Greetings, all!
I have a somewhat unusual first project with Emscripten. I need to get a
domain-specific C-like language generating WebAssembly. This is not too
bad, because the DSL compiles into C code, which I then feed to the C
compiler for the relevant platform. At this level, WebAssembly is "just
another platform" but as always, the details are more complicated.
I need to provide declarations for the platform's C run-time library
functions, types, constants, and so on to the DSL. This is a fairly routine
task, given the platform's C headers, but there are a few things about the
Emscripten headers that are puzzling me.
To forestall the obvious question, no, I can't just use the provided
headers. The DSL is C-like, but has some syntax differences. Unlike C++,
many C programs are not valid programs in the DSL, which gives much more
freedom in the language design. It's a separate development that split from
normal C in the mid-eighties and is still very much worth using for its
specialised role. Yes, new hires have to learn it, which takes about two
days for someone who knows C or C++. Learning about the specialised
application area it is used for takes much longer.
I'm running Emscripten on Linux. I started by looking at Emscripten 3.1.41,
since that's the version used by a couple of other product teams that work
on my site. That is fairly simple when I run a few emcc compiles with -H to
get a report of what files are referenced. The top-level headers come from
emsdk/upstream/emscripten/cache/sysroot/include. A few Clang-associated
ones come from emsdk/upstream/lib/clang/17/include. That's all fine with
me.
Then I decided to check the latest emsdk, and found that with 3.1.69, the
headers come from a different place. It builds a cache of the headers and
other files it uses under ~/.emscripten_cache. The problems with that are:
Storage: It will take 36MB in the user directory of everyone who ever
compiles with Emscripten. We keep all our user directories on a server
disk, because that's enormously convenient in many ways. But we really
don't want to burn space with duplicates of that cache.
Version lock: We need to be able to have several Emscripten versions in use
simultaneously, by the same account, without conflicts. Our reason for this
is that we plan to release products on WebAssembly, and from time to time,
update the version of Emscripten we use, to get access to new C and C++
standards, compiler bug fixes, and so on. But we will not update the tools
used to build a product version that's been released and is under
maintenance, because we'd have to re-do a lot of the QA that we do at a
release. So the service accounts that run our builds can't have caches of
version-specific headers in their user directories.
I need a way to tell Emscripten to put that cache somewhere else. I did
some grep'ing of the Emscripten scripts, and found this line, in both
3.1.41 and 3.1.69:
upstream/emscripten/tools/config.py: CACHE =
os.path.expanduser(os.path.join('~', '.emscripten_cache'))
I have not yet attempted to read the Python code and learn how it all
works, because that could take ages; I don't know Python well at all and am
not keen on it. I'm a naturally low-level programmer, much happier with
assembly code than object orientation
Is there an environment variable I can use to relocate that cache?
Thanks very much,
John
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