macOS Apple Silicon (maca64): mex compilation fails due to new Apple linker (`_mexCreateMexFunction` undefined)

[hurak]

macOS Apple Silicon (maca64): mex compilation fails due to new Apple linker (_mexCreateMexFunction undefined)

Summary

On macOS Apple Silicon (arm64 / maca64) with recent Xcode / Command Line Tools (clang ≥ 17), compilation of Breach MEX files fails with linker errors such as:

Undefined symbols for architecture arm64:
  "_mexCreateMexFunction"
  "_mexDestroyMexFunction"
  "_mexFunctionAdapter"
referenced from: <initial-undefines>

This affects both legacy and online monitoring MEX files (e.g. during InstallBreach, CompileRobusthom, compile_stl_mex).

Root cause

Recent Apple linkers are stricter and no longer tolerate MATLAB’s C++ MEX adapter symbols being left unresolved at link time, even when using -undefined dynamic_lookup.

This is a known incompatibility between:

• MATLAB’s MEX C++ adapter mechanism, and
• the new Apple linker shipped with recent Xcode / CLT.

As a result, plain mex fails on macOS arm64, even for trivial examples, unless a workaround is applied.

Workaround (confirmed)

Force the classic Apple linker by adding the following to every mex invocation:

'LDFLAGS=\$LDFLAGS -ld_classic'

Example:

mex file.cpp 'LDFLAGS=\$LDFLAGS -ld_classic'

This resolves the linker errors. A deprecation warning is expected:

ld: warning: -ld_classic is deprecated and will be removed in a future release

Changes required in Breach installation scripts

Any script that constructs mex command strings must explicitly add this flag. In particular:

• InstallBreach.m
• CompileRobusthom.m
• compile_stl_mex.m

Minimal hard-wired fix pattern used successfully:

if ismac && strcmp(computer('arch'),'maca64')
    FLAGS = [FLAGS ' ''LDFLAGS=\$LDFLAGS -ld_classic'' '];
end

or, in scripts using sprintf('mex ...'), inject the same quoted argument into each mex command.

Scope

• Platform: macOS Apple Silicon (arm64 / maca64)
• MATLAB: confirmed on R2025b (likely earlier recent releases too)
• Xcode / CLT: recent versions (clang 17)

Intel macOS, Linux, and Windows are unaffected.

Recommendation for maintainers

Short term:

• Automatically append -ld_classic to LDFLAGS for ismac && maca64.

Long term:

• Detect toolchain / linker compatibility more robustly, or
• Track MathWorks updates addressing this linker incompatibility.

Until then, without this workaround, fresh installs on Apple Silicon macOS will fail.

[hurak]

Almost surely related to the issue #12.