From 7b29ae7e8ceaa13a10213324792fe5e32b06f186 Mon Sep 17 00:00:00 2001
From: Hood Chatham <roberthoodchatham@gmail.com>
Date: Fri, 31 Jul 2026 16:48:39 -0700
Subject: [PATCH] Turn off shared library signature check when configuring

`AC_CHECK_LIB(somelib, func)` writes the following to conftest.c:

```C
char func ();

int main(void) {
   return func ();
}
```
Then runs `emcc conftest.c -lsomelib` and checks whether it succeeds. With
static libraries the signature mismatch is just a warning but with shared
libraries it is a hard error. Before emcc 6.0.0, even if configure tried to make
a shared library it would make a fake dynamic library instead and we didn't see
this problem. But since then it makes a real dynamic library and then fails to
configure.

The fix is if we are configuring pass `-Wl,--no-shlib-sigcheck` to disable the
signature check.
---
 test/test_other.py | 15 +++++++++++++++
 tools/link.py      |  7 +++++++
 2 files changed, 22 insertions(+)

diff --git a/test/test_other.py b/test/test_other.py
index af0d39f4fb216..ddd8cd37342f0 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -12406,6 +12406,21 @@ def test_autoconf_mode(self):
     output = self.run_process([os.path.abspath('a.out')], stdout=PIPE).stdout
     self.assertContained('Hello, world!', output)
 
+  def test_autoconf_check_lib_side_module(self):
+    # AC_CHECK_LIB probes for a symbol using an unprototyped declaration and a
+    # zero-argument call.  Verify that the signature mismatch against the real
+    # function does not make the probe fail when the library is a side module.
+    create_file('libtest.c', 'int identity(int x) { return x; }\n')
+    self.run_process([EMCC, '-fPIC', '-shared', '-o', 'libtest.so', 'libtest.c'])
+    create_file('conftest.c', '''
+      char identity ();
+
+      int main(void) {
+        return identity ();
+      }
+    ''')
+    self.run_process([EMCC, 'conftest.c', 'libtest.so', '-o', 'conftest.js'])
+
   def test_standalone_export_main(self):
     # Tests that explicitly exported `_main` does not fail, even though `_start` is the entry
     # point.
diff --git a/tools/link.py b/tools/link.py
index daad61cc53659..fdf489889ae30 100644
--- a/tools/link.py
+++ b/tools/link.py
@@ -889,6 +889,13 @@ def phase_linker_setup(linker_args):  # ruff: ignore[complex-structure, too-many
     # autoconf declares functions without their proper signatures, and STRICT causes that to trip up by passing --fatal-warnings to the linker.
     if settings.STRICT:
       exit_with_error('autoconfiguring is not compatible with STRICT')
+    # AC_CHECK_LIB probes for a symbol by declaring it without a prototype
+    # (`char foo ();`) and then calling it with no arguments.  When the symbol
+    # comes from an object file or an archive lld only warns about the
+    # resulting signature mismatch and synthesizes a thunk, but when it comes
+    # from a shared library the mismatch is a hard error. This turns off the
+    # shared library check.
+    linker_args.append('--no-shlib-sigcheck')
 
   if settings.OPT_LEVEL >= 1:
     default_setting('ASSERTIONS', 0)