[Tcl-bugs] [ tcl-Feature Requests-565088 ] Bring tcl.h into the 1990s

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Feature Requests item #565088, was opened at 2002-06-05 16:33
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Category: 48. Portability Support
Group: None
Status: Open
Resolution: None
Priority: 3
Submitted By: Joe English (jenglish)
Assigned to: Joe English (jenglish)
Summary: Bring tcl.h into the 1990s

Initial Comment:
Brief summary: <tcl.h> contains several preprocessor
macros intended to aid source-level compatibility
between ANSI C and K&R C compilers.  However, the
#ifdef logic tends to assume K&R C by default and
enables modern features based on the presence of
__STDC__ and/or other special cases.

Since pre-ANSI compilers are a rarity nowadays (2002),
and many compilers do not define __STDC__ in the
default (non-strict-ansi) mode even though they do in
fact support all the relevant features, it would make
more sense to assume modern C by default and only
enable the backwards-compatibility macros on a
special-case basis.

I'll put together a patch in the next couple of days.

Detailed analysis (from a post to comp.lang.tcl):

From: jen...@fl... (Joe English)
Newsgroups: comp.lang.tcl
Subject: Re: Build problem using aCC on HP-UX
Date: 5 Jun 2002 15:32:02 GMT
Message-ID: <adl...@en...>

[...]
The Tcl core was C89-compatible until the recent
(8.4a2? 3?)
addition of "long long", which is a C99 feature. 
(Actually,
I'm not sure under which conditions 'long long' is
actually
used -- it's tough to decipher what's going on in the
#ifdef
chain in tcl.h).

However, the core also includes backwards-compatibility
#defines for several features which were not present in
K&R C (VOID, CONST, USE_PROTOTYPE, USE_STDARG, etc.).
The choice of whether to use C89 or K&R features is
determined by whether or not __STDC__ is #define'd,
or, in many cases, with an additional list of special
cases, e.g.

    #   if defined(__STDC__) || defined(__cplusplus) ||
defined(__BORLANDC__)

Most compilers nowadays (in my experience) are C89, but
do not yet fully support C99.  Many of these support
C9X features like "long long", but only when invoked in
"extensions-enabled" mode.

Here's where the problem comes up: "long long" is
standard C99,
but it's an extension to C89; thus 20th century
compilers don't
enable it in "strict ANSI" mode and Tcl 8.4 can't be
compiled
in that mode.  Most compilers -- GCC being a notable
exception --
don't define __STDC__ when extensions are enabled, so
all of
the backwards-compatibility cruft in tcl.h gets
activated
(except on platforms which have been accounted for by a
special-case preprocessor test as above).

The correct way to proceed is IMO to bring tcl.h into
the 1990s:
it should assume C89 features are present by default
(void, const,
stdarg.h, etc.), and only enable the
backwards-compatibility
cruft on a special-case basis, instead of the other way
around.
For example, instead of:

    #if ((defined(__STDC__) || defined(SABER)) &&
!defined(NO_PROTOTYPE)) \
        || defined(__cplusplus) ||
defined(USE_PROTOTYPE)
    #   define _USING_PROTOTYPES_ 1
    #   define _ANSI_ARGS_(x)   x
    #   define CONST const
    #else
    #   define _ANSI_ARGS_(x)   ()
    #   define CONST
    #endif

use something like:

        #if defined(ANCIENT_COMPILER)
        #       define CONST
        #       define _ANSI_ARGS_(x)
        #else
        #       define _ANSI_ARGS_(x) x
        #       define CONST const
        #endif

where ANCIENT_COMPILER is undefined by default.

----------------------------------------------------------------------

>Comment By: Joe English (jenglish)
Date: 2002-06-07 14:31

Message:
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OK, patch attached.  Tested on Linux (SuSE 6.3, gcc), HP-UX
10.20 (cc -Ae), and AIX 4 (native cc).

----------------------------------------------------------------------

Comment By: Joe English (jenglish)
Date: 2002-06-07 08:40

Message:
Logged In: YES 
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Other changes:

In r1.8 of tcl.h, the definition of STRINGIFY:

#define STRINGIFY(x) STRINGIFY1(x)
#define STRINGIFY1(x) #x

was replaced with a messy #ifdef chain.  The current version  uses the above (correct) definition when _MSC_VER or RESOURCE_INCLUDED is defined, an incorrect version if __STDC__ is defined, and a fourth version (which may or may not work on older compilers -- this wasn't possible to do portably pre-ANSI).  It also added a JOIN() macro, which is not used anywhere in the core. STRINGIFY() appears in two places (win/tclWinPipe.c, win/tcl.rc).

I plan to revert to the older definition. (On the tcl'ers chat, Jeff said this would be OK.)

I also plan to remove references to <sys/type.h>: this was added as part of the TIP 72 implementation, but it turns out not to be needed (confirmed by DKF [private email], who added it initially), and causes problems compiling on non-POSIX systems.  (<sys/types.h> is a POSIX header file that defines various typedefs like dev_t, pid_t, ino_t, etc., none of which are used elsewhere).

----------------------------------------------------------------------

Comment By: Joe English (jenglish)
Date: 2002-06-07 07:56

Message:
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> [...] please avoid any macro like "ANCIENT_COMPILER".

Naturally :-)

Currently I'm using the following CPP symbols:
NO_PROTOTYPES
NO_CONST
NO_VOID
NO_STDARG

All of which are undefined by default.  This should allow the current build system to work unchanged on any modern compiler.  As far as I can tell, all of the platforms mentioned in tcl.m4/configure.in, as well as Windows (MSVC, Borland C, GCC+CYGWIN, GCC+MINGW) and the Mac satisfy this criteria but please someone correct me if I'm wrong.  I'm unable to test this patch on a pre-ANSI compiler, since I no longer have one available.

----------------------------------------------------------------------

Comment By: Don Porter (dgp)
Date: 2002-06-07 07:33

Message:
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The basic idea seems fine, but please avoid any macro
like "ANCIENT_COMPILER".  By the time 2010 gets here,
every compiler we're using now will be an "ancient compiler",
but that's not what we mean.  Use macro names that say
precisely what they mean.

----------------------------------------------------------------------

Comment By: Donal K. Fellows (dkf)
Date: 2002-06-07 02:44

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David: No.  It increases the maintenance cost too much, and
I am not convinced the benefits are worth it.  If the new
compilers cope with the source as it is now, there's no good
reason to change it.  Especially as I don't fancy typing all
that stuff in every time I create a new function!

And I think old style function defns are more readable
anyway.  It is one of the few places where ANSI, by tailing
C++,  did not improve things IMHO...

----------------------------------------------------------------------

Comment By: David Gravereaux (davygrvy)
Date: 2002-06-06 18:07

Message:
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Bravo!  How about we hit the source, too?

int
#if OLD_CRUFTY_PRE_ANSIC89_COMPILER
Tcl_Stat(path, oldStyleBuf)
    CONST char *path;		/* Path of file to stat (in 
current CP). */
    struct stat *oldStyleBuf;	/* Filled with results of 
stat call. */
#else
Tcl_Stat(
    CONST char *path,		/* Path of file to stat (in 
current CP). */
    struct stat *oldStyleBuf)	/* Filled with results of 
stat call. */
#endif
{
    ....

----------------------------------------------------------------------

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[Tcl-bugs] [ tcl-Feature Requests-565088 ] Bring tcl.h into the 1990s

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[Tcl-bugs] [ tcl-Feature Requests-565088 ] Bring tcl.h into the 1990s