This is Info file gcc.info, produced by Makeinfo version 1.68 from the
input file ./gcc.texi.

INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc). The GNU Compiler Collection.
END-INFO-DIR-ENTRY
This file documents the use and the internals of the GNU compiler.

Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

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

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License" and "Funding for Free Software", 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: Temporaries, Next: Copy Assignment, Prev: Static Definitions, Up: C++ Misunderstandings

Temporaries May Vanish Before You Expect
----------------------------------------

It is dangerous to use pointers or references to *portions* of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type `char *' or
`const char *' - which is one reason why the standard `string' class
requires you to call the `c_str' member function. However, any class
that returns a pointer to some internal structure is potentially
subject to this problem.

For example, a program may use a function `strfunc' that returns
`string' objects, and another function `charfunc' that operates on
pointers to `char':

string strfunc ();
void charfunc (const char *);

void
f ()
{
const char *p = strfunc().c_str();
...
charfunc (p);
...
charfunc (p);
}

In this situation, it may seem reasonable to save a pointer to the C
string returned by the `c_str' member function and use that rather than
call `c_str' repeatedly. However, the temporary string created by the
call to `strfunc' is destroyed after `p' is initialized, at which point
`p' is left pointing to freed memory.

Code like this may run successfully under some other compilers,
particularly obsolete cfront-based compilers that delete temporaries
along with normal local variables. However, the GNU C++ behavior is
standard-conforming, so if your program depends on late destruction of
temporaries it is not portable.

The safe way to write such code is to give the temporary a name,
which forces it to remain until the end of the scope of the name. For
example:

string& tmp = strfunc ();
charfunc (tmp.c_str ());

File: gcc.info, Node: Copy Assignment, Prev: Temporaries, Up: C++ Misunderstandings

Implicit Copy-Assignment for Virtual Bases
------------------------------------------

When a base class is virtual, only one subobject of the base class
belongs to each full object. Also, the constructors and destructors are
invoked only once, and called from the most-derived class. However, such
objects behave unspecified when being assigned. For example:

struct Base{
char *name;
Base(char *n) : name(strdup(n)){}
Base& operator= (const Base& other){
free (name);
name = strdup (other.name);
}
};

struct A:virtual Base{
int val;
A():Base("A"){}
};

struct B:virtual Base{
int bval;
B():Base("B"){}
};

struct Derived:public A, public B{
Derived():Base("Derived"){}
};

void func(Derived &d1, Derived &d2)
{
d1 = d2;
}

The C++ standard specifies that `Base::Base' is only called once
when constructing or copy-constructing a Derived object. It is
unspecified whether `Base::operator=' is called more than once when the
implicit copy-assignment for Derived objects is invoked (as it is
inside `func' in the example).

g++ implements the "intuitive" algorithm for copy-assignment: assign
all direct bases, then assign all members. In that algorithm, the
virtual base subobject can be encountered many times. In the example,
copying proceeds in the following order: `val', `name' (via `strdup'),
`bval', and `name' again.

If application code relies on copy-assignment, a user-defined
copy-assignment operator removes any uncertainties. With such an
operator, the application can define whether and how the virtual base
subobject is assigned.

File: gcc.info, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble

Caveats of using `protoize'
===========================

The conversion programs `protoize' and `unprotoize' can sometimes
change a source file in a way that won't work unless you rearrange it.

* `protoize' can insert references to a type name or type tag before
the definition, or in a file where they are not defined.

If this happens, compiler error messages should show you where the
new references are, so fixing the file by hand is straightforward.

* There are some C constructs which `protoize' cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. `protoize'
inserts a comment containing `???' each time it finds such a
variable; so you can find all such variables by searching for this
string. ANSI C does not require declaring the argument types of
pointer-to-function types.

* Using `unprotoize' can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure `unprotoize' is safe is when you
are removing prototypes that were made with `protoize'; if the
program worked before without any prototypes, it will work again
without them.

You can find all the places where this problem might occur by
compiling the program with the `-Wconversion' option. It prints a
warning whenever an argument is converted.

* Both conversion programs can be confused if there are macro calls
in and around the text to be converted. In other words, the
standard syntax for a declaration or definition must not result
from expanding a macro. This problem is inherent in the design of
C and cannot be fixed. If only a few functions have confusing
macro calls, you can easily convert them manually.

* `protoize' cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing
conditionals. When this happens, `protoize' changes nothing in
regard to such a function. `protoize' tries to detect such
instances and warn about them.

You can generally work around this problem by using `protoize' step
by step, each time specifying a different set of `-D' options for
compilation, until all of the functions have been converted.
There is no automatic way to verify that you have got them all,
however.

* Confusion may result if there is an occasion to convert a function
declaration or definition in a region of source code where there
is more than one formal parameter list present. Thus, attempts to
convert code containing multiple (conditionally compiled) versions
of a single function header (in the same vicinity) may not produce
the desired (or expected) results.

If you plan on converting source files which contain such code, it
is recommended that you first make sure that each conditionally
compiled region of source code which contains an alternative
function header also contains at least one additional follower
token (past the final right parenthesis of the function header).
This should circumvent the problem.

* `unprotoize' can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such
choices of formal parameter names.

* You might also want to correct some of the indentation by hand and
break long lines. (The conversion programs don't write lines
longer than eighty characters in any case.)

File: gcc.info, Node: Non-bugs, Next: Warnings and Errors, Prev: Protoize Caveats, Up: Trouble

Certain Changes We Don't Want to Make
=====================================

This section lists changes that people frequently request, but which
we do not make because we think GCC is better without them.

* Checking the number and type of arguments to a function which has
an old-fashioned definition and no prototype.

Such a feature would work only occasionally--only for calls that
appear in the same file as the called function, following the
definition. The only way to check all calls reliably is to add a
prototype for the function. But adding a prototype eliminates the
motivation for this feature. So the feature is not worthwhile.

* Warning about using an expression whose type is signed as a shift
count.

Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.

* Warning about assigning a signed value to an unsigned variable.

Such assignments must be very common; warning about them would
cause more annoyance than good.

* Warning about unreachable code.

It's very common to have unreachable code in machine-generated
programs. For example, this happens normally in some files of GNU
C itself.

* Warning when a non-void function value is ignored.

Coming as I do from a Lisp background, I balk at the idea that
there is something dangerous about discarding a value. There are
functions that return values which some callers may find useful;
it makes no sense to clutter the program with a cast to `void'
whenever the value isn't useful.

* Assuming (for optimization) that the address of an external symbol
is never zero.

This assumption is false on certain systems when `#pragma weak' is
used.

* Making `-fshort-enums' the default.

This would cause storage layout to be incompatible with most other
C compilers. And it doesn't seem very important, given that you
can get the same result in other ways. The case where it matters
most is when the enumeration-valued object is inside a structure,
and in that case you can specify a field width explicitly.

* Making bitfields unsigned by default on particular machines where
"the ABI standard" says to do so.

The ANSI C standard leaves it up to the implementation whether a
bitfield declared plain `int' is signed or not. This in effect
creates two alternative dialects of C.

The GNU C compiler supports both dialects; you can specify the
signed dialect with `-fsigned-bitfields' and the unsigned dialect
with `-funsigned-bitfields'. However, this leaves open the
question of which dialect to use by default.

Currently, the preferred dialect makes plain bitfields signed,
because this is simplest. Since `int' is the same as `signed int'
in every other context, it is cleanest for them to be the same in
bitfields as well.

Some computer manufacturers have published Application Binary
Interface standards which specify that plain bitfields should be
unsigned. It is a mistake, however, to say anything about this
issue in an ABI. This is because the handling of plain bitfields
distinguishes two dialects of C. Both dialects are meaningful on
every type of machine. Whether a particular object file was
compiled using signed bitfields or unsigned is of no concern to
other object files, even if they access the same bitfields in the
same data structures.

A given program is written in one or the other of these two
dialects. The program stands a chance to work on most any machine
if it is compiled with the proper dialect. It is unlikely to work
at all if compiled with the wrong dialect.

Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently
on certain machines.

Occasionally users write programs intended only for a particular
machine type. On these occasions, the users would benefit if the
GNU C compiler were to support by default the same dialect as the
other compilers on that machine. But such applications are rare.
And users writing a program to run on more than one type of
machine cannot possibly benefit from this kind of compatibility.

This is why GCC does and will treat plain bitfields in the same
fashion on all types of machines (by default).

There are some arguments for making bitfields unsigned by default
on all machines. If, for example, this becomes a universal de
facto standard, it would make sense for GCC to go along with it.
This is something to be considered in the future.

(Of course, users strongly concerned about portability should
indicate explicitly in each bitfield whether it is signed or not.
In this way, they write programs which have the same meaning in
both C dialects.)

* Undefining `__STDC__' when `-ansi' is not used.

Currently, GCC defines `__STDC__' as long as you don't use
`-traditional'. This provides good results in practice.

Programmers normally use conditionals on `__STDC__' to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain `gcc' supports
all the features of ANSI C, the correct answer to these questions
is "yes".

Some users try to use `__STDC__' to check for the availability of
certain library facilities. This is actually incorrect usage in
an ANSI C program, because the ANSI C standard says that a
conforming freestanding implementation should define `__STDC__'
even though it does not have the library facilities. `gcc -ansi
-pedantic' is a conforming freestanding implementation, and it is
therefore required to define `__STDC__', even though it does not
come with an ANSI C library.

Sometimes people say that defining `__STDC__' in a compiler that
does not completely conform to the ANSI C standard somehow
violates the standard. This is illogical. The standard is a
standard for compilers that claim to support ANSI C, such as `gcc
-ansi'--not for other compilers such as plain `gcc'. Whatever the
ANSI C standard says is relevant to the design of plain `gcc'
without `-ansi' only for pragmatic reasons, not as a requirement.

GCC normally defines `__STDC__' to be 1, and in addition defines
`__STRICT_ANSI__' if you specify the `-ansi' option. On some
hosts, system include files use a different convention, where
`__STDC__' is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention
when processing system include files, but when processing user
files it follows the usual GNU C convention.

* Undefining `__STDC__' in C++.

Programs written to compile with C++-to-C translators get the
value of `__STDC__' that goes with the C compiler that is
subsequently used. These programs must test `__STDC__' to
determine what kind of C preprocessor that compiler uses: whether
they should concatenate tokens in the ANSI C fashion or in the
traditional fashion.

These programs work properly with GNU C++ if `__STDC__' is defined.
They would not work otherwise.

In addition, many header files are written to provide prototypes
in ANSI C but not in traditional C. Many of these header files
can work without change in C++ provided `__STDC__' is defined. If
`__STDC__' is not defined, they will all fail, and will all need
to be changed to test explicitly for C++ as well.

* Deleting "empty" loops.

Historically, GCC has not deleted "empty" loops under the
assumption that the most likely reason you would put one in a
program is to have a delay, so deleting them will not make real
programs run any faster.

However, the rationale here is that optimization of a nonempty loop
cannot produce an empty one, which holds for C but is not always
the case for C++.

Moreover, with `-funroll-loops' small "empty" loops are already
removed, so the current behavior is both sub-optimal and
inconsistent and will change in the future.

* Making side effects happen in the same order as in some other
compiler.

It is never safe to depend on the order of evaluation of side
effects. For example, a function call like this may very well
behave differently from one compiler to another:

void func (int, int);

int i = 2;
func (i++, i++);

There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any
particular order. Either increment might happen first. `func'
might get the arguments `2, 3', or it might get `3, 2', or even
`2, 2'.

* Not allowing structures with volatile fields in registers.

Strictly speaking, there is no prohibition in the ANSI C standard
against allowing structures with volatile fields in registers, but
it does not seem to make any sense and is probably not what you
wanted to do. So the compiler will give an error message in this
case.

File: gcc.info, Node: Warnings and Errors, Prev: Non-bugs, Up: Trouble

Warning Messages and Error Messages
===================================

The GNU compiler can produce two kinds of diagnostics: errors and
warnings. Each kind has a different purpose:

*Errors* report problems that make it impossible to compile your
program. GCC reports errors with the source file name and line
number where the problem is apparent.

*Warnings* report other unusual conditions in your code that *may*
indicate a problem, although compilation can (and does) proceed.
Warning messages also report the source file name and line number,
but include the text `warning:' to distinguish them from error
messages.

Warnings may indicate danger points where you should check to make
sure that your program really does what you intend; or the use of
obsolete features; or the use of nonstandard features of GNU C or C++.
Many warnings are issued only if you ask for them, with one of the `-W'
options (for instance, `-Wall' requests a variety of useful warnings).

GCC always tries to compile your program if possible; it never
gratuitously rejects a program whose meaning is clear merely because
(for instance) it fails to conform to a standard. In some cases,
however, the C and C++ standards specify that certain extensions are
forbidden, and a diagnostic *must* be issued by a conforming compiler.
The `-pedantic' option tells GCC to issue warnings in such cases;
`-pedantic-errors' says to make them errors instead. This does not
mean that *all* non-ANSI constructs get warnings or errors.

*Note Options to Request or Suppress Warnings: Warning Options, for
more detail on these and related command-line options.

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

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

Your bug reports play an essential role in making GCC reliable.

When you encounter a problem, the first thing to do is to see if it
is already known. *Note Trouble::. 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 GCC work
better. Bug reports are your contribution to the maintenance of GCC.

Since the maintainers are very overloaded, we cannot respond to every
bug report. However, if the bug has not been fixed, we are likely to
send you a patch and ask you to tell us whether it works.

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?
* Where: Bug Lists. Where to send your bug report.
* Reporting: Bug Reporting. How to report a bug effectively.
* Patches: Sending Patches. How to send a patch for GCC.
* Known: Trouble. Known problems.
* Help: Service. Where to ask for help.

File: gcc.info, Node: Bug Criteria, Next: Bug Lists, 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 or
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 GCC 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; GCC 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.

* 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 or C++ (or Fortran or
Objective-C) compilers, your suggestions for improvement of GCC
are welcome in any case.

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

Where to Report Bugs
====================

Send bug reports for the GNU Compiler Collection to
`gcc-bugs@gcc.gnu.org'. In accordance with the GNU-wide convention, in
which bug reports for tool "foo" are sent to `bug-foo@gnu.org', the
address `bug-gcc@gnu.org' may also be used; it will forward to the
address given above.

Please read `<URL:http://www.gnu.org/software/gcc/bugs.html>' for
bug reporting instructions before you post a bug report.

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 maintainers need more information, they may be unable
to reach you. For this reason, you should always send bug reports by
mail to the proper mailing list.

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

GNU Compiler Bugs
Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA

File: gcc.info, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs

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

You may find additional and/or more up-to-date instructions at
`<URL:http://www.gnu.org/software/gcc/bugs.html>'.

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 someone 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?" This cannot help us fix a bug, so it is basically useless. We
respond by asking for enough details to enable us to investigate. You
might as well expedite matters by sending them to begin with.

Try to make your bug report self-contained. If we have to ask you
for more information, it is best if you include all the previous
information in your response, as well as the information that was
missing.

Please report each bug in a separate message. This makes it easier
for us to track which bugs have been fixed and to forward your bugs
reports to the appropriate maintainer.

To enable someone to investigate the bug, you should include all
these things:

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

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

* A complete input file that will reproduce the bug. If the bug is
in the C preprocessor, send a source file and any header files
that it requires. If the bug is in the compiler proper (`cc1'),
send the preprocessor output generated by adding `-save-temps' to
the compilation command (*note Debugging Options::.). When you do
this, use the same `-I', `-D' or `-U' options that you used in
actual compilation. Then send the INPUT.i or INPUT.ii files
generated.

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

Without a real example one can compile, all anyone 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.

Even if the input file that fails comes from a GNU program, you
should still send the complete test case. Don't ask the GCC
maintainers to do the extra work of obtaining the program in
question--they are all overworked as it is. Also, the problem may
depend on what is in the header files on your system; it is
unreliable for the GCC maintainers to try the problem with the
header files available to them. By sending CPP output, you can
eliminate this source of uncertainty and save us a certain
percentage of wild goose chases.

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

If we were to try to guess the arguments, we would probably guess
wrong and then we 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 complete list of any modifications you have made to the compiler
source. (We don't promise to investigate the bug unless it
happens in an unmodified compiler. But if you've made
modifications and don't tell us, then you are sending us on a wild
goose chase.)

Be precise about these changes. A description in English is not
enough--send a context diff for them.

Adding files of your own (such as a machine description for a
machine we don't support) is a modification of the compiler source.

* Details of any other deviations from the standard procedure for
installing GCC.

* A description of what behavior you observe that you believe is
incorrect. For example, "The compiler gets a fatal signal," or,
"The assembler instruction at line 208 in the output is incorrect."

Of course, if the bug is that the compiler gets a fatal signal,
then one can't miss it. But if the bug is incorrect output, the
maintainer might not notice unless it is glaringly wrong. None of
us has time to study all the assembler code from a 50-line C
program just on the chance that one instruction might be wrong.
We need *you* to do this part!

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 the copy here would not. If
you said to expect a crash, then when the compiler here fails to
crash, we would know that the bug was not happening. If you don't
say to expect a crash, then we would not know whether the bug was
happening. We would not be able to draw any conclusion from our
observations.

If the problem is a diagnostic when compiling GCC with some other
compiler, say whether it is a warning or an error.

Often the observed symptom is incorrect output when your program
is run. Sad to say, this is not enough information unless the
program is short and simple. None of us has time to study a large
program 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 us 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 finding a bug
in the program itself.

* If you send examples of assembler code output from GCC, 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 mention something in the GCC source, refer to it by
context, not by line number.

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

* Additional information from a debugger might enable someone to
find a problem on a machine which he does not have available.
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 GCC 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.

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
we will find the bug is by running a single example under the
debugger with breakpoints, not by pure deduction from a series of
examples. You might as well 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. Errors in the output
will be easier to spot, running under the debugger will take less
time, etc. Most GCC 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 the entire test case you used.

* In particular, some people insert conditionals `#ifdef BUG' around
a statement which, if removed, makes the bug not happen. These
are just clutter; we won't pay any attention to them anyway.
Besides, you should send us cpp output, and that can't have
conditionals.

* A patch for the bug.

A patch for the bug is useful 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 we need. We might see problems
with your patch and decide to fix the problem another way, or we
might not understand it at all.

Sometimes with a program as complicated as GCC 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 the example, we won't be
able to construct one, so we won't be able to verify that the bug
is fixed.

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

*Note Sending Patches::, for guidelines on how to make it easy for
us to understand and install your patches.

* 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.

* A core dump file.

We have no way of examining a core dump for your type of machine
unless we have an identical system--and if we do have one, we
should be able to reproduce the crash ourselves.

File: gcc.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs

Sending Patches for GCC
=======================

If you would like to write bug fixes or improvements for the GNU C
compiler, that is very helpful. Send suggested fixes to the patches
mailing list, `gcc-patches@gcc.gnu.org'.

Please follow these guidelines so we can study your patches
efficiently. If you don't follow these guidelines, your information
might still be useful, but using it will take extra work. Maintaining
GNU C is a lot of work in the best of circumstances, and we can't keep
up unless you do your best to help.

* Send an explanation with your changes of what problem they fix or
what improvement they bring about. For a bug fix, just include a
copy of the bug report, and explain why the change fixes the bug.

(Referring to a bug report is not as good as including it, because
then we will have to look it up, and we have probably already
deleted it if we've already fixed the bug.)

* Always include a proper bug report for the problem you think you
have fixed. We need to convince ourselves that the change is
right before installing it. Even if it is right, we might have
trouble judging it if we don't have a way to reproduce the problem.

* Include all the comments that are appropriate to help people
reading the source in the future understand why this change was
needed.

* Don't mix together changes made for different reasons. Send them
*individually*.

If you make two changes for separate reasons, then we might not
want to install them both. We might want to install just one. If
you send them all jumbled together in a single set of diffs, we
have to do extra work to disentangle them--to figure out which
parts of the change serve which purpose. If we don't have time
for this, we might have to ignore your changes entirely.

If you send each change as soon as you have written it, with its
own explanation, then the two changes never get tangled up, and we
can consider each one properly without any extra work to
disentangle them.

Ideally, each change you send should be impossible to subdivide
into parts that we might want to consider separately, because each
of its parts gets its motivation from the other parts.

* Send each change as soon as that change is finished. Sometimes
people think they are helping us by accumulating many changes to
send them all together. As explained above, this is absolutely
the worst thing you could do.

Since you should send each change separately, you might as well
send it right away. That gives us the option of installing it
immediately if it is important.

* Use `diff -c' to make your diffs. Diffs without context are hard
for us to install reliably. More than that, they make it hard for
us to study the diffs to decide whether we want to install them.
Unidiff format is better than contextless diffs, but not as easy
to read as `-c' format.

If you have GNU diff, use `diff -cp', which shows the name of the
function that each change occurs in.

* Write the change log entries for your changes. We get lots of
changes, and we don't have time to do all the change log writing
ourselves.

Read the `ChangeLog' file to see what sorts of information to put
in, and to learn the style that we use. The purpose of the change
log is to show people where to find what was changed. So you need
to be specific about what functions you changed; in large
functions, it's often helpful to indicate where within the
function the change was.

On the other hand, once you have shown people where to find the
change, you need not explain its purpose. Thus, if you add a new
function, all you need to say about it is that it is new. If you
feel that the purpose needs explaining, it probably does--but the
explanation will be much more useful if you put it in comments in
the code.

If you would like your name to appear in the header line for who
made the change, send us the header line.

* When you write the fix, keep in mind that we can't install a
change that would break other systems.

People often suggest fixing a problem by changing
machine-independent files such as `toplev.c' to do something
special that a particular system needs. Sometimes it is totally
obvious that such changes would break GCC for almost all users.
We can't possibly make a change like that. At best it might tell
us how to write another patch that would solve the problem
acceptably.

Sometimes people send fixes that *might* be an improvement in
general--but it is hard to be sure of this. It's hard to install
such changes because we have to study them very carefully. Of
course, a good explanation of the reasoning by which you concluded
the change was correct can help convince us.

The safest changes are changes to the configuration files for a
particular machine. These are safe because they can't create new
bugs on other machines.

Please help us keep up with the workload by designing the patch in
a form that is good to install.

File: gcc.info, Node: Service, Next: Contributing, Prev: Bugs, Up: Top

How To Get Help with GCC
************************

If you need help installing, using or changing GCC, there are two
ways to find it:

* Send a message to a suitable network mailing list. First try
`gcc-bugs@gcc.gnu.org' or `bug-gcc@gnu.org', and if that brings no
response, try `gcc@gcc.gnu.org'.

* Look in the service directory for someone who might help you for a
fee. The service directory is found in the file named `SERVICE'
in the GCC distribution.

File: gcc.info, Node: Contributing, Next: VMS, Prev: Service, Up: Top

Contributing to GCC Development
*******************************

If you would like to help pretest GCC releases to assure they work
well, or if you would like to work on improving GCC, please contact the
maintainers at `gcc@gcc.gnu.org'. A pretester should be willing to try
to investigate bugs as well as report them.

If you'd like to work on improvements, please ask for suggested
projects or suggest your own ideas. If you have already written an
improvement, please tell us about it. If you have not yet started
work, it is useful to contact `gcc@gcc.gnu.org' before you start; the
maintainers may be able to suggest ways to make your extension fit in
better with the rest of GCC and with other development plans.

File: gcc.info, Node: VMS, Next: Portability, Prev: Contributing, Up: Top

Using GCC on VMS
****************

Here is how to use GCC on VMS.

* Menu:

* Include Files and VMS:: Where the preprocessor looks for the include files.
* Global Declarations:: How to do globaldef, globalref and globalvalue with
GCC.
* VMS Misc:: Misc information.

File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Up: VMS

Include Files and VMS
=====================

Due to the differences between the filesystems of Unix and VMS, GCC
attempts to translate file names in `#include' into names that VMS will
understand. The basic strategy is to prepend a prefix to the
specification of the include file, convert the whole filename to a VMS
filename, and then try to open the file. GCC tries various prefixes
one by one until one of them succeeds:

1. The first prefix is the `GNU_CC_INCLUDE:' logical name: this is
where GNU C header files are traditionally stored. If you wish to
store header files in non-standard locations, then you can assign
the logical `GNU_CC_INCLUDE' to be a search list, where each
element of the list is suitable for use with a rooted logical.

2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'. This is where
VAX-C header files are traditionally stored.

3. If the include file specification by itself is a valid VMS
filename, the preprocessor then uses this name with no prefix in
an attempt to open the include file.

4. If the file specification is not a valid VMS filename (i.e. does
not contain a device or a directory specifier, and contains a `/'
character), the preprocessor tries to convert it from Unix syntax
to VMS syntax.

Conversion works like this: the first directory name becomes a
device, and the rest of the directories are converted into
VMS-format directory names. For example, the name `X11/foobar.h'
is translated to `X11:[000000]foobar.h' or `X11:foobar.h',
whichever one can be opened. This strategy allows you to assign a
logical name to point to the actual location of the header files.

5. If none of these strategies succeeds, the `#include' fails.

Include directives of the form:

#include foobar

are a common source of incompatibility between VAX-C and GCC. VAX-C
treats this much like a standard `#include <foobar.h>' directive. That
is incompatible with the ANSI C behavior implemented by GCC: to expand
the name `foobar' as a macro. Macro expansion should eventually yield
one of the two standard formats for `#include':

#include "FILE"
#include <FILE>

If you have this problem, the best solution is to modify the source
to convert the `#include' directives to one of the two standard forms.
That will work with either compiler. If you want a quick and dirty fix,
define the file names as macros with the proper expansion, like this:

#define stdio <stdio.h>

This will work, as long as the name doesn't conflict with anything else
in the program.

Another source of incompatibility is that VAX-C assumes that:

#include "foobar"

is actually asking for the file `foobar.h'. GCC does not make this
assumption, and instead takes what you ask for literally; it tries to
read the file `foobar'. The best way to avoid this problem is to
always specify the desired file extension in your include directives.

GCC for VMS is distributed with a set of include files that is
sufficient to compile most general purpose programs. Even though the
GCC distribution does not contain header files to define constants and
structures for some VMS system-specific functions, there is no reason
why you cannot use GCC with any of these functions. You first may have
to generate or create header files, either by using the public domain
utility `UNSDL' (which can be found on a DECUS tape), or by extracting
the relevant modules from one of the system macro libraries, and using
an editor to construct a C header file.

A `#include' file name cannot contain a DECNET node name. The
preprocessor reports an I/O error if you attempt to use a node name,
whether explicitly, or implicitly via a logical name.