make POP_SAVEARRAY() safe

[perl5.git] / hints / README.hints

=head1 NAME

README.hints - hint files used by Configure

=head1 DESCRIPTION

These files are used by Configure to set things which Configure either
can't or doesn't guess properly.  Most of these hint files have been
tested with at least some version of perl5, but some are still left
over from perl4.

Please send any problems or suggested changes to perlbug@perl.org.

=head1 Hint file naming convention.

Each hint file name should have only
one '.'.  (This is for portability to non-unix file systems.)  Names
should also fit in <= 14 characters, for portability to older SVR3
systems.  File names are of the form $osname_$osvers.sh, with all '.'
changed to '_', and all characters (such as '/') that don't belong in
Unix filenames omitted.

For example, consider Sun OS 4.1.3.  Configure determines $osname=sunos
(all names are converted to lower case) and $osvers=4.1.3.  Configure
will search for an appropriate hint file in the following order:

	sunos_4_1_3.sh
	sunos_4_1.sh
	sunos_4.sh
	sunos.sh

If you need to create a hint file, please try to use as general a name
as possible and include minor version differences inside case or test
statements.  For example, for IRIX 6.X, we have the following hints
files:

	irix_6_0.sh
	irix_6_1.sh
	irix_6.sh

That is, 6.0 and 6.1 have their own special hints, but 6.2, 6.3, and
up are all handled by the same irix_6.sh.  That way, we don't have to
make a new hint file every time the IRIX O/S is upgraded.

If you need to test for specific minor version differences in your
hints file, be sure to include a default choice.  (See aix.sh for one
example.) That way, if you write a hint file for foonix 3.2, it might
still work without any changes when foonix 3.3 is released.

Please also comment carefully on why the different hints are needed.
That way, a future version of Configure may be able to automatically
detect what is needed.

A glossary of config.sh variables is in the file Porting/Glossary.

=head1 Setting variables

=head2 Optimizer

If you want to set a variable, try to allow for Configure command-line
overrides.  For example, suppose you think the default optimizer
setting to be -O2 for a particular platform.  You should allow for
command line overrides with something like

	case "$optimize" in
	'') optimize='-O2' ;;
	esac

or, if your system has a decent test(1) command,

	test -z "$optimize" && optimize='-O2'

This allows the user to select a different optimization level, e.g.
-O6 or -g.

=head2 Compiler and Linker flags

If you want to set $ccflags or $ldflags, you should append to the existing
value to allow Configure command-line settings, e.g. use

	ccflags="$ccflags -DANOTHER_OPTION_I_NEED"

so that the user can do something like

	sh Configure -Dccflags='FIX_NEGATIVE_ZERO'

and have the FIX_NEGATIVE_ZERO value preserved by the hints file.

=head2 Libraries

Configure will attempt to use the libraries listed in the variable
$libswanted.  If necessary, you should remove broken libraries from
that list, or add additional libraries to that list.  You should
*not* simply set $libs -- that ignores the possibilities of local
variations.  For example, a setting of libs='-lgdbm -lm -lc' would
fail if another user were to try to compile Perl on a system without
GDBM but with Berkeley DB.  See hints/dec_osf.sh and hints/solaris_2.sh
for examples.

=head2 Other

In general, try to avoid hard-wiring something that Configure will
figure out anyway.  Also try to allow for Configure command-line
overrides.

=head1 Working around compiler bugs

Occasionally, the root cause of a bug in perl turns out to be due to a bug
in the compiler.  Often, changing the compilation options (particularly the
optimization level) can work around the bug.  However, if you try to do
this on the command line, you will be changing the compilation options for
every component of perl, which can really hurt perl's performance.
Instead, consider placing a test case into the hints directory to detect
whether the compiler bug is present, and add logic to the hints file to
take a specific and appropriate action

=head2 Test-case conventions

Test cases should be named "tNNN.c", where NNN is the next unused sequence
number.  The test case must be executable and should display a message
containing the word "fails" when the compiler bug is present.  It should
display the word "works" with the compiler bug is not present.  The test
cases should be liberally commented and may be used by any hints file that
needs them.  See the first hints file (t001.c) for an example.

=head2 Hint file processing

The hint file must define a call-back unit (see below) that will compile,
link, and run the test case, and then check for the presence of the string
"fails" in the output.  If it finds this string, it sets a special variable
to specify the compilation option(s) for the specific perl source file that
is affected by the bug.

The special variable is named "XXX_cflags" where "XXX" is the name of
the source file (without the ".c" suffix).  The value of this variable
is the string "optimize=YYY", where "YYY" is the compilation option
necessary to work around the bug.  The default value of this variable
is "-O" (letter O), which specifies that the C compiler should compile
the source program at the default optimization level.  If you can
avoid the compiler bug by disabling optimization, just reset the
"optimize" variable to the null string.  Sometimes a bug is present at
a higher optimization level (say, O3) and not present at a lower
optimization level (say, O1).  In this case, you should specify the
highest optimization level at which the bug is not present, so that
you will retain as many of the benefits of code optimization as
possible.

For example, if the pp_pack.c source file must be compiled at
optimization level 0 to work around a problem on a particular
platform, one of the statements

	pp_pack_cflags="optimize=-O0"	or
	pp_pack_cflags="optimize="

will do the trick, since level 0 is equivalent to no optimization.
(In case your printer or display device does not distinguish the
letter O from the digit 0, that is the letter O followed by the digit
0).  You can specify any compiler option or set of options here, not
just optimizer options.  These options are appended to the list of all
other compiler options, so you should be able to override almost any
compiler option prepared by Configure.  (Obviously this depends on how
the compiler treats conflicting options, but most seem to go with the
last value specified on the command line).

You should also allow for the XXX_cflags variable to be overridden on the
command line.

See the vos.sh hints file for an extended example of these techniques.

=head1 Hint file tricks

=head2 Printing critical messages

[This is still experimental]

If you have a *REALLY* important message that the user ought to see at
the end of the Configure run, you can store it in the file
'config.msg'.  At the end of the Configure run, Configure will display
the contents of this file.  Currently, the only place this is used is
in Configure itself to warn about the need to set LD_LIBRARY_PATH if
you are building a shared libperl.so.

To use this feature, just do something like the following

	$cat <<EOM  | $tee -a ../config.msg >&4

    This is a really important message.  Be sure to read it
    before you type 'make'.
    EOM

This message will appear on the screen as the hint file is being
processed and again at the end of Configure.

Please use this sparingly.

=head2 Propagating variables to config.sh

Sometimes, you want an extra variable to appear in config.sh.  For
example, if your system can't compile toke.c with the optimizer on,
you can put

    toke_cflags='optimize=""'

at the beginning of a line in your hints file.  Configure will then
extract that variable and place it in your config.sh file.  Later,
while compiling toke.c, the cflags shell script will eval $toke_cflags
and hence compile toke.c without optimization.

Note that for this to work, the variable you want to propagate must
appear in the first column of the hint file.  It is extracted by
Configure with a simple sed script, so beware that surrounding case
statements aren't any help.

By contrast, if you don't want Configure to propagate your temporary
variable, simply indent it by a leading tab in your hint file.

For example, prior to 5.002, a bug in scope.c led to perl crashing
when compiled with -O in AIX 4.1.1.  The following "obvious"
workaround in hints/aix.sh wouldn't work as expected:

    case "$osvers" in
	4.1.1)
    scope_cflags='optimize=""'
	;;
    esac

because Configure doesn't parse the surrounding 'case' statement, it
just blindly propagates any variable that starts in the first column.
For this particular case, that's probably harmless anyway.

Three possible fixes are:

=over

=item 1

Create an aix_4_1_1.sh hint file that contains the scope_cflags
line and then sources the regular aix hints file for the rest of
the information.

=item 2

Do the following trick:

    scope_cflags='case "$osvers" in 4.1*) optimize=" ";; esac'

Now when $scope_cflags is eval'd by the cflags shell script, the
case statement is executed.  Of course writing scripts to be eval'd is
tricky, especially if there is complex quoting.  Or,

=item 3

Write directly to Configure's temporary file UU/config.sh.
You can do this with

    case "$osvers" in
	4.1.1)
	echo "scope_cflags='optimize=\"\"'" >> UU/config.sh
	scope_cflags='optimize=""'
	;;
    esac

Note you have to both write the definition to the temporary
UU/config.sh file and set the variable to the appropriate value.

This is sneaky, but it works.  Still, if you need anything this
complex, perhaps you should create the separate hint file for
aix 4.1.1.

=back

=head2 Call-backs

=over 4

=item Compiler-related flags

The settings of some things, such as optimization flags, may depend on
the particular compiler used.  For example, consider the following:

    case "$cc" in
    *gcc*)  ccflags="$ccflags -posix"
	    ldflags="$ldflags -posix"
	    ;;
    *)      ccflags="$ccflags -Xp -D_POSIX_SOURCE"
	    ldflags="$ldflags -Xp"
	    ;;
    esac

However, the hints file is processed before the user is asked which
compiler should be used.  Thus in order for these hints to be useful,
the user must specify  sh Configure -Dcc=gcc on the command line, as
advised by the INSTALL file.

For versions of perl later than 5.004_61, this problem can
be circumvented by the use of "call-back units".  That is, the hints
file can tuck this information away into a file UU/cc.cbu.  Then,
after Configure prompts the user for the C compiler, it will load in
and run the UU/cc.cbu "call-back" unit.  See hints/solaris_2.sh for an
example. Some callbacks exist for other variables than cc, such as for
uselongdouble. At the present time, these callbacks are only called if the
variable in question is defined; however, this may change, so the scheme in
hints/solaris_2.sh of checking to see if uselongdouble is defined is a good
idea.

=item Call status

Call-backs are only called always, even if the value for the call-back is
uset: UU/usethreads.cbu is called when Configure is about to deal with
threads. All created call-backs from hints should thus check the status
of the variable, and act upon it.

=item Future status

I hope this "call-back" scheme is simple enough to use but powerful
enough to deal with most situations.  Still, there are certainly cases
where it's not enough.  For example, for aix we actually change
compilers if we are using threads.

I'd appreciate feedback on whether this is sufficiently general to be
helpful, or whether we ought to simply continue to require folks to
say things like "sh Configure -Dcc=gcc -Dusethreads" on the command line.

=back

Have the appropriate amount of fun :-)

    Andy Dougherty		doughera@lafayette.edu (author)
    Paul Green			paul.green@stratus.com (compiler bugs)