diff --git a/build/detect-fnm-extmatch.c b/build/detect-fnm-extmatch.c
new file mode 100644
index 000000000..afa35b633
--- /dev/null
+++ b/build/detect-fnm-extmatch.c
@@ -0,0 +1,6 @@
+#include <fnmatch.h>
+
+int main(void) {
+  int x = FNM_EXTMATCH;
+  return 0;
+}
diff --git a/build/detect-glob-period.c b/build/detect-glob-period.c
new file mode 100644
index 000000000..aa9665681
--- /dev/null
+++ b/build/detect-glob-period.c
@@ -0,0 +1,6 @@
+#include <glob.h>
+
+int main(void) {
+  int x = GLOB_PERIOD;
+  return 0;
+}
diff --git a/configure b/configure
index a7c0f1b9d..a740276ca 100755
--- a/configure
+++ b/configure
@@ -445,6 +445,24 @@ echo_cpp() {
   else
     echo '/* #undef HAVE_PWENT */'
   fi
+
+  # Check if non-POSIX GLOB_PERIOD is available
+  if cc_quiet build/detect-glob-period.c; then
+    echo '/* libc defines GLOB_PERIOD */'
+  else
+    # INVALID value that we can detect in code
+    # e.g. Android does not support this, but glibc and musl libc do
+    echo '#define GLOB_PERIOD 0'
+  fi
+
+  # Check if non-POSIX GLOB_PERIOD is available
+  if cc_quiet build/detect-fnm-extmatch.c; then
+    echo '/* libc defines FNM_EXTMATCH */'
+  else
+    # INVALID value that we can detect in code
+    # e.g. musl libc does not support this, but glibc does
+    echo '#define FNM_EXTMATCH 0'
+  fi
 }
 
 # Another way of working: set detected-config.mk ?
diff --git a/doc/portability.md b/doc/portability.md
index 085e75a5e..d1f7d675c 100644
--- a/doc/portability.md
+++ b/doc/portability.md
@@ -14,16 +14,24 @@ These are some notes that supplement [INSTALL](INSTALL.html).
 
 ## Issues in the core of Oils
 
-### GNU libc for extended globs
+### libc - `FNM_EXTMATCH` is not in POSIX
 
-For matching extended globs like `@(*.cc|*.h)`, Oils relies on GNU libc
-support.
+To match extended globs like `@(*.cc|*.h)`, OSH relies on `FNM_EXTMATCH` from
+GNU libc.
 
-- This is not a POSIX feature.
-- It's also unlike bash, which has its own extended glob support.
+This is unlike bash, which has its own extended glob library.
 
 TODO: when using other libc, using this syntax should be an error.
 
+### libc - `GLOB_PERIOD` is not in POSIX
+
+To implement the bash feature `shopt -s dotglob`, OSH relies on `GLOB_PERIOD`,
+which some libc's implement.
+
+This is unlike bash, which has its own glob library.
+
+TODO: `shopt -s dotglob` should give a warning when turned on.
+
 ### Atomic Assignments
 
 The signal handler assumes that int and pointer assignments are atomic.  This
diff --git a/pyext/libc.c b/pyext/libc.c
index c6384dc09..0ad522497 100644
--- a/pyext/libc.c
+++ b/pyext/libc.c
@@ -411,6 +411,7 @@ void initlibc(void) {
   module = Py_InitModule("libc", methods);
   if (module != NULL) {
       PyModule_AddIntConstant(module, "GLOB_PERIOD", GLOB_PERIOD);
+      PyModule_AddIntConstant(module, "FNM_EXTMATCH", FNM_EXTMATCH);
       PyModule_AddIntConstant(module, "FNM_CASEFOLD", FNM_CASEFOLD);
       PyModule_AddIntConstant(module, "REG_ICASE", REG_ICASE);
       PyModule_AddIntConstant(module, "REG_NEWLINE", REG_NEWLINE);
diff --git a/pyext/libc_test.py b/pyext/libc_test.py
index 345299b8e..f8826ac5e 100755
--- a/pyext/libc_test.py
+++ b/pyext/libc_test.py
@@ -20,6 +20,10 @@
 
 class LibcTest(unittest.TestCase):
 
+  def testConstants(self):
+      print('GLOB_PERIOD %d' % libc.GLOB_PERIOD)
+      print('FNM_EXTMATCH %d' % libc.FNM_EXTMATCH)
+
   def testFnmatch(self):
 
     cases = [