diff --git a/README.md b/README.md
index e03e98b..a9ec17b 100644
--- a/README.md
+++ b/README.md
@@ -179,6 +179,13 @@ pass `NULL` to inherit the parent's current working directory. On Windows,
 custom environment entries and the current working directory string are
 interpreted as UTF-8.
 
+Not every platform can honour a custom working directory. Where it cannot,
+`SUBPROCESS_HAVE_CWD` is defined to `0` and passing a non-`NULL` working
+directory fails with `ENOSYS`; this currently affects glibc older than 2.29,
+macOS older than 10.15, and iOS, tvOS and watchOS, where Apple marks the
+underlying call unavailable. Define `SUBPROCESS_HAVE_CWD` yourself to override
+the detection, for instance on musl older than 1.1.24.
+
 Note though that you **cannot** specify `subprocess_option_inherit_environment`
 with a custom environment. If you want to merge some custom environment with the
 parent process environment then it is up to you as the user to query the original
diff --git a/subprocess.h b/subprocess.h
index 5e80902..a81f45a 100644
--- a/subprocess.h
+++ b/subprocess.h
@@ -274,6 +274,25 @@ subprocess_weak int subprocess_alive(struct subprocess_s *const process);
 #include <unistd.h>
 #endif
 
+/* Whether subprocess_create_ex can honour process_cwd. glibc only gained
+   posix_spawn_file_actions_addchdir_np in 2.29, and macOS in 10.15; the SDKs
+   mark it unavailable on iOS, tvOS and watchOS, where the undefined version
+   macro folds to 0 and so answers correctly. Define this yourself to override
+   the detection, for instance on musl older than 1.1.24. */
+#if !defined(SUBPROCESS_HAVE_CWD)
+#if defined(__GLIBC__)
+#if __GLIBC_PREREQ(2, 29)
+#define SUBPROCESS_HAVE_CWD 1
+#else
+#define SUBPROCESS_HAVE_CWD 0
+#endif
+#elif defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED < 101500
+#define SUBPROCESS_HAVE_CWD 0
+#else
+#define SUBPROCESS_HAVE_CWD 1
+#endif
+#endif
+
 #if defined(_WIN32)
 
 #include <wchar.h>
@@ -1207,6 +1226,8 @@ int subprocess_create_ex(const char *const commandLine[], int options,
   if (process_cwd) {
 #if defined(__APPLE__) && MAC_OS_X_VERSION_MIN_REQUIRED >= 260000
     posix_error = posix_spawn_file_actions_addchdir(&actions, process_cwd);
+#elif !SUBPROCESS_HAVE_CWD
+    posix_error = ENOSYS;
 #else
 #if defined(__APPLE__) && defined(__clang__)
 #pragma clang diagnostic push
diff --git a/test/test_shared.h b/test/test_shared.h
index a694ed6..b6a9706 100644
--- a/test/test_shared.h
+++ b/test/test_shared.h
@@ -1116,6 +1116,7 @@ SUBPROCESS_TEST(environment, specify_environment) {
   ASSERT_EQ(0, subprocess_destroy(&process));
 }
 
+#if SUBPROCESS_HAVE_CWD
 SUBPROCESS_TEST(create_ex, subprocess_cwd) {
   char current_path[4096];
   char target_path[4096];
@@ -1156,6 +1157,7 @@ SUBPROCESS_TEST(create_ex, subprocess_cwd) {
 
   ASSERT_EQ(0, subprocess_destroy(&process));
 }
+#endif
 
 #if !defined(_MSC_VER)
 SUBPROCESS_TEST(executable_resolve, no_slashes_with_environment) {