+#if defined(LISP_FEATURE_SB_THREAD) && !defined(LISP_FEATURE_SB_LUTEX) && !defined(LISP_FEATURE_SB_PTHREAD_FUTEX)
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <errno.h>
+
+/* values taken from the kernel's linux/futex.h. This header file
+ doesn't exist in userspace, which is our excuse for not grovelling
+ them automatically */
+#define FUTEX_WAIT 0
+#define FUTEX_WAKE 1
+/* This is also copied from linux/futex.h so that a binary compiled on
+ * a not so recent Linux system can still take advantage of private
+ * futexes when available.*/
+#define FUTEX_WAIT_PRIVATE (0+128)
+#define FUTEX_WAKE_PRIVATE (1+128)
+#define FUTEX_FD (2)
+#define FUTEX_REQUEUE (3)
+
+/* Not static so that Lisp may query it. */
+boolean futex_private_supported_p;
+
+static inline int
+futex_wait_op()
+{
+ return (futex_private_supported_p ? FUTEX_WAIT_PRIVATE : FUTEX_WAIT);
+}
+
+static inline int
+futex_wake_op()
+{
+ return (futex_private_supported_p ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE);
+}
+
+#define sys_futex sbcl_sys_futex
+static inline int sys_futex (void *futex, int op, int val, struct timespec *rel)
+{
+ return syscall (SYS_futex, futex, op, val, rel);
+}
+
+static void
+futex_init()
+{
+ int x = 0;
+ sys_futex(&x, FUTEX_WAIT, 1, 0);
+ if (errno == ENOSYS)
+ lose("This version of SBCL is compiled with threading support, but your kernel\n"
+ "is too old to support this. Please use a more recent kernel or\n"
+ "a version of SBCL without threading support.\n");
+ sys_futex(&x, FUTEX_WAIT_PRIVATE, 1, 0);
+ if (errno == EWOULDBLOCK) {
+ futex_private_supported_p = 1;
+ } else {
+ futex_private_supported_p = 0;
+ SHOW("No futex private suppport\n");
+ }
+}
+
+int
+futex_wait(int *lock_word, int oldval, long sec, unsigned long usec)
+{
+ struct timespec timeout;
+ int t;
+
+ if (sec<0) {
+ t = sys_futex(lock_word, futex_wait_op(), oldval, 0);
+ }
+ else {
+ timeout.tv_sec = sec;
+ timeout.tv_nsec = usec * 1000;
+ t = sys_futex(lock_word, futex_wait_op(), oldval, &timeout);
+ }
+ if (t==0)
+ return 0;
+ else if (errno==ETIMEDOUT)
+ return 1;
+ else if (errno==EINTR)
+ return 2;
+ else
+ /* EWOULDBLOCK and others, need to check the lock */
+ return -1;
+}
+
+int
+futex_wake(int *lock_word, int n)
+{
+ return sys_futex(lock_word, futex_wake_op(),n,0);
+}
+#endif
+