path: root/src/thread
diff options
authorRich Felker <>2012-07-24 00:51:36 -0400
committerRich Felker <>2012-07-24 00:51:36 -0400
commit07e62953c768f9f09485388c22ffaed98d11d676 (patch)
tree5590ab7130d72e33cd5ef3be1659ea18367121e8 /src/thread
parent5d26d5d15bf6c2fc8c0e58b76fb418b51e207af7 (diff)
retry on cas failures in sem_trywait
this seems counter-intuitive since sem_trywait is supposed to just try once, not wait for the semaphore. however, the retry loop is not a wait. instead, it's to handle the case where the value changes due to a simultaneous post or wait from another thread while the semaphore value remains positive. in such a case, it's absolutely wrong for sem_trywait to fail with EAGAIN because the semaphore is not busy.
Diffstat (limited to 'src/thread')
1 files changed, 2 insertions, 2 deletions
diff --git a/src/thread/sem_trywait.c b/src/thread/sem_trywait.c
index 55d90075..04edf46b 100644
--- a/src/thread/sem_trywait.c
+++ b/src/thread/sem_trywait.c
@@ -3,8 +3,8 @@
int sem_trywait(sem_t *sem)
- int val = sem->__val[0];
- if (val>0) {
+ 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;