No supported way to negate `-pthread`: `USE_PTHREADS` is deprecated but is the only working off-switch

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Đánh giá

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1-2 ngày
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Tính năng
Độ rõ ràng
Khá rõ ràng
Mức độ hoạt động
Sôi nổi
Công nghệ
wasm
Lĩnh vực
compilers

Hướng nghiên cứu

Start with the user_settings handling in emcc.py and the LEGACY_SETTINGS entry in tools/settings.py, then reproduce the emsdk 6.0.9 commands from the issue. Determine how a supported negation of -pthread should interact with USE_PTHREADS and STRICT. Done means a thread-enabled command can be reliably disabled without the deprecated setting, or the documented workaround remains accepted under STRICT.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.

Mô tả

Summary

USE_PTHREADS is deprecated in favour of -pthread, but -pthread has no negation, so a
project that consumes a build system which passes -pthread unconditionally has no
supported way to turn threads off. The only mechanism that works is the deprecated setting
itself, which is a hard error under STRICT.

I am filing because the deprecation message explicitly invites it ("Please open a bug if
you have a continuing need for this setting"), and a tracker search for -no-pthread,
for negating -pthread, and for the USE_PTHREADS deprecation turned up nothing similar.

What I tried, on emsdk 6.0.9

Baseline, matching what the upstream build passes:

emcc -pthread -sALLOW_MEMORY_GROWTH=1 -o out.js hello.c
# -> (import "env" "memory" (memory $mimport$0 256 32768 shared))

Then each candidate negation, injected via EMCC_CFLAGS so it lands after the
command line (the same shape a downstream build system leaves you with):

Flag Result
-no-pthread still shared -- the string does not occur in emcc.py, tools/ or docs/emcc.txt; it is forwarded to clang, which reports it unused
-sSHARED_MEMORY=0 still shared
-sPTHREADS=0 hard error: PTHREADS is an internal setting and cannot be set from command line
-sUSE_PTHREADS=0 works -- (memory $0 258 32768), non-shared, no PThread/Worker references in the glue

emcc.py (around the user_settings handling) maps USE_PTHREADS onto the internal
PTHREADS, and that appears to be the only path that can negate it. tools/settings.py
lists USE_PTHREADS in LEGACY_SETTINGS with "No longer needed. Use -pthread instead",
and legacy settings are fatal when STRICT is on.

Why this matters downstream

We are building Lean 4 for WebAssembly. Its CMake sets

set(EMSCRIPTEN_SETTINGS "-s ALLOW_MEMORY_GROWTH=1 -fwasm-exceptions -pthread -flto")
string(APPEND LEANC_EXTRA_CC_FLAGS " -pthread")

unconditionally in its Emscripten branch, with no option to disable it. We want a
thread-free comparison build (no SharedArrayBuffer requirement, so the artifact can load
without cross-origin isolation). Our only route is -sUSE_PTHREADS=0, i.e. a deprecated
setting that a future emsdk may remove and that EMCC_STRICT already rejects.

What would help

Any one of these would remove the dead end, in rough order of preference:

  1. A supported negation of -pthread -- -no-pthread is the spelling most people try
    first (clang accepts it for the host, which is why it fails silently here).
  2. Keep -sUSE_PTHREADS=0 working, or exempt it from STRICT, until a negation exists.
  3. Failing both, a documentation note in docs/emcc.txt stating that -pthread cannot be
    negated and that -sUSE_PTHREADS=0 is the workaround, so the deprecation warning does
    not lead people to remove their only working option.

Happy to open a PR for (3) if that would be welcome, and happy to test a patch for (1).

Versions

emsdk / emscripten 6.0.9, Linux 6.18 (Manjaro), node 24.

Disclosure: this report was researched and drafted by Claude Opus 5 (Claude Code) working
at my direction. Every flag result in the table above is from a real local probe on 6.0.9,
not a hypothetical, and the cited source behaviour was checked against the installed
emscripten before filing.

Ngôn ngữ chính
C++
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