Enable real dynamic linking with `-shared` by default by sbc100 · Pull Request #25930 · emscripten-core/emscripten

sbc100 · 2025-12-11T00:38:48Z

This change essentially disables FAKE_DYLIBS by default, which in
turn means -shared will now produce dynamic libraries by default.

If you want the old behaviour you now need -sFAKE_DYLIBS.

dschuff

I wonder what would happen if we tested this with Binaryen. it doesn't seem prepared to force the Emscripten build to be static (assuming that's the behavior we actually want).

This test was first added in c735bb6. Hopefully these changes make it more obvious what is being tested, and also test more commonly used flags (`-shared` vs `-r`). Split out from #25930.

sbc100 · 2026-05-15T19:18:57Z

I wonder what would happen if we tested this with Binaryen. it doesn't seem prepared to force the Emscripten build to be static (assuming that's the behavior we actually want).

I actually ended up leaving the cmake TARGET_SUPPORTS_SHARED_LIBS setting as False until we resolve https://gitlab.kitware.com/cmake/cmake/-/work_items/27240

sbc100 · 2026-05-15T19:20:07Z

OK, I think this change is in pretty good shape now

sbc100 · 2026-05-16T00:48:40Z

@kripken @dschuff this change is pretty small now. No codesize changes anymore.

In particular, mention the `FAKE_DYLIBS` setting. This is important especially since we are about the change the default behavior. See emscripten-core#25930

sbc100 · 2026-05-18T22:32:56Z

Given that we have a sensible behavior for dynamic linking, I think that having the -shared flag enable that behavior is a more sensible default than having the -shared flag do nothing. (Note that we are not enabling dynamic linking itself by default the way it is on Linux). Speaking of what the behavior is, before we ship this in a release, we should update our documentation with a description of the expected way to use and enable dynamic linking.

Good idea. I started by updated the docs here: #26976.

I can then include another update with this PR when it lands.

In particular, mention the `FAKE_DYLIBS` setting. This is important especially since we are about the change the default behavior. See emscripten-core#25930

In particular, mention the `FAKE_DYLIBS` setting. This is important especially since we are about the change the default behavior. See #25930

sbc100 · 2026-05-19T22:39:05Z

OK, this change now has corresponding update to Dynamic-Linking.rst.

@kripken was I able to persuade you this is a direction worth going in?

kripken · 2026-05-19T22:41:04Z

Yes, I'm not opposed to the direction. I do worry about some breakage, but you're probably right that there are few users of this feature, and eventually this might lead to simplifications.

sbc100 · 2026-05-19T23:59:34Z

Any last concerns before landing this?

This should have been part of emscripten-core#25930

This should have been part of #25930

This change essentially disables `FAKE_DYLIBS` by default, which in turn means `-shared` will now produce dynamic libraries by default. If you want the old behaviour you now need `-sFAKE_DYLIBS`.

sbc100 · 2026-05-20T20:59:51Z

OK, cmake issue is now resolved I believe.

kripken · 2026-05-20T23:31:08Z

+    self.run_process([EMCC, 'libdir/libfile.c', '-sFAKE_DYLIBS', '-shared', '-fPIC', '-o', 'libdir/libfile.so' + lib_suffix])
    # Build libother and dynamically link it to libfile
-    self.run_process([EMCC, '-Llibdir', 'libdir/libother.c'] + link_flags + ['-shared', '-o', 'libdir/libother.so'])
+    self.run_process([EMCC, '-Llibdir', 'libdir/libother.c'] + link_flags + ['-sFAKE_DYLIBS', '-shared', '-fPIC', '-o', 'libdir/libother.so'])


Here I think we do need a comment on the new flag? I don't see one in this function.

I split out a change to remove this redundant test: #26986

sbc100 force-pushed the shared_flag_part2 branch from 655b473 to e01726b Compare December 11, 2025 00:39

sbc100 commented Dec 11, 2025

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sbc100 force-pushed the shared_flag_part2 branch from 3b0addc to 2a98976 Compare May 12, 2026 01:15

sbc100 changed the title ~~Disable FAKE_DYLIBS by default~~ Enable real dynamic linking by default May 12, 2026

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sbc100 force-pushed the shared_flag_part2 branch 2 times, most recently from dbf81cc to 2a061bd Compare May 14, 2026 01:07

sbc100 mentioned this pull request May 14, 2026

Improve test_multiply_defined_libsymbols #26953

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sbc100 force-pushed the shared_flag_part2 branch from 19b4b57 to 28ec96d Compare May 15, 2026 19:31

sbc100 changed the title ~~Enable real dynamic linking by default~~ Enable real dynamic linking with -shared by default May 15, 2026

sbc100 force-pushed the shared_flag_part2 branch from 28ec96d to 6b35408 Compare May 15, 2026 21:40

sbc100 requested review from dschuff and kripken May 16, 2026 00:48

sbc100 mentioned this pull request May 18, 2026

Update documentation on the -shared flag #26976

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sbc100 added a commit that referenced this pull request May 19, 2026

Update documentation on the -shared flag (#26976)

952a74e

In particular, mention the `FAKE_DYLIBS` setting. This is important especially since we are about the change the default behavior. See #25930

sbc100 force-pushed the shared_flag_part2 branch from 2901ff3 to 524ceee Compare May 19, 2026 22:37

sbc100 mentioned this pull request May 20, 2026

Add testing for audio worklets + pthreads. NFC #26982

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Add testing for audio worklets + pthreads. NFC

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This should have been part of emscripten-core#25930

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Add testing for audio worklets + pthreads. NFC (#26982)

54b97db

This should have been part of #25930

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sbc100 force-pushed the shared_flag_part2 branch from 8d7ffbb to b594e80 Compare May 20, 2026 20:49

Enable real dynamic linking with -shared by default

efe9645

This change essentially disables `FAKE_DYLIBS` by default, which in turn means `-shared` will now produce dynamic libraries by default. If you want the old behaviour you now need `-sFAKE_DYLIBS`.

sbc100 force-pushed the shared_flag_part2 branch from b594e80 to efe9645 Compare May 20, 2026 20:58

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sbc100 added 2 commits May 20, 2026 15:41

feedback

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Merge remote-tracking branch 'upstream/main' into shared_flag_part2

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sbc100 commented Dec 11, 2025 •

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