add BUS_SEMAPHORES_ARE_SYNCHRONOUS + explain some syntax in doc/protocol

Signed-off-by: Mattias Andrée <maandree@operamail.com>

2 files changed, 28 insertions, 4 deletions

diff --git a/doc/protocol b/doc/protocol
index def8f6f..a846581 100644
--- a/doc/protocol
+++ b/doc/protocol
@@ -14,10 +14,14 @@ broadcast:
 	with P(X):
 	  Z(W)
 	  Write NUL-terminate message to shared memory
-	  with V(N):
+	  with V(N): -- (1)
 	    Q := 0
 	    Z(S)
 
+	-- (1) may be omitted if semaphores are known that
+	   P(·), Z(·), V(·) cannot create a race condition
+	   with a processes running Z(·).
+
 
 listen:
 	with V(S):
@@ -29,6 +33,17 @@ listen:
 	      break
 	    with V(W):
 	      with P(S):
-	         Z(S)
-	         Z(N)
+	        Z(S)
+	        Z(N)
+
+
+`V(a)` means that semaphore a is released.
+`P(a)` means that semaphore a is acquired.
+`Z(a)` means that the process waits for semaphore a to become 0.
+`with P(a)` that `P(a)` is done before the entering the scope,
+and `V(a)` is done when exiting the scope. It also means that
+these actions [P(a) and V(a)] are undone when the process exits,
+or if the call fails.
+`with V(a)` that `V(a)` is like `with P(a)` that `P(a)` and
+`V(a)` are exchanged.
 
diff --git a/src/bus.c b/src/bus.c
index 614f7ad..5737fd7 100644
--- a/src/bus.c
+++ b/src/bus.c
@@ -623,22 +623,31 @@ fail:
 int
 bus_write(const bus_t *bus, const char *message, int flags /* TODO document in man page */)
 {
-	int state = 0, saved_errno;
+	int saved_errno;
+#ifndef BUS_SEMAPHORES_ARE_SYNCHRONOUS
+	int state = 0;
+#endif
 	if (acquire_semaphore(bus, X, SEM_UNDO | F(BUS_NOWAIT, IPC_NOWAIT)) == -1)
 		return -1;
 	t(zero_semaphore(bus, W, 0));
 	write_shared_memory(bus, message);
+#ifndef BUS_SEMAPHORES_ARE_SYNCHRONOUS
 	t(release_semaphore(bus, N, SEM_UNDO));  state++;
+#endif
 	t(write_semaphore(bus, Q, 0));
 	t(zero_semaphore(bus, S, 0));
+#ifndef BUS_SEMAPHORES_ARE_SYNCHRONOUS
 	t(acquire_semaphore(bus, N, SEM_UNDO));  state--;
+#endif
 	t(release_semaphore(bus, X, SEM_UNDO));
 	return 0;
 
 fail:
 	saved_errno = errno;
+#ifndef BUS_SEMAPHORES_ARE_SYNCHRONOUS
 	if (state > 0)
 		acquire_semaphore(bus, N, SEM_UNDO);
+#endif
 	release_semaphore(bus, X, SEM_UNDO);
 	errno = saved_errno;
 	return -1;