diff --git a/include/limits.h b/include/limits.h
index 53a27b9d..c4d3f42b 100644
--- a/include/limits.h
+++ b/include/limits.h
@@ -66,7 +66,7 @@
 #define PTHREAD_KEYS_MAX 128
 #define PTHREAD_STACK_MIN 2048
 #define PTHREAD_DESTRUCTOR_ITERATIONS 4
-#define SEM_VALUE_MAX 0x7fffffff
+#define SEM_VALUE_MAX 0x3fffffff
 #define SEM_NSEMS_MAX 256
 #define DELAYTIMER_MAX 0x7fffffff
 #define MQ_PRIO_MAX 32768
diff --git a/src/thread/sem_getvalue.c b/src/thread/sem_getvalue.c
index d9d83071..c0b7762d 100644
--- a/src/thread/sem_getvalue.c
+++ b/src/thread/sem_getvalue.c
@@ -1,8 +1,9 @@
 #include <semaphore.h>
+#include <limits.h>
 
 int sem_getvalue(sem_t *restrict sem, int *restrict valp)
 {
 	int val = sem->__val[0];
-	*valp = val < 0 ? 0 : val;
+	*valp = val & SEM_VALUE_MAX;
 	return 0;
 }
diff --git a/src/thread/sem_post.c b/src/thread/sem_post.c
index 31e3293d..5051ec97 100644
--- a/src/thread/sem_post.c
+++ b/src/thread/sem_post.c
@@ -1,17 +1,38 @@
 #include <semaphore.h>
+#include <limits.h>
 #include "pthread_impl.h"
 
 int sem_post(sem_t *sem)
 {
-	int val, waiters, priv = sem->__val[2];
-	do {
+	int val, new, priv = sem->__val[2], sync = 0;
+	for (;;) {
+		int cnt;
 		val = sem->__val[0];
-		waiters = sem->__val[1];
-		if (val == SEM_VALUE_MAX) {
-			errno = EOVERFLOW;
-			return -1;
+		cnt = val & SEM_VALUE_MAX;
+		if (cnt < SEM_VALUE_MAX) {
+			if (!sync && val < 0 && !(val & 0x40000000) && !sem->__val[1]) {
+				new = cnt | 0x40000000;
+				if (a_cas(sem->__val, val, new) != val)
+					continue;
+				val = new;
+				sync = 1;
+			}
+			new = val + 1;
+		} else {
+			new = val;
 		}
-	} while (a_cas(sem->__val, val, val+1+(val<0)) != val);
-	if (val<0 || waiters) __wake(sem->__val, 1, priv);
+		if (sync) {
+			if (sem->__val[1])
+				new |= 0x80000000;
+			new &= ~0x40000000;
+		}
+		if (a_cas(sem->__val, val, new) == val)
+			break;
+	}
+	if ((val & SEM_VALUE_MAX) == SEM_VALUE_MAX) {
+		errno = EOVERFLOW;
+		return -1;
+	}
+	if (new < 0 || (new & 0x40000000)) __wake(sem->__val, 1, priv);
 	return 0;
 }
diff --git a/src/thread/sem_timedwait.c b/src/thread/sem_timedwait.c
index 58d3ebfe..51f6a474 100644
--- a/src/thread/sem_timedwait.c
+++ b/src/thread/sem_timedwait.c
@@ -1,4 +1,5 @@
 #include <semaphore.h>
+#include <limits.h>
 #include "pthread_impl.h"
 
 static void cleanup(void *p)
@@ -13,14 +14,18 @@ int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
 	if (!sem_trywait(sem)) return 0;
 
 	int spins = 100;
-	while (spins-- && sem->__val[0] <= 0 && !sem->__val[1]) a_spin();
+	while (spins-- && !(sem->__val[0] & SEM_VALUE_MAX) && !sem->__val[1])
+		a_spin();
 
 	while (sem_trywait(sem)) {
-		int r;
+		int r, t, val = sem->__val[0];
+		if (val & SEM_VALUE_MAX)
+			continue;
 		a_inc(sem->__val+1);
-		a_cas(sem->__val, 0, -1);
+		t = val | 0x80000000;
+		a_cas(sem->__val, val, t);
 		pthread_cleanup_push(cleanup, (void *)(sem->__val+1));
-		r = __timedwait_cp(sem->__val, -1, CLOCK_REALTIME, at, sem->__val[2]);
+		r = __timedwait_cp(sem->__val, t, CLOCK_REALTIME, at, sem->__val[2]);
 		pthread_cleanup_pop(1);
 		if (r) {
 			errno = r;
diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index 04edf46b..beb435da 100644
--- a/src/thread/sem_trywait.c
+++ b/src/thread/sem_trywait.c
@@ -1,12 +1,12 @@
 #include <semaphore.h>
+#include <limits.h>
 #include "pthread_impl.h"
 
 int sem_trywait(sem_t *sem)
 {
 	int val;
-	while ((val=sem->__val[0]) > 0) {
-		int new = val-1-(val==1 && sem->__val[1]);
-		if (a_cas(sem->__val, val, new)==val) return 0;
+	while ((val=sem->__val[0]) & SEM_VALUE_MAX) {
+		if (a_cas(sem->__val, val, val-1)==val) return 0;
 	}
 	errno = EAGAIN;
 	return -1;