diff --git a/CMakeLists.txt b/CMakeLists.txt
index a0446b1..308268f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -142,6 +142,9 @@
     if (CMAKE_SYSTEM_VERSION MATCHES "Microsoft")
         add_definitions (-DNN_HAVE_WSL)
     endif()
+elseif (CMAKE_SYSTEM_NAME MATCHES "Android")
+    add_definitions (-DNN_HAVE_ANDROID)
+    add_definitions (-DNN_HAVE_LINUX)
 elseif (CMAKE_SYSTEM_NAME MATCHES "Darwin")
     add_definitions (-DNN_HAVE_OSX)
 elseif (CMAKE_SYSTEM_NAME MATCHES "Windows")
@@ -248,7 +251,10 @@
     nn_check_sym (backtrace_symbols_fd execinfo.h NN_HAVE_BACKTRACE)
     nn_check_struct_member(msghdr msg_control sys/socket.h NN_HAVE_MSG_CONTROL)
     if (NN_HAVE_SEMAPHORE_RT OR NN_HAVE_SEMAPHORE_PTHREAD)
-        add_definitions (-DNN_HAVE_SEMAPHORE)
+        if (NOT CMAKE_SYSTEM_NAME MATCHES "Darwin")
+            # macOS doesn't have unnamed semaphores
+            add_definitions (-DNN_HAVE_SEMAPHORE)
+        endif()
     endif ()
 endif ()
 
diff --git a/src/utils/sem.c b/src/utils/sem.c
index 17943d5..06a0128 100644
--- a/src/utils/sem.c
+++ b/src/utils/sem.c
@@ -24,74 +24,8 @@
 #include "err.h"
 #include "fast.h"
 
-#if defined NN_HAVE_OSX
 
-void nn_sem_init (struct nn_sem *self)
-{
-    int rc;
-
-    rc = pthread_mutex_init (&self->mutex, NULL);
-    errnum_assert (rc == 0, rc);
-    rc = pthread_cond_init (&self->cond, NULL);
-    errnum_assert (rc == 0, rc);
-    self->signaled = 0;
-}
-
-void nn_sem_term (struct nn_sem *self)
-{
-    int rc;
-
-    rc = pthread_cond_destroy (&self->cond);
-    errnum_assert (rc == 0, rc);
-    rc = pthread_mutex_destroy (&self->mutex);
-    errnum_assert (rc == 0, rc);
-}
-
-void nn_sem_post (struct nn_sem *self)
-{
-    int rc;
-
-    rc = pthread_mutex_lock (&self->mutex);
-    errnum_assert (rc == 0, rc);
-    nn_assert (self->signaled == 0);
-    self->signaled = 1;
-    rc = pthread_cond_signal (&self->cond);
-    errnum_assert (rc == 0, rc);
-    rc = pthread_mutex_unlock (&self->mutex);
-    errnum_assert (rc == 0, rc);
-}
-
-int nn_sem_wait (struct nn_sem *self)
-{
-    int rc;
-
-    /*  With OSX, semaphores are global named objects. They are not useful for
-        our use case. To get a similar object we exploit the implementation
-        detail of pthread_cond_wait() in Darwin kernel: It exits if signal is
-        caught. Note that this behaviour is not mandated by POSIX
-        and may break with future versions of Darwin. */
-    rc = pthread_mutex_lock (&self->mutex);
-    errnum_assert (rc == 0, rc);
-    if (nn_fast (self->signaled)) {
-        rc = pthread_mutex_unlock (&self->mutex);
-        errnum_assert (rc == 0, rc);
-        return 0;
-    }
-    rc = pthread_cond_wait (&self->cond, &self->mutex);
-    errnum_assert (rc == 0, rc);
-    if (nn_slow (!self->signaled)) {
-        rc = pthread_mutex_unlock (&self->mutex);
-        errnum_assert (rc == 0, rc);
-        return -EINTR;
-    }
-    self->signaled = 0;
-    rc = pthread_mutex_unlock (&self->mutex);
-    errnum_assert (rc == 0, rc);
-
-    return 0;
-}
-
-#elif defined NN_HAVE_WINDOWS
+#if defined NN_HAVE_WINDOWS
 
 void nn_sem_init (struct nn_sem *self)
 {
@@ -164,6 +98,74 @@
 }
 
 #else
-#error
+
+/*  Simulate semaphores with condition variables. */
+
+void nn_sem_init (struct nn_sem *self)
+{
+    int rc;
+
+    rc = pthread_mutex_init (&self->mutex, NULL);
+    errnum_assert (rc == 0, rc);
+    rc = pthread_cond_init (&self->cond, NULL);
+    errnum_assert (rc == 0, rc);
+    self->signaled = 0;
+}
+
+void nn_sem_term (struct nn_sem *self)
+{
+    int rc;
+
+    rc = pthread_cond_destroy (&self->cond);
+    errnum_assert (rc == 0, rc);
+    rc = pthread_mutex_destroy (&self->mutex);
+    errnum_assert (rc == 0, rc);
+}
+
+void nn_sem_post (struct nn_sem *self)
+{
+    int rc;
+
+    rc = pthread_mutex_lock (&self->mutex);
+    errnum_assert (rc == 0, rc);
+    nn_assert (self->signaled == 0);
+    self->signaled = 1;
+    rc = pthread_cond_signal (&self->cond);
+    errnum_assert (rc == 0, rc);
+    rc = pthread_mutex_unlock (&self->mutex);
+    errnum_assert (rc == 0, rc);
+}
+
+int nn_sem_wait (struct nn_sem *self)
+{
+    int rc;
+
+    /*  With OSX, semaphores are global named objects. They are not useful for
+        our use case. To get a similar object we exploit the implementation
+        detail of pthread_cond_wait() in Darwin kernel: It exits if signal is
+        caught. Note that this behaviour is not mandated by POSIX
+        and may break with future versions of Darwin. */
+    rc = pthread_mutex_lock (&self->mutex);
+    errnum_assert (rc == 0, rc);
+    if (nn_fast (self->signaled)) {
+        rc = pthread_mutex_unlock (&self->mutex);
+        errnum_assert (rc == 0, rc);
+        return 0;
+    }
+    rc = pthread_cond_wait (&self->cond, &self->mutex);
+    errnum_assert (rc == 0, rc);
+    if (nn_slow (!self->signaled)) {
+        rc = pthread_mutex_unlock (&self->mutex);
+        errnum_assert (rc == 0, rc);
+        return -EINTR;
+    }
+    self->signaled = 0;
+    rc = pthread_mutex_unlock (&self->mutex);
+    errnum_assert (rc == 0, rc);
+
+    return 0;
+}
+
+
 #endif
 
diff --git a/src/utils/sem.h b/src/utils/sem.h
index 6cfc021..34c0428 100644
--- a/src/utils/sem.h
+++ b/src/utils/sem.h
@@ -39,17 +39,7 @@
 /*  Waits till sem object becomes unlocked and locks it. */
 int nn_sem_wait (struct nn_sem *self);
 
-#if defined NN_HAVE_OSX
-
-#include <pthread.h>
-
-struct nn_sem {
-    pthread_mutex_t mutex;
-    pthread_cond_t cond;
-    int signaled;
-};
-
-#elif defined NN_HAVE_WINDOWS
+#if defined NN_HAVE_WINDOWS
 
 #include "win.h"
 
@@ -65,6 +55,16 @@
     sem_t sem;
 };
 
+#else /*  Simulate semaphore with condition variable. */
+
+#include <pthread.h>
+
+struct nn_sem {
+    pthread_mutex_t mutex;
+    pthread_cond_t cond;
+    int signaled;
+};
+
 #endif
 
 #endif