diff --git a/libc/bionic/semaphore.cpp b/libc/bionic/semaphore.cpp
new file mode 100644
index 0000000..c23eb75
--- /dev/null
+++ b/libc/bionic/semaphore.cpp
@@ -0,0 +1,322 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <semaphore.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/time.h>
+#include <time.h>
+
+#include "private/bionic_atomic_inline.h"
+#include "private/bionic_constants.h"
+#include "private/bionic_futex.h"
+#include "private/bionic_time_conversions.h"
+
+// In this implementation, a semaphore contains a
+// 31-bit signed value and a 1-bit 'shared' flag
+// (for process-sharing purpose).
+//
+// We use the value -1 to indicate contention on the
+// semaphore, 0 or more to indicate uncontended state,
+// any value lower than -2 is invalid at runtime.
+//
+// State diagram:
+//
+// post(1)  ==> 2
+// post(0)  ==> 1
+// post(-1) ==> 1, then wake all waiters
+//
+// wait(2)  ==> 1
+// wait(1)  ==> 0
+// wait(0)  ==> -1 then wait for a wake up + loop
+// wait(-1) ==> -1 then wait for a wake up + loop
+
+// Use the upper 31-bits for the counter, and the lower one
+// for the shared flag.
+#define SEMCOUNT_SHARED_MASK      0x00000001
+#define SEMCOUNT_VALUE_MASK       0xfffffffe
+#define SEMCOUNT_VALUE_SHIFT      1
+
+// Convert a value into the corresponding sem->count bit pattern.
+#define SEMCOUNT_FROM_VALUE(val)    (((val) << SEMCOUNT_VALUE_SHIFT) & SEMCOUNT_VALUE_MASK)
+
+// Convert a sem->count bit pattern into the corresponding signed value.
+static inline int SEMCOUNT_TO_VALUE(uint32_t sval) {
+  return (static_cast<int>(sval) >> SEMCOUNT_VALUE_SHIFT);
+}
+
+// The value +1 as a sem->count bit-pattern.
+#define SEMCOUNT_ONE              SEMCOUNT_FROM_VALUE(1)
+
+// The value -1 as a sem->count bit-pattern.
+#define SEMCOUNT_MINUS_ONE        SEMCOUNT_FROM_VALUE(-1)
+
+#define SEMCOUNT_DECREMENT(sval)    (((sval) - (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
+#define SEMCOUNT_INCREMENT(sval)    (((sval) + (1U << SEMCOUNT_VALUE_SHIFT)) & SEMCOUNT_VALUE_MASK)
+
+// Return the shared bitflag from a semaphore.
+static inline uint32_t SEM_GET_SHARED(sem_t* sem) {
+  return (sem->count & SEMCOUNT_SHARED_MASK);
+}
+
+
+int sem_init(sem_t* sem, int pshared, unsigned int value) {
+  if (sem == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  // Ensure that 'value' can be stored in the semaphore.
+  if (value > SEM_VALUE_MAX) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  sem->count = SEMCOUNT_FROM_VALUE(value);
+  if (pshared != 0) {
+    sem->count |= SEMCOUNT_SHARED_MASK;
+  }
+  return 0;
+}
+
+int sem_destroy(sem_t* sem) {
+  if (sem == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+  return 0;
+}
+
+sem_t* sem_open(const char*, int, ...) {
+  errno = ENOSYS;
+  return SEM_FAILED;
+}
+
+int sem_close(sem_t*) {
+  errno = ENOSYS;
+  return -1;
+}
+
+int sem_unlink(const char*) {
+  errno = ENOSYS;
+  return -1;
+}
+
+// Decrement a semaphore's value atomically,
+// and return the old one. As a special case,
+// this returns immediately if the value is
+// negative (i.e. -1)
+static int __sem_dec(volatile uint32_t* sem) {
+  volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
+  uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
+  uint32_t old_value, new_value;
+  int ret;
+
+  do {
+    old_value = (*sem & SEMCOUNT_VALUE_MASK);
+    ret = SEMCOUNT_TO_VALUE(old_value);
+    if (ret < 0) {
+      break;
+    }
+
+    new_value = SEMCOUNT_DECREMENT(old_value);
+  } while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
+
+  return ret;
+}
+
+// Same as __sem_dec, but will not touch anything if the
+// value is already negative *or* 0. Returns the old value.
+static int __sem_trydec(volatile uint32_t* sem) {
+  volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
+  uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
+  uint32_t old_value, new_value;
+  int          ret;
+
+  do {
+    old_value = (*sem & SEMCOUNT_VALUE_MASK);
+    ret = SEMCOUNT_TO_VALUE(old_value);
+    if (ret <= 0) {
+      break;
+    }
+
+    new_value = SEMCOUNT_DECREMENT(old_value);
+  } while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
+
+  return ret;
+}
+
+
+// "Increment" the value of a semaphore atomically and
+// return its old value. Note that this implements
+// the special case of "incrementing" any negative
+// value to +1 directly.
+//
+// NOTE: The value will _not_ wrap above SEM_VALUE_MAX
+static int __sem_inc(volatile uint32_t* sem) {
+  volatile int32_t* ptr = reinterpret_cast<volatile int32_t*>(sem);
+  uint32_t shared = (*sem & SEMCOUNT_SHARED_MASK);
+  uint32_t old_value, new_value;
+  int ret;
+
+  do {
+    old_value = (*sem & SEMCOUNT_VALUE_MASK);
+    ret = SEMCOUNT_TO_VALUE(old_value);
+
+    // Can't go higher than SEM_VALUE_MAX.
+    if (ret == SEM_VALUE_MAX) {
+      break;
+    }
+
+    // If the counter is negative, go directly to +1, otherwise just increment.
+    if (ret < 0) {
+        new_value = SEMCOUNT_ONE;
+    } else {
+      new_value = SEMCOUNT_INCREMENT(old_value);
+    }
+  } while (__bionic_cmpxchg((old_value|shared), (new_value|shared), ptr) != 0);
+
+  return ret;
+}
+
+int sem_wait(sem_t* sem) {
+  if (sem == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  uint32_t shared = SEM_GET_SHARED(sem);
+
+  while (true) {
+    if (__sem_dec(&sem->count) > 0) {
+      ANDROID_MEMBAR_FULL();
+      return 0;
+    }
+
+    __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, NULL);
+  }
+}
+
+int sem_timedwait(sem_t* sem, const timespec* abs_timeout) {
+  if (sem == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  // POSIX says we need to try to decrement the semaphore
+  // before checking the timeout value. Note that if the
+  // value is currently 0, __sem_trydec() does nothing.
+  if (__sem_trydec(&sem->count) > 0) {
+    ANDROID_MEMBAR_FULL();
+    return 0;
+  }
+
+  // Check it as per POSIX.
+  if (abs_timeout == NULL || abs_timeout->tv_sec < 0 || abs_timeout->tv_nsec < 0 || abs_timeout->tv_nsec >= NS_PER_S) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  uint32_t shared = SEM_GET_SHARED(sem);
+
+  while (true) {
+    // POSIX mandates CLOCK_REALTIME here.
+    timespec ts;
+    if (!timespec_from_absolute_timespec(ts, *abs_timeout, CLOCK_REALTIME)) {
+      errno = ETIMEDOUT;
+      return -1;
+    }
+
+    // Try to grab the semaphore. If the value was 0, this will also change it to -1.
+    if (__sem_dec(&sem->count) > 0) {
+      ANDROID_MEMBAR_FULL();
+      break;
+    }
+
+    // Contention detected. Wait for a wakeup event.
+    int ret = __futex_wait_ex(&sem->count, shared, shared|SEMCOUNT_MINUS_ONE, &ts);
+
+    // Return in case of timeout or interrupt.
+    if (ret == -ETIMEDOUT || ret == -EINTR) {
+      errno = -ret;
+      return -1;
+    }
+  }
+  return 0;
+}
+
+int sem_post(sem_t* sem) {
+  if (sem == NULL) {
+    return EINVAL;
+  }
+
+  uint32_t shared = SEM_GET_SHARED(sem);
+
+  ANDROID_MEMBAR_FULL();
+  int old_value = __sem_inc(&sem->count);
+  if (old_value < 0) {
+    // Contention on the semaphore. Wake up all waiters.
+    __futex_wake_ex(&sem->count, shared, INT_MAX);
+  } else if (old_value == SEM_VALUE_MAX) {
+    // Overflow detected.
+    errno = EOVERFLOW;
+    return -1;
+  }
+
+  return 0;
+}
+
+int sem_trywait(sem_t* sem) {
+  if (sem == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  if (__sem_trydec(&sem->count) > 0) {
+    ANDROID_MEMBAR_FULL();
+    return 0;
+  } else {
+    errno = EAGAIN;
+    return -1;
+  }
+}
+
+int sem_getvalue(sem_t* sem, int* sval) {
+  if (sem == NULL || sval == NULL) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  int val = SEMCOUNT_TO_VALUE(sem->count);
+  if (val < 0) {
+    val = 0;
+  }
+
+  *sval = val;
+  return 0;
+}