Issue #8407: The signal handler writes the signal number as a single byte
instead of a nul byte into the wakeup file descriptor. So it is possible to
wait more than one signal and know which signals were raised.
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 473eda2..68a5d2c 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -262,13 +262,17 @@
.. function:: set_wakeup_fd(fd)
- Set the wakeup fd to *fd*. When a signal is received, a ``'\0'`` byte is
- written to the fd. This can be used by a library to wakeup a poll or select
- call, allowing the signal to be fully processed.
+ Set the wakeup file descriptor to *fd*. When a signal is received, the
+ signal number is written as a single byte into the fd. This can be used by
+ a library to wakeup a poll or select call, allowing the signal to be fully
+ processed.
The old wakeup fd is returned. *fd* must be non-blocking. It is up to the
library to remove any bytes before calling poll or select again.
+ Use for example ``struct.unpack('%uB' % len(data), data)`` to decode the
+ signal numbers list.
+
When threads are enabled, this function can only be called from the main thread;
attempting to call it from other threads will cause a :exc:`ValueError`
exception to be raised.