Emit the emscripten entry point as `__main_argc_argv` by guybedford · Pull Request #158937 · rust-lang/rust

guybedford · 2026-07-08T01:33:05Z

Fixes support for building wasm32-unknown-emscripten under the -sPROXY_TO_PTHREAD linking option.

The wasm32-unknown-emscripten target uses the argc/argv form of main but emitted the entry point under the raw name main. That links on the default entry path (emscripten calls it via callMain), but fails on entry paths that reference the mangled symbol directly from wasm. In particular -sPROXY_TO_PTHREAD links crt1_proxy_main.o, whose call chain bottoms out at __main_argc_argv, so linking a Rust binary fails with:

    wasm-ld: error: crt1_proxy_main.o: undefined symbol: main

This sets entry_name = "__main_argc_argv" so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases.

The JS-visible name of the entry point stays _main, since emscripten bridges _main to the wasm __main_argc_argv export internally (whereas it wouldn't for ___main_argc_argv).

The `wasm32-unknown-emscripten` target uses the argc/argv form of main but emitted the entry point under the raw name `main`. That links on the default entry path (emscripten calls it via `callMain`), but fails on entry paths that reference the mangled symbol directly from wasm. In particular `-sPROXY_TO_PTHREAD` links `crt1_proxy_main.o`, whose call chain bottoms out at `__main_argc_argv`, so linking a Rust binary fails with: ``` wasm-ld: error: crt1_proxy_main.o: undefined symbol: main ``` This sets `entry_name = "__main_argc_argv"` so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases. The JS-visible name of the entry point stays `_main`, since emscripten bridges `_main` to the wasm `__main_argc_argv` export internally (whereas it wouldn't for `___main_argc_argv`).

rustbot · 2026-07-08T01:33:09Z

These commits modify compiler targets.
(See the Target Tier Policy.)

rustbot · 2026-07-08T01:33:11Z

r? @jieyouxu

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jieyouxu · 2026-07-08T11:45:26Z

Maybe r? @bjorn3 (since you are already looking at it and actually know stuff about wasm)

bjorn3

Makes sense. Code changes LGTM. And I will trust your judgement as someone with write permissions to Emscripten that this is a change that should be done.

r=me with or without the comment changed.

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bjorn3 · 2026-07-08T17:40:29Z

@bors r+

rust-bors · 2026-07-08T17:40:32Z

📌 Commit 15f9829 has been approved by bjorn3

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

…, r=bjorn3 Emit the emscripten entry point as `__main_argc_argv` Fixes support for building `wasm32-unknown-emscripten` under the `-sPROXY_TO_PTHREAD` linking option. The `wasm32-unknown-emscripten` target uses the argc/argv form of main but emitted the entry point under the raw name `main`. That links on the default entry path (emscripten calls it via `callMain`), but fails on entry paths that reference the mangled symbol directly from wasm. In particular `-sPROXY_TO_PTHREAD` links `crt1_proxy_main.o`, whose call chain bottoms out at `__main_argc_argv`, so linking a Rust binary fails with: ``` wasm-ld: error: crt1_proxy_main.o: undefined symbol: main ``` This sets `entry_name = "__main_argc_argv"` so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases. The JS-visible name of the entry point stays `_main`, since emscripten bridges `_main` to the wasm `__main_argc_argv` export internally (whereas it wouldn't for `___main_argc_argv`).

…uwer Rollup of 25 pull requests Successful merges: - #158871 (add relnotes for 1.97.0) - #158968 (stdarch subtree update) - #154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - #156370 (Reject linked dylib EII default overrides) - #157153 (allow `Allocator`s to be used as `#[global_allocator]`s) - #158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - #158617 (allow mGCA const arguments to fall back to anon consts) - #158645 (Fix splat ICEs and ban it in closures) - #158655 (Fix coroutine MIR saved local remapping) - #158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - #158912 (Introduce new bootstrap config section for PGO configuration) - #158920 (Update wasm-component-ld to 0.5.26) - #158926 (wrapping_sh* methods: clarify underspecified reference) - #158927 (add core test run with `-Zforce-intrinsic-fallback`) - #158932 (Do not build the compiler when invoking `x perf compare`) - #158937 (Emit the emscripten entry point as `__main_argc_argv`) - #151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - #156548 ( Library support for aarch64-unknown-linux-pauthtest target) - #158307 (CI job for parallel frontend ui tests) - #158347 (Improve generic parameters handling for #[diagnostic::on_const]) - #158722 (delegation: do not always inherit `ConstArgHasType` predicates) - #158741 (Simplify `Option::into_flat_iter` signature) - #158807 (Add regression test for CString::clone_into unwind safety) - #158862 (Fix the span for parameter suggestion ) - #158883 (tests: fix enum-match.rs to handle LLVM 23)

…, r=bjorn3 Emit the emscripten entry point as `__main_argc_argv` Fixes support for building `wasm32-unknown-emscripten` under the `-sPROXY_TO_PTHREAD` linking option. The `wasm32-unknown-emscripten` target uses the argc/argv form of main but emitted the entry point under the raw name `main`. That links on the default entry path (emscripten calls it via `callMain`), but fails on entry paths that reference the mangled symbol directly from wasm. In particular `-sPROXY_TO_PTHREAD` links `crt1_proxy_main.o`, whose call chain bottoms out at `__main_argc_argv`, so linking a Rust binary fails with: ``` wasm-ld: error: crt1_proxy_main.o: undefined symbol: main ``` This sets `entry_name = "__main_argc_argv"` so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases. The JS-visible name of the entry point stays `_main`, since emscripten bridges `_main` to the wasm `__main_argc_argv` export internally (whereas it wouldn't for `___main_argc_argv`).

Rollup of 25 pull requests Successful merges: - #158871 (add relnotes for 1.97.0) - #158968 (stdarch subtree update) - #157690 (codegen_ssa: pack small const aggregates into immediate stores) - #158541 (Move `std::io::Write` to `core::io`) - #154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - #156370 (Reject linked dylib EII default overrides) - #157153 (allow `Allocator`s to be used as `#[global_allocator]`s) - #158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - #158617 (allow mGCA const arguments to fall back to anon consts) - #158645 (Fix splat ICEs and ban it in closures) - #158655 (Fix coroutine MIR saved local remapping) - #158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - #158870 (std: merge the unix-like io::error modules into one file) - #158920 (Update wasm-component-ld to 0.5.26) - #158926 (wrapping_sh* methods: clarify underspecified reference) - #158927 (add core test run with `-Zforce-intrinsic-fallback`) - #158932 (Do not build the compiler when invoking `x perf compare`) - #158937 (Emit the emscripten entry point as `__main_argc_argv`) - #151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - #156144 (Better docs for PartialEq (includes macro rename)) - #156548 ( Library support for aarch64-unknown-linux-pauthtest target) - #158307 (CI job for parallel frontend ui tests) - #158347 (Improve generic parameters handling for #[diagnostic::on_const]) - #158722 (delegation: do not always inherit `ConstArgHasType` predicates) - #158741 (Simplify `Option::into_flat_iter` signature)

…, r=bjorn3 Emit the emscripten entry point as `__main_argc_argv` Fixes support for building `wasm32-unknown-emscripten` under the `-sPROXY_TO_PTHREAD` linking option. The `wasm32-unknown-emscripten` target uses the argc/argv form of main but emitted the entry point under the raw name `main`. That links on the default entry path (emscripten calls it via `callMain`), but fails on entry paths that reference the mangled symbol directly from wasm. In particular `-sPROXY_TO_PTHREAD` links `crt1_proxy_main.o`, whose call chain bottoms out at `__main_argc_argv`, so linking a Rust binary fails with: ``` wasm-ld: error: crt1_proxy_main.o: undefined symbol: main ``` This sets `entry_name = "__main_argc_argv"` so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases. The JS-visible name of the entry point stays `_main`, since emscripten bridges `_main` to the wasm `__main_argc_argv` export internally (whereas it wouldn't for `___main_argc_argv`).

Rollup of 23 pull requests Successful merges: - #158968 (stdarch subtree update) - #154445 (rustdoc: Represent `--output-format=json` coverage and ir differently) - #158495 (Rename HAS_CT_PROJECTION to HAS_CONST_ALIAS) - #158666 (Carry the `b_offset` inside `BackendRepr::ScalarPair`) - #158870 (std: merge the unix-like io::error modules into one file) - #158920 (Update wasm-component-ld to 0.5.26) - #158926 (wrapping_sh* methods: clarify underspecified reference) - #158927 (add core test run with `-Zforce-intrinsic-fallback`) - #158932 (Do not build the compiler when invoking `x perf compare`) - #158937 (Emit the emscripten entry point as `__main_argc_argv`) - #151379 (Stabilize `VecDeque::retain_back` from `truncate_front`) - #156144 (Better docs for PartialEq (includes macro rename)) - #156548 ( Library support for aarch64-unknown-linux-pauthtest target) - #157995 (`Vec::dedup_by` docs explicit function argument order) - #158307 (CI job for parallel frontend ui tests) - #158741 (Simplify `Option::into_flat_iter` signature) - #158807 (Add regression test for CString::clone_into unwind safety) - #158862 (Fix the span for parameter suggestion ) - #158894 (Make the ordering of non-terminal binds in ambiguity error messages deterministic) - #158902 (add codegen test for range length bound propagation) - #158913 (Update `browser-ui-test` version to `0.24.1`) - #158935 (std: support real fd methods on Emscripten) - #158978 (Add regression test for too-big by-value ABI args)

Rollup merge of #158937 - guybedford:emscripten-pthread-main, r=bjorn3 Emit the emscripten entry point as `__main_argc_argv` Fixes support for building `wasm32-unknown-emscripten` under the `-sPROXY_TO_PTHREAD` linking option. The `wasm32-unknown-emscripten` target uses the argc/argv form of main but emitted the entry point under the raw name `main`. That links on the default entry path (emscripten calls it via `callMain`), but fails on entry paths that reference the mangled symbol directly from wasm. In particular `-sPROXY_TO_PTHREAD` links `crt1_proxy_main.o`, whose call chain bottoms out at `__main_argc_argv`, so linking a Rust binary fails with: ``` wasm-ld: error: crt1_proxy_main.o: undefined symbol: main ``` This sets `entry_name = "__main_argc_argv"` so the entry point is emitted under the C-ABI name emscripten's crt/libc expects, and then works in both cases. The JS-visible name of the entry point stays `_main`, since emscripten bridges `_main` to the wasm `__main_argc_argv` export internally (whereas it wouldn't for `___main_argc_argv`).

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 8, 2026

rustbot assigned jieyouxu Jul 8, 2026

bjorn3 reviewed Jul 8, 2026

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Comment thread compiler/rustc_codegen_ssa/src/back/linker.rs Outdated

rustbot assigned bjorn3 and unassigned jieyouxu Jul 8, 2026

bjorn3 approved these changes Jul 8, 2026

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15f9829

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 8, 2026

JonathanBrouwer mentioned this pull request Jul 8, 2026

Rollup of 14 pull requests #158971

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Rollup of 25 pull requests #158973

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JonathanBrouwer mentioned this pull request Jul 8, 2026

Rollup of 25 pull requests #158977

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jhpratt mentioned this pull request Jul 8, 2026

Rollup of 25 pull requests #158986

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Rollup of 25 pull requests #158989

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Rollup of 23 pull requests #158991

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rust-bors Bot merged commit c706c84 into rust-lang:main Jul 9, 2026
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rustbot added this to the 1.99.0 milestone Jul 9, 2026

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Emit the emscripten entry point as `__main_argc_argv`#158937

Emit the emscripten entry point as `__main_argc_argv`#158937
rust-bors[bot] merged 2 commits into
rust-lang:mainfrom
guybedford:emscripten-pthread-main

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