Enable real dynamic linking by default

This change essentially disables `FAKE_DYLIBS` by default, which in
turn means `-shared` will now produce dynamic libraries by default.

If you want the old behaviour you now need `-sFAKE_DYLIBS`.

Author: sbc100

ChangeLog.md

@@ -43,6 +43,10 @@ See docs/process.md for more on how version tagging works.
   notified of all IDBFS sync start and end events. (#26895)
 - google-closure-compiler was updated to 20260429.0.0. (#26869)
   Closure compiler now provides a native macOS arm64 binary for Apple Silicon.
+- The FAKE_DYLIBS setting is now disabled by default. This means that `-shared`
+  will now produce real dynamic libraries by default. It also means that
+  `-sMAIN_MODULE=2` is not implicit if any real dynamic libraries are included
+  in the link. (#25930)
 
 5.0.7 - 04/30/26
 ----------------

cmake/Modules/Platform/Emscripten.cmake

@@ -18,7 +18,7 @@ set(CMAKE_SYSTEM_NAME Emscripten)
 set(CMAKE_SYSTEM_VERSION 1)
 
 set(CMAKE_CROSSCOMPILING TRUE)
-set_property(GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS FALSE)
+set_property(GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS TRUE)
 
 # Advertise Emscripten as a 32-bit platform (as opposed to
 # CMAKE_SYSTEM_PROCESSOR=x86_64 for 64-bit platform), since some projects (e.g.

site/source/docs/tools_reference/settings_reference.rst

@@ -3354,13 +3354,12 @@ Default value: false
 FAKE_DYLIBS
 ===========
 
-This setting changes the behaviour of the ``-shared`` flag.  The default
-setting of ``true`` means the ``-shared`` flag actually produces a normal
-object file (i.e. ``ld -r``).  Setting this to false will cause ``-shared``
-to behave like :ref:`SIDE_MODULE` and produce a dynamically linked
-library.
+This setting changes the behaviour of the ``-shared`` flag.  When set to true
+you get the old emscripten behaivour where the ``-shared`` flag actually
+produces a normal object file (i.e. ``ld -r``).  The default behaviour is that
+`-shared` is the as :ref:`SIDE_MODULE`.
 
-Default value: true
+Default value: false
 
 .. _executable:
 

src/settings.js

@@ -2199,12 +2199,11 @@ var GROWABLE_ARRAYBUFFERS = false;
 // indirectly using `importScripts`
 var CROSS_ORIGIN = false;
 
-// This setting changes the behaviour of the ``-shared`` flag.  The default
-// setting of ``true`` means the ``-shared`` flag actually produces a normal
-// object file (i.e. ``ld -r``).  Setting this to false will cause ``-shared``
-// to behave like :ref:`SIDE_MODULE` and produce a dynamically linked
-// library.
-var FAKE_DYLIBS = true;
+// This setting changes the behaviour of the ``-shared`` flag.  When set to true
+// you get the old emscripten behaivour where the ``-shared`` flag actually
+// produces a normal object file (i.e. ``ld -r``).  The default behaviour is that
+// `-shared` is the as :ref:`SIDE_MODULE`.
+var FAKE_DYLIBS = false;
 
 // Add a #! line to generated JS file and make it executable.  This is useful
 // for building command line tools that run under node.

test/test_core.py

@@ -4059,8 +4059,8 @@ def dylink_testf(self, main, side=None, expected=None, force_c=False, main_cflag
 
     shutil.move(so_file, so_file + '.orig')
 
-    # Verify that building with -sSIDE_MODULE is essentially the same as building with `-shared -fPIC -sFAKE_DYLIBS=0`.
-    flags = ['-shared', '-fPIC', '-sFAKE_DYLIBS=0']
+    # Verify that building with -sSIDE_MODULE is essentially the same as building with `-shared -fPIC`
+    flags = ['-shared', '-fPIC']
     if isinstance(side, list):
       # side is just a library
       self.run_process([EMCC] + side + self.get_cflags() + flags + ['-o', so_file])

test/test_other.py

@@ -1173,7 +1173,7 @@ def test_odd_suffixes(self):
     for suffix in ('lo',):
       self.clear()
       print(suffix)
-      self.run_process([EMCC, test_file('hello_world.c'), '-shared', '-o', 'binary.' + suffix])
+      self.run_process([EMCC, test_file('hello_world.c'), '-sFAKE_DYLIBS', '-shared', '-o', 'binary.' + suffix])
       self.run_process([EMCC, 'binary.' + suffix])
       self.assertContained('Hello, world!', self.run_js('a.out.js'))
 
@@ -1354,7 +1354,7 @@ def test_multiply_defined_libsymbols(self):
     ''')
 
     self.cflags.remove('-Werror')
-    self.emcc('libA.c', ['-shared', '-o', 'libA.so'])
+    self.emcc('libA.c', ['-shared', '-sFAKE_DYLIBS', '-o', 'libA.so'])
 
     self.emcc('a2.c', ['-r', '-L.', '-lA', '-o', 'a2.o'])
     self.emcc('b2.c', ['-r', '-L.', '-lA', '-o', 'b2.o'])
@@ -1487,7 +1487,12 @@ def test_link_group_bitcode(self):
   # We deliberately ignore duplicate input files in order to allow
   # "libA.so" on the command line twice. This is not really .so support
   # and the .so files are really object files.
-  def test_redundant_link(self):
+  @parameterized({
+    '': ([],),
+    'fake_dylibs': (['-sFAKE_DYLIBS'],),
+  })
+  def test_redundant_link(self, args):
+    self.cflags += args
     create_file('libA.c', 'int mult() { return 1; }')
     create_file('main.c', r'''
       #include <stdio.h>
@@ -1499,7 +1504,7 @@ def test_redundant_link(self):
     ''')
 
     self.cflags.remove('-Werror')
-    self.emcc('libA.c', ['-shared', '-o', 'libA.so'])
+    self.emcc('libA.c', ['-fPIC', '-shared', '-o', 'libA.so'])
     self.emcc('main.c', ['libA.so', 'libA.so', '-o', 'a.out.js'])
     self.assertContained('result: 1', self.run_js('a.out.js'))
 
@@ -2147,9 +2152,9 @@ def test_multidynamic_link(self, link_flags, lib_suffix):
     ''')
 
     # Build libfile normally into an .so
-    self.run_process([EMCC, 'libdir/libfile.c', '-shared', '-o', 'libdir/libfile.so' + lib_suffix])
+    self.run_process([EMCC, 'libdir/libfile.c', '-sFAKE_DYLIBS', '-shared', '-fPIC', '-o', 'libdir/libfile.so' + lib_suffix])
     # Build libother and dynamically link it to libfile
-    self.run_process([EMCC, '-Llibdir', 'libdir/libother.c'] + link_flags + ['-shared', '-o', 'libdir/libother.so'])
+    self.run_process([EMCC, '-Llibdir', 'libdir/libother.c'] + link_flags + ['-sFAKE_DYLIBS', '-shared', '-fPIC', '-o', 'libdir/libother.so'])
     # Build the main file, linking in both the libs
     self.run_process([EMCC, '-Llibdir', os.path.join('main.c')] + link_flags + ['-lother', '-c'])
     print('...')
@@ -4940,20 +4945,6 @@ def test_valid_abspath_2(self):
     self.run_process(cmd)
     self.assertContained('Hello, world!', self.run_js('a.out.js'))
 
-  def test_warn_dylibs(self):
-    shared_suffixes = ['.so', '.dylib', '.dll']
-
-    for suffix in ('.o', '.bc', '.so', '.dylib', '.js', '.html'):
-      print(suffix)
-      cmd = [EMCC, test_file('hello_world.c'), '-o', 'out' + suffix]
-      if suffix in {'.o', '.bc'}:
-        cmd.append('-c')
-      if suffix in {'.dylib', '.so'}:
-        cmd.append('-shared')
-      err = self.run_process(cmd, stderr=PIPE).stderr
-      warning = 'linking a library with `-shared` will emit a static object file'
-      self.assertContainedIf(warning, err, suffix in shared_suffixes)
-
   @crossplatform
   @parameterized({
     'O2': [['-O2']],
@@ -8446,19 +8437,6 @@ def test_side_module_folder_deps(self):
     self.run_process([EMCC, test_file('hello_world.c'), '-sSIDE_MODULE', '-o', 'subdir/libside2.so', '-L', 'subdir', '-lside1'])
     self.run_process([EMCC, test_file('hello_world.c'), '-sMAIN_MODULE', '-o', 'main.js', '-L', 'subdir', '-lside2'])
 
-  @crossplatform
-  def test_side_module_ignore(self):
-    self.run_process([EMCC, test_file('hello_world.c'), '-sSIDE_MODULE', '-o', 'libside.so'])
-
-    # Attempting to link statically against a side module (libside.so) should fail.
-    self.assert_fail([EMCC, '-L.', '-lside'], 'wasm-ld: error: unable to find library -lside')
-
-    # But a static library in the same location (libside.a) should take precedence.
-    self.run_process([EMCC, test_file('hello_world.c'), '-c'])
-    self.run_process([EMAR, 'cr', 'libside.a', 'hello_world.o'])
-    self.run_process([EMCC, '-L.', '-lside'])
-    self.assertContained('Hello, world!', self.run_js('a.out.js'))
-
   @is_slow_test
   @parameterized({
     '': ([],),
@@ -12114,42 +12092,40 @@ def test_err(self):
   def test_euidaccess(self):
     self.do_other_test('test_euidaccess.c')
 
-  def test_shared_flag(self):
-    create_file('side.c', 'int foo;')
-    self.run_process([EMCC, '-shared', 'side.c', '-o', 'libother.so'])
+  def test_fake_dylibs(self):
+    create_file('other.c', 'int foo = 10;')
+    self.run_process([EMCC, '-shared', '-sFAKE_DYLIBS', '-fPIC', 'other.c', '-o', 'libother.so'])
+    self.assertIsObjectFile('libother.so')
 
-    # Test that `-shared` flag causes object file generation but gives a warning
-    err = self.run_process([EMCC, '-shared', test_file('hello_world.c'), '-o', 'out.foo', 'libother.so'], stderr=PIPE).stderr
-    self.assertContained('linking a library with `-shared` will emit a static object', err)
+    # Test that `-sFAKE_DYLIBS` flag causes object file generation and will generate a warning about
+    # dylink dependencies being ignored.
+    err = self.run_process([EMCC, '-shared', '-sFAKE_DYLIBS', '-fPIC', test_file('hello_world.c'), '-o', 'out.foo', 'libother.so'], stderr=PIPE).stderr
     self.assertContained('emcc: warning: ignoring dynamic library libother.so when generating an object file, this will need to be included explicitly in the final link', err)
     self.assertIsObjectFile('out.foo')
 
     # Test that adding `-sFAKE_DYLIBS=0` build a real side module
     err = self.run_process([EMCC, '-shared', '-fPIC', '-sFAKE_DYLIBS=0', test_file('hello_world.c'), '-o', 'out.foo', 'libother.so'], stderr=PIPE).stderr
-    self.assertNotContained('linking a library with `-shared` will emit a static object', err)
     self.assertNotContained('emcc: warning: ignoring dynamic library libother.so when generating an object file, this will need to be included explicitly in the final link', err)
     self.assertIsWasmDylib('out.foo')
 
     # Test that using an executable output name overrides the `-shared` flag, but produces a warning.
-    err = self.run_process([EMCC, '-shared', test_file('hello_world.c'), '-o', 'out.js'],
+    err = self.run_process([EMCC, '-shared', '-sFAKE_DYLIBS', '-fPIC', test_file('hello_world.c'), '-o', 'out.js'],
                            stderr=PIPE).stderr
     self.assertContained('warning: -shared/-r used with executable output suffix', err)
     self.run_js('out.js')
 
   def test_shared_soname(self):
-    self.run_process([EMCC, '-shared', '-Wl,-soname', '-Wl,libfoo.so.13', test_file('hello_world.c'), '-lc', '-o', 'libfoo.so'])
+    self.run_process([EMCC, '-shared', '-sFAKE_DYLIBS', '-Wl,-soname', '-Wl,libfoo.so.13', test_file('hello_world.c'), '-lc', '-o', 'libfoo.so'])
     self.run_process([EMCC, '-sSTRICT', 'libfoo.so'])
     self.assertContained('Hello, world!', self.run_js('a.out.js'))
 
-  def test_shared_and_side_module_flag(self):
-    # Test that `-shared` and `-sSIDE_MODULE` flag causes wasm dylib generation without a warning.
-    err = self.run_process([EMCC, '-shared', '-sSIDE_MODULE', test_file('hello_world.c'), '-o', 'out.foo'], stderr=PIPE).stderr
-    self.assertNotContained('linking a library with `-shared` will emit a static object', err)
+  def test_shared_flag(self):
+    # Test that `-shared` flag causes wasm dylib generation
+    self.run_process([EMCC, '-shared', '-fPIC', test_file('hello_world.c'), '-o', 'out.foo'])
     self.assertIsWasmDylib('out.foo')
 
-    # Test that `-shared` and `-sSIDE_MODULE` flag causes wasm dylib generation without a warning even if given executable output name.
-    err = self.run_process([EMCC, '-shared', '-sSIDE_MODULE', test_file('hello_world.c'), '-o', 'out.wasm'],
-                           stderr=PIPE).stderr
+    # Test that `-shared` causes wasm dylib generation warning even if given executable output name.
+    err = self.run_process([EMCC, '-shared', '-fPIC', test_file('hello_world.c'), '-o', 'out.wasm'], stderr=PIPE).stderr
     self.assertNotContained('warning: -shared/-r used with executable output suffix', err)
     self.assertIsWasmDylib('out.wasm')
 

tools/building.py

@@ -165,6 +165,12 @@ def lld_flags_for_executable(external_symbols):
     stub = create_stub_object(external_symbols)
     cmd.append(stub)
 
+  if settings.FAKE_DYLIBS:
+    cmd.append('-Bstatic')
+  else:
+    # wasm-ld still default to static linking by default. If that ever changes we can remove this line.
+    cmd.append('-Bdynamic')
+
   if not settings.ERROR_ON_UNDEFINED_SYMBOLS:
     cmd.append('--import-undefined')
 
@@ -203,7 +209,6 @@ def lld_flags_for_executable(external_symbols):
   c_exports = [e for e in c_exports if e not in external_symbols]
   c_exports += settings.REQUIRED_EXPORTS
   if settings.MAIN_MODULE:
-    cmd.append('-Bdynamic')
     c_exports += side_module_external_deps(external_symbols)
   for export in c_exports:
     if settings.ERROR_ON_UNDEFINED_SYMBOLS:

tools/link.py

@@ -805,17 +805,14 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
 
   apply_library_settings(linker_args)
 
-  if settings.SIDE_MODULE or settings.MAIN_MODULE:
-    default_setting('FAKE_DYLIBS', 0)
-
   if options.shared and not settings.FAKE_DYLIBS:
     default_setting('SIDE_MODULE', 1)
 
-  if not settings.FAKE_DYLIBS:
+  if not settings.SIDE_MODULE and not settings.FAKE_DYLIBS:
     options.dylibs = get_dylibs(options, linker_args)
-    # If there are any dynamically linked libraries on the command line then
-    # need to enable `MAIN_MODULE` in order to produce JS code that can load them.
-    if not settings.MAIN_MODULE and not settings.SIDE_MODULE and options.dylibs:
+    # If there are any dynamic libraries on the command line then enable
+    # `MAIN_MODULE` by default in order to produce JS code that can load them.
+    if options.dylibs and not settings.MAIN_MODULE:
       default_setting('MAIN_MODULE', 2)
 
   linker_args += calc_extra_ldflags(options)
@@ -917,8 +914,6 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
     if final_suffix in EXECUTABLE_EXTENSIONS:
       diagnostics.warning('emcc', '-shared/-r used with executable output suffix. This behaviour is deprecated.  Please remove -shared/-r to build an executable or avoid the executable suffix (%s) when building object files.' % final_suffix)
     else:
-      if options.shared and 'FAKE_DYLIBS' not in user_settings:
-        diagnostics.warning('emcc', 'linking a library with `-shared` will emit a static object file (FAKE_DYLIBS defaults to true).  If you want to build a runtime shared library use the SIDE_MODULE or FAKE_DYLIBS=0.')
       options.oformat = OFormat.OBJECT
 
   if not options.oformat:
@@ -3016,8 +3011,7 @@ def phase_calculate_linker_inputs(options, linker_args):
   else:
     linker_args = filter_out_duplicate_fake_dynamic_libs(linker_args)
 
-  if settings.MAIN_MODULE:
-    process_dynamic_libs(options.dylibs, options.lib_dirs)
+  process_dynamic_libs(options.dylibs, options.lib_dirs)
 
   return linker_args