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From: David Abrahams (david.abrahams_at_[hidden])
Date: 2002-01-06 17:04:27

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To: "David Abrahams" <david.abrahams_at_[hidden]>
Cc: "Craig McPheeters" <cmcpheeters_at_[hidden]>,
"Richard Geiger" <opensource_at_[hidden]>
Subject: Re: Possibility of merged codebase?
References: <0aa501c196dd$8cd48000$0500a8c0_at_[hidden]>
Date: Sun, 06 Jan 2002 13:28:02 -0700
In-Reply-To: <0aa501c196dd$8cd48000$0500a8c0_at_[hidden]> ("David
Abrahams"'s message of "Sun, 6 Jan 2002 13:07:30 -0500")
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From: "Matt Armstrong" <matt+dated+1010780889.ea82e9_at_[hidden]>
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"David Abrahams" <david.abrahams_at_[hidden]> writes:

> Gentlemen,
> We seem to be the ones currently pushing the frontiers of the core
> Jam functionality, and given the promising recent activity from
> Perforce I have a renewed interest in moving the Boost.Jam source
> code much closer to what's at Perforce. I am quite interested in
> having many of your improvements, and I honestly believe that many
> of the ones I've implemented could be of real use to you. I think a
> merged codebase, if done in a principled way, could make it easier
> for Perforce to accept/adopt our extensions, and result in a better
> world for the Jam community at large.
> By the way, I am /not/ suggesting that we do shared work on the
> build system itself (e.g. Jambase), beyond trivial maintenance of
> the builtin rules.
> What do you say?
> Dave

I don't think I'll have much time to contribute. My interest in Jam
is mainly rooted in my job, and I am nearing the end of my slotted
time to fix the build system up. Soon I'll be doing other things.

Because of this, my primary interest is in keeping our local
patches/fixes minimal and well documented (in code with #ifdefs and
externally in a LOCAL_CHANGES.txt file), with the idea that this will
ease future merges of the stock upstream code base.

Ironically, the fact that "stock jam" changes so infrequently is
another argument for continuing in this vein. Any local changes we
make will likely remain maintainable simply because upstream jam
doesn't change frequently or radically. This makes it safe to base
changes off stock jam.

My personal take is that if I were to spend significant time hacking
on a build tool, it'd be a re-write of jam, not incremental changes to
it. I'd dump Jam's rule language for something both more traditional
and powerful, since I am almost constantly annoyed by both the
limitation and quirks of the Jam language. I think there is a reason
many of the newer build tools are written directly in popular
scripting languages like Perl and Python -- when you need to do real
work you're in a language that can handle it. I have been thinking
something based on scheme, since a small scheme interpreter optimized
for Jam (i.e. perhaps no garbage collection, all strings in a newstr
style pool, etc.) would be pretty small. But, I won't be leading this
kind of project, since I have other stuff I consider more interesting.

I'll append my LOCAL_CHANGES.txt, which lists all our local change to
jam in detail. Some of these I've already posted about:


This file details the differences between our copy of Jam and the
stock upstream version as distributed by Perforce.

* Conventions Used for Jam Patches

All changes we've made to Jam C source are surrounded by an
#ifdef. The #ifdefs are constructed such that it is possible to
remove each feature independent of the other. This greatly eases
maintenance costs, since the next time an upstream version of jam
is merged in it'll be very easy to see why each change was made.
It also makes it easy to assess how big a tweak to jam each change
actually is.

The naming convention for the #defines is:

OPT_BUILTIN_..._EXT -- a new builtin rule
OPT_IMPROVE_..._EXT -- a new general improvement, but no real
change in functionaltiy
OPT_FIX_..._EXT -- a bug fix to Jam (suitable for including in
the upstream Jam).
OPT_..._EXT -- anything that doesn't fall neatly in the above.

All changes made to Jamfiles or Jamrules are surrounded by obvious
comments of the form:


* The builtin Jambase is slightly changed.

The builtin Jambase has a few tweaks to make it nicer under NT.

It sets the MSVCNT var from MSVCDIR if MSVCDIR is set. MSVCDIR is
the variable Visual C++ 6.0's version of vcvars32.bat sets, while
MSVCNT seems to be a Visual C++ 5.0 thing. This change has been
sent upstream.

If MSVCNT is still unset, it uses W32_GETREG and W32_SHORTNAME to
grab the installation location of Visual C++, and sets MSVCNT
appropriately. This is merely a matter of convenience for people.

It doesn't complain if it can't find a compiler under NT.

It doesn't announce that it is using Visual C++.

In all other ways (variables set, rules and actions defined) the
built in Jambase is identical to stock Jam.

* New Builtin Rules

** PWD

A new rule PWD returns the current working directory. Used like

pwd = [ PWD ];

This, together with some Jam logic, can be used to generate a
fully qualified path name. Currently it is only used to fully
qualify the tools/bin directry before changing the PATH.

This option is controlled by the OPT_BUILTIN_PWD_EXT #define.


A new rule MATCH does regexp matching on a string, returning the
result as a list of matches. Used like so:

matches = [ MATCH string : pattern ] ;

matches[1] is the portion of 'string' that 'pattern' matched.
matches[2], matches[3], etc. hold the portion of 'string' matched
by parenthesized sub expressions within 'pattern'.

The syntax of the pattern regexp is identical to that of the
HDRSCAN variable, since this rule uses Jam's internal regexp

The initial purpose of this rule is to allow the implementation of
a Split rule within Jam, so things like path names can be easily

This option is controlled by the OPT_BUILTIN_MATCH_EXT #define.


Available only under WinNT (Win2k as well). Gets a value from the
registry, like so:

value = [ W32_GETREG list ] ;

This is primarily so Jam can find the location of the Visual C++
installation from the registry, which makes it a bit easier to get
a build environment up and running. Otherwise, they would have to
set the MSVCDIR environment variable, either at Visual C++ install
time or by running the vcvars32.bat file that comes with Visual

This option is controlled by the OPT_BUILTIN_W32_GETREG_EXT


Available only under WinNT (Win2k as well). Takes a string
holding a file name and returns its short name. E.g. "Program
Files" -> "PROGRA~1" etc. Used like so:

short = [ W32_SHORTNAME longname ] ;

This is primarily useful for shortening the long path name
supplied by W32_GETREG, which often contains things like "Program
Files" in it, etc., which confuses Jam later on.

This option is controlled by the OPT_BUILTIN_W32_SHORTNAME_EXT

* New Features

** Header Caching

This code is taken from //guest/craig_mcpheeters/jam/src/ on the
Perforce public depot. Many thanks to Craig McPheeters for making his
code available. It is delimited by the OPT_HEADER_CACHE_EXT #define
within the code.

Jam has a facility to scan source files for other files they might
include. This code implements a cache of these scans, so the entire
source tree need not be scanned each time jam is run. This brings the
following benefits:

- If a file would otherwise be scanned multiple times in a
single jam run (because the same file is represented by
multiple targets, perhaps each with a different grist), it
will now be scanned only once. In this way, things are
faster even if the cache file is not present when Jam is

- If a cache entry is present in the cache file when Jam
starts, and the file has not changed since the last time it
was scanned, Jam will not bother to re-scan it. This
markedly increaces Jam startup times for large projects.

This code has improvements over Craig McPheeters' original
version. I've described all of these changes to Craig and he
intends to incorporate them back into his version. The changes

- The actual name of the cache file is controlled by the
HCACHEFILE Jam variable. If HCACHEFILE is left unset (the
default), reading and writing of a cache file is not
performed. The cache is always used internally regardless
of HCACHEFILE, which helps when HDRGRIST causes the same
file to be scanned multiple times.

Setting LOCATE and SEARCH on the the HCACHEFILE works as
well, so you can place anywhere on disk you like or even
search for it in several directories. You may also set it
in your environment to share it amongst all your projects.

- The .jamdeps file is in a new format that allows binary data
to be in any of the fields, in particular the file names.
The original code would break if a file name contained the
'@' or '\n' characters. The format is also versioned,
allowing upgrades to automatically ignore old .jamdeps
files. The format remains human readable. In addition,
care has been taken to not add the entry into the header
cache until the entire record has been successfully read from
the file.

- The cache stores the value of HDRPATTERN with each cache
entry, and it is compared along with the file's date to
determine if there is a cache hit. If the HDRPATTERN does
not match, it is treated as a cache miss. This allows
HDRPATTERN to change without worrying about stale cache
entries. It also allows the same file to be scanned
multiple times with different HDRPATTERN values.

- Each cache entry is given an "age" which is the maximum
number of times a given header cache entry can go unused
before it is purged from the cache. This helps clean up old
entries in the .jamdeps file when files move around or are
removed from your project.

You control the maximum age with the HCACHEMAXAGE variable.
If set to 0, no cache aging is performed. Otherwise it is
the number of times a jam must be run before an unused cache
entry is purged. The default for HCACHEMAXAGE if left unset
is 100.

- Jambase itself is changed.

SubDir now always sets HDRGRIST to $(SOURCE_GRIST) so header
scanning can deal with multiple header files of the same
name in different directories. With the header cache, this
does no longer incurs a performance penalty -- a given file
will still only be scanned once.

The FGristSourceFiles rule is now just an alias for
FGristFiles. Header files do not necessarily have global
visibility, and the header cache eliminates any performance
penalty this might otherwise incur.

Because of all these improvements, the following claims can be
made about this header cache implementation that can not be made
about Craig McPheeters' original version.

- The semantics of a Jam run will never be different because of
the header cache (the HDRPATTERN check ensures this).

- It will never be necessary to delete .jamdeps to fix obscure
jam problems or purge old entries.

** Exporting Jam variables to the environment using ENVEXPORT.

This change causes the global value of the ENVEXPORT variable to
take on special meaning. It becomes a list of Jam variables that
are to be exported into the environment.

For example, if ENVEXPORT is equal to the list FOO BAR BAZ, then
the environment variables FOO, BAR, and BAZ will be set to
whatever values the Jam global variables of the same name were set

If a Jam global variable holds a list, the entire list is exported
to the environment. When the variable's name ends with "PATH",
"Path" or "path", then the list elements are concatenated together
with the SPLITPATH character separating elements (SPLITPATH is ';'
under Windows and ':' under Unix), otherwise the list elements are
concatenated with a single space.

By default, the value of ENVEXPORT is the empty list, so no
environment variables are exported by default.

This option is controlled by the OPT_ENVIRONMENT_EXPORT_EXT

** The :X variable expansion

Expanding a variable with :X will change all \ chars in the
variable to / chars.


foo = "a\\b\\c"
bar = $(foo:X)
# bar is now "a/b/c"

This is useful when dealing with cygwin tools that expect path
elements to be unix style, I guess.

FIXME: is this truly necessary? Or can it be solved in Jam?
E.g. we might be able to use the Split rule to get around this.

This feature is enabled with the OPT_EXPAND_UNIXPATH_EXT #define.

** Human Readable Dependency Output

This code is taken from from //guest/craig_mcpheeters/jam/src/ on the
Perforce public depot. Many thanks to Craig McPheeters for making his
code available. It is delimited by the OPT_GRAPH_DEBUG_EXT #define
within the code.

With this option, debug level 10 will print out the entire dependency
tree in a form that is more easily understood than jam's debug level

** Target Fate Change Debugging

This code is taken from from //guest/craig_mcpheeters/jam/src/ on the
Perforce public depot. Many thanks to Craig McPheeters for making his
code available. It is delimited by the OPT_GRAPH_DEBUG_EXT #define
within the code.

With this option, debug level 11 prints out target fate changes as
they occur (and why they occur). This helps debug mysterious "why
is THAT file getting rebuilt" problems.

** Improved ...patience...

This changes the ...patience... lines to be printed out after the
first 100 and every subsequent 1000 files have been header scanned.
Previously, ...patience... was printed out for every 1000 targets.

This change both reduces the number of ...patience... lines printed,
and makes them more accurately reflect the work being done.

This change is enabled with the OPT_IMPROVED_PATIENCE_EXT #define.

** Improved debug level help

This change is delimited by the OPT_IMPROVE_DEBUG_LEVEL_HELP_EXT
#define within the code.

The -h option to jam now prints out what each of the debug levels do.

** Print Total Time

This change is delimited by the OPT_PRINT_TOTAL_TIME_EXT #define
within the code.

If the total time jam runs is greater than 10 seconds, the time is
printed when jam exits. This helps people back up claims that the
build is too slow and they need a faster machine. ;-)

** Improved HdrRule treatment

A new 3rd argument to HdrRule is the bound name of the 1st
argument to HdrRule. This allows HdrRule to extend the search
path for headers to include all directories headers have been
found in so far.

E.g. if a source file does "#include <foo/bar/baz.h>" and the
baz.h header is found in $(TOP)/include, this change allows
HdrRule to add $(TOP)/include/foo/bar to the HDRSEARCH path. This
way, if baz.h does #include "goo.h", any goo.h in
$(TOP)/include/foo/bar will be found.

The default Jambase makes use of this new argument to extend
HDRSEARCH on the header files.

This feature is enabled with the OPT_HDRRULE_BOUNDNAME_ARG_EXT

** Improved "compile" debug output.

With level 5 jam debugging, a jam rule execution trace is
printed. This extends that debugging output to include:

- when a new rule is defined (with a special note when the new
rule re-defines a pre-existing rule).

- when a new actions is defined (with a special note when the
new actions re-defines a pre-existing actions).

- when an included Jamfile ends.

This makes it possible to write scripts that process Jam debugging
output that look for potential errors, such as re-defining a rule
or action that is part of Jambase.

This feature is enabled with OPT_IMPROVE_DEBUG_COMPILE_EXT.

** "Lazy" targets

Because the Windows NT shell (cmd.exe) sucks, it is often best to
break up complex operations into many actions. Examples include
creating various response files and linker definition files for
the link step of a compile.

The problem with this is that these files may not always be
rebuilt when necessary. It is difficult to construct a
straightforward chain of actions that guarantees that all the
response files that need to get built whenever the final link
makes use of them.

Stock Jam provides two main ways to accomplish this:

- Mark the response files TEMPORARY and remove them with
RmTemps after the link. This is problematic since removing
them just adds mystery to the final link process for the
typical engineer. People often want to look at the files to
see exactly how the link occurs.

- Perform the final link with several actions that take a list
of the final image and all the response files in $(<). Each
action would build one of the elements in $(<). This is an
obtuse hack that is difficult to explain and maintain.

The solution presented here is a new built in rule LAZY. When
called like this:

LAZY target ;

"target" is marked "lazy".

When Jam decides that a given target is to be built, it now checks
all direct dependents to see if they are marked lazy. If they
are, the lazy dependents are also marked for rebuilding, and their
direct dependents are similarly considered, and so on.

This affords the benefits of marking targets TEMPORARY (that they
will be rebuilt whenever the targets they depend on are rebuilt)
without the negatives (that they get deleted after the build).


There is a bug in this implementation that I do not believe will
lead to practical problems. Consider the following set of

d -> b* (d depends on b*)
c -> b*
b* -> a

Consider b* to be marked "lazy". The current implementation will
correctly rebuild b whenever either d or c is rebuilt. However,
it does not guarantee that BOTH d and c get built whenever b* is
updated. If b* is updated only because it is lazy, some of its
dependents may not be updated. For example, if c is updated and
b* is marked for updated because it is lazy, then d may not be
updated. If d is marked for update and b* is marked for updated
because it is lazy, c may not be marked for update. I call this a
bug since it shouldn't be necessary to run Jam twice to satisfy
all dependencies.

A simple way to work around this is to mark b* with NOTFILE. This
will cause b*'s time stamp to no longer be considered. This is
arguably a reasonable thing to do, since these files are rarely
edited by hand and whenever they are used they are rebuilt.
Another workaround is to mark the final linked image with LEAF,
which will usually has a similar effect of removing b*'s time
stamp from consideration. Another workaround is to avoid having a
LAZY file with more than one dependent target (this is usually the
case anyway, which is the major reason I don't consider this
problem serious).

* Operational Changes

** Versioning

We add a PATCHED_VERSION variable that indicates the local version
of custom jam is in use.

The variable is a list. PATCHED_VERSION[1] is the major version,
PATCHED_VERSION[2] is the minor version.

As you might expect, major version increments indicate
non-backwards compatible changes (elimination of builtin rules or
other features, changing features in an incompatible way, etc.).
Minor version increments indicate the addition of backwards
compatible features and bug fixes.

It is expected that a project's Jamrules will check the
PATCHED_VERSION variable and check for a major version mismatch,
and ensure the minor version is not too low.

This option is enabled with the OPT_PATCHED_VERSION_VAR_EXT

** Maximum Command Length for NT

Jam ships with a maximum command line length of 996 for Windows
NT. Windows NT 4.0 and greater can handle command line lengths of
at least 10240 characters long (perhaps longer, no tests have been

This change increaces the maximum command line length to 10240 for
Windows 4.0 and greater.

Caveat: the default Windows 4.0 command shell can only handle
commands up to 1-2k bytes long for many of its own internal
commands, such as del and echo. So this feature has spurred the
implementation of jamshell.c, a simple shell that lives in

This option is enabled with the OPT_FIX_NT_BATCH_EXT #define.

* Bug Fixes

** Windows NT Batch File Naming Bug

This code is taken from from //guest/craig_mcpheeters/jam/src/ on
the Perforce public depot. Many thanks to Craig McPheeters for
making his code available. It is delimited by the
OPT_FIX_NT_BATCH_EXT #define within the code.

Running jam multiple times on the same machine could break because
jam's temp batch file names were of the form jamtmpXX.bat, where
XX begins at 00 and increaces numerically.

This fix adds the jam processes own PID to the temp batch file
name, allowing multiple copies of jam.exe to run simultaneously
without interfering with each other.

** Improper handling of "on target" values during header scanning

Setting any "on target" variables for $(<) within a HdrRule will
actually set the global values for those variables and the "on
target" values will remain unchanged.

Why? Jam implements "on target" variables by swapping the current
global values with the target specific values (see pushsettings()
in rules.c) and then unswapping them when the target is no longer
in scope (see popsettings() in rules.c).

This works fine if the target variables are not changed between
calls to pushsettings() and popsettings(). But, when scanning for
header file dependencies, the HDRRULE is run, and so the "on
target" variables of $(<) can be set.

Doing so will actually cause the global value of the variable to
be set. Why? Because the target's value will be swapped with the
global value in the popsettings() call after the HdrRule is
called. The value set on $(<) will either not change (if the same
variable was previously set on the target), or be taken from the
global setting (if the variable had never been set on the target

This problem occurs with the default Jambase's HdrRule when any
file includes itself. In this case, $(<) will also be present in

This fix makes a copy of the target's "on target" variables and
uses the copy with pushsettings() and popsettings() in make.c's
make0() function. An alternate fix would be to freeze the "on
target" variables of $(<) within a HdrRule, disalowing any

This code is enabled with the OPT_FIX_TARGET_VARIABLES_EXT

matt ------=_NextPart_000_0B32_01C196D4.32EC3AA0-- 

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