with breakpoints, not by pure deduction from a series of
     examples.  I recommend that you save your time for something else.

     Of course, if you can find a simpler example to report *instead*
     of the original one, that is a convenience for me.  Errors in the
     output will be easier to spot, running under the debugger will
     take less time, etc.  Most GNU CC bugs involve just one function,
     so the most straightforward way to simplify an example is to
     delete all the function definitions except the one where the bug
     occurs.  Those earlier in the file may be replaced by external
     declarations if the crucial function depends on them.
     (Exception: inline functions may affect compilation of functions
     defined later in the file.)

     However, simplification is not vital; if you don't want to do
     this, report the bug anyway and send me the entire test case you
     used.

   * A patch for the bug.

     A patch for the bug does help me if it is a good one.  But don't
     omit the necessary information, such as the test case, on the
     assumption that a patch is all I need.  I might see problems with
     your patch and decide to fix the problem another way, or I might
     not understand it at all.

     Sometimes with a program as complicated as GNU CC it is very hard
     to construct an example that will make the program follow a
     certain path through the code.  If you don't send me the example,
     I won't be able to construct one, so I won't be able to verify
     that the bug is fixed.

     And if I can't understand what bug you are trying to fix, or why
     your patch should be an improvement, I won't install it.  A test
     case will help me to understand.

   * A guess about what the bug is or what it depends on.

     Such guesses are usually wrong.  Even I can't guess right about
     such things without first using the debugger to find the facts.