1 files changed, 50 insertions, 0 deletions
diff --git a/doc/HACKING/design/01e-os-compat.md b/doc/HACKING/design/01e-os-compat.md
new file mode 100644
index 0000000000..072e95bc8a
--- /dev/null
+++ b/doc/HACKING/design/01e-os-compat.md
@@ -0,0 +1,50 @@
+
+## OS compatibility functions ##
+
+We've got a bunch of functions to wrap differences between various
+operating systems where we run.
+
+### The filesystem ###
+
+We wrap the most important filesystem functions with load-file,
+save-file, and map-file abstractions declared in util.c or compat.c.  If
+you're messing about with file descriptors yourself, you might be doing
+things wrong.  Most of the time, write_str_to_file() and
+read_str_from_file() are all you need.
+
+Use the check_private_directory() function to create or verify the
+presence of directories, and tor_listdir() to list the files in a
+directory.
+
+Those modules also have functions for manipulating paths a bit.
+
+### Networking ###
+
+Nearly all the world is on a Berkeley sockets API, except for
+windows, whose version of the Berkeley API was corrupted by late-90s
+insistence on backward compatibility with the
+sort-of-berkeley-sort-of-not add-on *thing* that was WinSocks.
+
+What's more, everybody who implemented sockets realized that select()
+wasn't a very good way to do nonblocking IO... and then the various
+implementations all decided to so something different.
+
+You can forget about most of these differences, fortunately: We use
+libevent to hide most of the differences between the various networking
+backends, and we add a few of our own functions to hide the differences
+that Libevent doesn't.
+
+To create a network connection, the right level of abstraction to look
+at is probably the connection_t system in connection.c.  Most of the
+lower level work has already been done for you.  If you need to
+instantiate something that doesn't fit well with connection_t, you
+should see whether you can instantiate it with connection_t anyway -- or
+you might need to refactor connection.c a little.
+
+Whenever possible, represent network addresses as tor_addr_t.
+
+### Process launch and monitoring ###
+
+Launching and/or monitoring a process is tricky business. You can use
+the mechanisms in procmon.c and tor_spawn_background(), but they're both
+a bit wonky.  A refactoring would not be out of order.