gh-99948: Support ctypes.util.find_library in emscripten environment

Lib/ctypes/util.py

@@ -173,6 +173,28 @@ def find_library(name):
         fname = f"{directory}/lib{name}.so"
         return fname if os.path.isfile(fname) else None
 
+elif sys.platform == "emscripten":
+    def _is_wasm(filename):
+        # Return True if the given file is an WASM module
+        wasm_header = b"\x00asm"
+        with open(filename, 'br') as thefile:
+            return thefile.read(4) == wasm_header
+
+    def find_library(name):
+        candidates = [f"lib{name}.so", f"lib{name}.wasm"]
+        paths = os.environ.get("LD_LIBRARY_PATH", "")
+        for libdir in paths.split(":"):
+            for name in candidates:
+                libfile = os.path.join(libdir, name)
+
+                if os.path.isfile(libfile):
+                    if not _is_wasm(libfile):
+                        continue
+
+                    return libfile
+
+        return None
+
 elif os.name == "posix":
     # Andreas Degert's find functions, using gcc, /sbin/ldconfig, objdump
     import re, tempfile

Lib/test/test_ctypes/test_find.py

@@ -153,5 +153,37 @@ def test_find(self):
                 self.assertIsNone(find_library(name))
 
 
+@unittest.skipUnless(test.support.is_emscripten,
+                     'Test only valid for Emscripten')
+class FindLibraryEmscripten(unittest.TestCase):
+    def test_find_on_libpath(self):
+        import tempfile
+
+        # A very simple wasm module
+        # In WAT format: (module)
+        wasm_module = b'\x00asm\x01\x00\x00\x00\x00\x08\x04name\x02\x01\x00'
+
+        with tempfile.TemporaryDirectory() as d:
+            libname = 'dummy'
+            dstname = os.path.join(d, 'lib%s.so' % libname)
+            with open(dstname, 'wb') as f:
+                f.write(wasm_module)
+
+            # now check that the .so can't be found (since not in
+            # LD_LIBRARY_PATH)
+            self.assertIsNone(find_library(libname))
+            # now add the location to LD_LIBRARY_PATH
+            with os_helper.EnvironmentVarGuard() as env:
+                KEY = 'LD_LIBRARY_PATH'
+                if KEY not in env:
+                    v = d
+                else:
+                    v = '%s:%s' % (env[KEY], d)
+                env.set(KEY, v)
+                # now check that the .so can be found (since in
+                # LD_LIBRARY_PATH)
+                self.assertEqual(find_library(libname), dstname)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2025-09-05-05-53-43.gh-issue-99948.KMSlG6.rst

@@ -0,0 +1 @@
+:func:`ctypes.util.find_library` now works in Emscripten build.