Disable signal handlers and limit threading for libraries via jl_options shim by gbaraldi · Pull Request #113 · JuliaLang/JuliaC.jl

gbaraldi · 2026-02-18T18:57:51Z

Summary

Libraries built with --output-lib now automatically set handle_signals=off and threads=1 via an __attribute__((constructor)) C shim that modifies jl_options before jl_init
Adds --jl-option <key=value> CLI flag using the same syntax as Julia CLI flags (e.g. --jl-option handle-signals=no, --jl-option threads=4,2)
Thread parsing mirrors jloptions.c semantics including interactive thread pool support
Uses dladdr/dlsym on Unix and direct &jl_options on Windows to resolve the correct symbol from the linked libjulia dependency
Validates option names against an allowlist (handle-signals, threads) and values against Julia's expected formats

Test plan

Unit tests for auto-population of library defaults
Unit tests for validation (unknown keys, invalid values)
CLI parsing tests for --jl-option (space/equals separators, multiple flags, error cases)
Runtime verification: compile exe with --jl-option, run it, check Base.JLOptions() fields
Runtime verification: compile privatized library, dlopen from fresh Julia process, call ccallable functions to verify options applied
Full test suite passes

Closes #112

🤖 Generated with Claude Code

…ons constructor shim Libraries built with --output-lib now automatically set handle_signals=off and threads=1 via an __attribute__((constructor)) C shim that modifies jl_options before jl_init. This prevents JuliaC libraries from installing signal handlers or spawning extra threads when loaded into a host process. The mechanism is exposed as --jl-option <key=value> with the same syntax as Julia CLI flags (e.g. --jl-option handle-signals=no, --jl-option threads=4,2). Thread parsing mirrors jloptions.c semantics including interactive thread pools. Uses dladdr/dlsym on Unix and direct &jl_options on Windows to resolve the correct symbol from the linked libjulia dependency. Closes #112 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

On Linux, dlfcn.h only exposes dladdr and Dl_info when _GNU_SOURCE is defined. macOS exposes them unconditionally, which is why local tests passed but CI on ubuntu failed. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

On Linux, dlopen(RTLD_NOLOAD) on the main executable returns NULL, causing the constructor to silently bail out for executables. On macOS, dlsym with a specific handle searches that image and its dependencies, so the dladdr/dlopen/dlsym approach works. Use RTLD_DEFAULT on Linux to search all loaded shared objects. This is safe because our libjulia is the only one loaded at constructor time. Keep the dladdr/dlopen approach on macOS for dependency-scoped resolution. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/compiling.jl

src/scripts/juliac-jl-options-shim.c

topolarity · 2026-02-18T21:04:01Z

I am happy with the C shim here, but is it possible to do the arg parsing, etc. on the Julia side via a jl_setup_args (or similar) function that updates a jl_options_t based on a provided args string?

gbaraldi · 2026-02-19T14:28:53Z

Not sure if get what you mean?

topolarity · 2026-02-19T14:46:34Z

I mean that we probably shouldn't re-implement argument parsing for Julia in JuliaC - it's a lot of maintenance burden and can lead to subtle bugs and inconsistencies, such as if Julia sets up the threadpools internally one way and then JuliaC decides to set up them up in another, no-longer-supported way (or any number of other divergences).

If you want to support translating CLI arguments to jl_options, we should add that functionality to libjulia and use it here IMO.

…ment Instead of generating C code that manually assigns jl_options struct fields, call jl_parse_opts with a synthetic argv. This reuses Julia's own option parsing logic and avoids duplicating the parsing for threads, handle-signals, etc. The constructor saves/restores getopt global state and image_file to avoid interfering with the host process. Options are validated at compile time by running a Julia subprocess with the same flags. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

topolarity

Looking much better without us parsing args ourselves.

A few questions w.r.t. the CLI help, etc.

src/JuliaC.jl

src/compiling.jl

- Fix out-of-date comments on jl_options field in ImageRecipe - Update help text to mention Julia CLI syntax and list supported options Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

topolarity · 2026-02-27T20:30:23Z

src/compiling.jl

+    # Generate and compile a C shim that sets jl_options fields via constructor
+    if !isempty(recipe.jl_options)
+        _validate_jl_options(recipe.jl_options)
+        obj = _compile_jl_options_shim(recipe.jl_options; verbose=recipe.verbose)


It may be worth compiling and linking this always.

Otherwise, you may hit mysterious bugs where jl_parse_opts, e.g., sets an unexpected default, but you only see the behavior based on whether you provided any jl_options flags or not.

topolarity · 2026-02-27T20:31:57Z

src/JuliaC.jl

    println(io, "  --privatize                 Privatize bundled libjulia (Unix)")
    println(io, "  --trim[=mode]               Strip IR/metadata (e.g. --trim=safe)")
    println(io, "  --compile-ccallable         Export ccallable entrypoints")
+    println(io, "  --jl-option <key=value>     Set a Julia option using CLI syntax (supported: handle-signals, threads)")


Suggested change

println(io, " --jl-option <key=value> Set a Julia option using CLI syntax (supported: handle-signals, threads)")

println(io, " --jl-option <key=value> Set a Julia option using CLI syntax (supported: handle-signals=[yes/no], threads=[N])")

This may not be required if you pipe the feedback from args validation back to the user - I just want to make sure that there's some way to learn the valid arguments

Right now Julia is unfortunately not that helpful:

ERROR: julia: invalid argument to --handle-signals (maybe)

topolarity · 2026-02-27T20:34:32Z

test/programatic.jl

+end
+
+@testset "jl_options applied at runtime (library)" begin
+    if Sys.isunix()


Why only unix?

topolarity

Looks largely good to me.

I might recommend --jl-cli-option instead of --jl-option since I think it makes it clearer these strings are piped to Julia's CLI args handler.

Either way, I think this is good enough to land if you add a test for the library functionality on Windows

gbaraldi and others added 2 commits February 18, 2026 15:56

gbaraldi force-pushed the gb/jl-options-shim branch from dfe3525 to 13215fb Compare February 18, 2026 19:07

topolarity reviewed Feb 18, 2026

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gbaraldi force-pushed the gb/jl-options-shim branch 4 times, most recently from bc12bcf to 6bdb164 Compare February 18, 2026 19:24

gbaraldi force-pushed the gb/jl-options-shim branch 2 times, most recently from 7908d36 to 3853d61 Compare February 19, 2026 16:39

gbaraldi force-pushed the gb/jl-options-shim branch from 3853d61 to 3319bce Compare February 19, 2026 16:58

topolarity reviewed Feb 20, 2026

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Address PR review comments: update comments and help text

d38ff84

- Fix out-of-date comments on jl_options field in ImageRecipe - Update help text to mention Julia CLI syntax and list supported options Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

topolarity reviewed Feb 27, 2026

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test/programatic.jl

end

@testset "jl_options applied at runtime (library)" begin

if Sys.isunix()

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Why only unix?

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Disable signal handlers and limit threading for libraries via jl_options shim#113

Disable signal handlers and limit threading for libraries via jl_options shim#113
gbaraldi wants to merge 5 commits intomainfrom
gb/jl-options-shim

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	println(io, " --jl-option <key=value> Set a Julia option using CLI syntax (supported: handle-signals, threads)")
	println(io, " --jl-option <key=value> Set a Julia option using CLI syntax (supported: handle-signals=[yes/no], threads=[N])")

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gbaraldi commented Feb 18, 2026

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topolarity commented Feb 18, 2026

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