From 484a1b61049ca9927979216503d4a08abf9600b3 Mon Sep 17 00:00:00 2001
From: Freddie Chopin <freddie_chopin@op.pl>
Date: Wed, 9 Nov 2016 07:01:49 -0600
Subject: [PATCH] sem_wait() and sem_trywait() no longer modify the errno value
 UNLESS an error occurs.  This allows these functions to be used internallly
 without clobbering the errno value.

---
 sched/semaphore/sem_trywait.c | 24 ++++++++++++------------
 sched/semaphore/sem_wait.c    |  4 ++--
 2 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/sched/semaphore/sem_trywait.c b/sched/semaphore/sem_trywait.c
index 37ba0d66e2..c55116a2b7 100644
--- a/sched/semaphore/sem_trywait.c
+++ b/sched/semaphore/sem_trywait.c
@@ -84,13 +84,9 @@ int sem_trywait(FAR sem_t *sem)
 
   /* This API should not be called from interrupt handlers */
 
-  DEBUGASSERT(up_interrupt_context() == false);
+  DEBUGASSERT(sem != NULL && up_interrupt_context() == false);
 
-  /* Assume any errors reported are due to invalid arguments. */
-
-  set_errno(EINVAL);
-
-  if (sem)
+  if (sem != NULL)
     {
       /* The following operations must be performed with interrupts disabled
        * because sem_post() may be called from an interrupt handler.
@@ -98,12 +94,6 @@ int sem_trywait(FAR sem_t *sem)
 
       flags = enter_critical_section();
 
-      /* Any further errors could only occurr because the semaphore is not
-       * available.
-       */
-
-      set_errno(EAGAIN);
-
       /* If the semaphore is available, give it to the requesting task */
 
       if (sem->semcount > 0)
@@ -114,11 +104,21 @@ int sem_trywait(FAR sem_t *sem)
           rtcb->waitsem = NULL;
           ret = OK;
         }
+      else
+        {
+          /* Semaphore is not available */
+
+          set_errno(EAGAIN);
+        }
 
       /* Interrupts may now be enabled. */
 
       leave_critical_section(flags);
     }
+  else
+    {
+      set_errno(EINVAL);
+    }
 
   return ret;
 }
diff --git a/sched/semaphore/sem_wait.c b/sched/semaphore/sem_wait.c
index 9c19d406d7..da855bc70c 100644
--- a/sched/semaphore/sem_wait.c
+++ b/sched/semaphore/sem_wait.c
@@ -84,11 +84,11 @@ int sem_wait(FAR sem_t *sem)
 
   /* This API should not be called from interrupt handlers */
 
-  DEBUGASSERT(up_interrupt_context() == false);
+  DEBUGASSERT(sem != NULL && up_interrupt_context() == false);
 
   /* Make sure we were supplied with a valid semaphore. */
 
-  if (sem)
+  if (sem != NULL)
     {
       /* The following operations must be performed with interrupts
        * disabled because sem_post() may be called from an interrupt