Coding conventions for Tor
--------------------------

tl;dr:

    * Run configure with '--enable-gcc-warnings'
    * Run 'make check-spaces' to catch whitespace errors
    * Document your functions
    * Write unit tests
    * Add a file in 'changes' for your branch.

Patch checklist
~~~~~~~~~~~~~~~

If possible, send your patch as one of these (in descending order of
preference)

   - A git branch we can pull from
   - Patches generated by git format-patch
   - A unified diff

Did you remember...

   - To build your code while configured with --enable-gcc-warnings?
   - To run "make check-spaces" on your code?
   - To run "make check-docs" to see whether all new options are on
     the manpage?
   - To write unit tests, as possible?
   - To base your code on the appropriate branch?
   - To include a file in the "changes" directory as appropriate?

How we use Git branches
-----------------------

Each main development series (like 0.2.1, 0.2.2, etc) has its main work
applied to a single branch.  At most one series can be the development series
at a time; all other series are maintenance series that get bug-fixes only.
The development series is built in a git branch called "master"; the
maintenance series are built in branches called "maint-0.2.0", "maint-0.2.1",
and so on.  We regularly merge the active maint branches forward.

For all series except the development series, we also have a "release" branch
(as in "release-0.2.1").  The release series is based on the corresponding
maintenance series, except that it deliberately lags the maint series for
most of its patches, so that bugfix patches are not typically included in a
maintenance release until they've been tested for a while in a development
release.  Occasionally, we'll merge an urgent bugfix into the release branch
before it gets merged into maint, but that's rare.

If you're working on a bugfix for a bug that occurs in a particular version,
base your bugfix branch on the "maint" branch for the first supported series
that has that bug.  (As of June 2013, we're supporting 0.2.3 and later.) If
you're working on a new feature, base it on the master branch.


How we log changes
------------------

When you do a commit that needs a ChangeLog entry, add a new file to
the "changes" toplevel subdirectory.  It should have the format of a
one-entry changelog section from the current ChangeLog file, as in

  o Major bugfixes:
    - Fix a potential buffer overflow. Fixes bug 99999; bugfix on
      0.3.1.4-beta.

To write a changes file, first categorize the change.  Some common categories
are: Minor bugfixes, Major bugfixes, Minor features, Major features, Code
simplifications and refactoring.  Then say what the change does.  If
it's a bugfix, mention what bug it fixes and when the bug was
introduced.  To find out which Git tag the change was introduced in,
you can use "git describe --contains <sha1 of commit>".

If at all possible, try to create this file in the same commit where you are
making the change.  Please give it a distinctive name that no other branch will
use for the lifetime of your change. To verify the format of the changes file,
you can use "make check-changes".

When we go to make a release, we will concatenate all the entries
in changes to make a draft changelog, and clear the directory. We'll
then edit the draft changelog into a nice readable format.

What needs a changes file?::
   A not-exhaustive list: Anything that might change user-visible
   behavior. Anything that changes internals, documentation, or the build
   system enough that somebody could notice.  Big or interesting code
   rewrites.  Anything about which somebody might plausibly wonder "when
   did that happen, and/or why did we do that" 6 months down the line.

Why use changes files instead of Git commit messages?::
   Git commit messages are written for developers, not users, and they
   are nigh-impossible to revise after the fact.

Why use changes files instead of entries in the ChangeLog?::
   Having every single commit touch the ChangeLog file tended to create
   zillions of merge conflicts.

Whitespace and C conformance
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Invoke "make check-spaces" from time to time, so it can tell you about
deviations from our C whitespace style.  Generally, we use:

    - Unix-style line endings
    - K&R-style indentation
    - No space before newlines
    - A blank line at the end of each file
    - Never more than one blank line in a row
    - Always spaces, never tabs
    - No more than 79-columns per line.
    - Two spaces per indent.
    - A space between control keywords and their corresponding paren
      "if (x)", "while (x)", and "switch (x)", never "if(x)", "while(x)", or
      "switch(x)".
    - A space between anything and an open brace.
    - No space between a function name and an opening paren. "puts(x)", not
      "puts (x)".
    - Function declarations at the start of the line.

We try hard to build without warnings everywhere.  In particular, if you're
using gcc, you should invoke the configure script with the option
"--enable-gcc-warnings".  This will give a bunch of extra warning flags to
the compiler, and help us find divergences from our preferred C style.

Functions to use; functions not to use
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We have some wrapper functions like tor_malloc, tor_free, tor_strdup, and
tor_gettimeofday; use them instead of their generic equivalents.  (They
always succeed or exit.)

You can get a full list of the compatibility functions that Tor provides by
looking through src/common/util*.h and src/common/compat*.h.  You can see the
available containers in src/common/containers*.h.  You should probably
familiarize yourself with these modules before you write too much code, or
else you'll wind up reinventing the wheel.

Use 'INLINE' instead of 'inline' -- it's a vestige of an old hack to make
sure that we worked on MSVC6.

We don't use strcat or strcpy or sprintf of any of those notoriously broken
old C functions.  Use strlcat, strlcpy, or tor_snprintf/tor_asprintf instead.

We don't call memcmp() directly.  Use fast_memeq(), fast_memneq(),
tor_memeq(), or tor_memneq() for most purposes.

Functions not to write
~~~~~~~~~~~~~~~~~~~~~~

Try to never hand-write new code to parse or generate binary
formats. Instead, use trunnel if at all possible.  See
    https://gitweb.torproject.org/trunnel.git/tree
for more information about trunnel.

For information on adding new trunnel code to Tor, see src/trunnel/README


Calling and naming conventions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Whenever possible, functions should return -1 on error and 0 on success.

For multi-word identifiers, use lowercase words combined with
underscores. (e.g., "multi_word_identifier").  Use ALL_CAPS for macros and
constants.

Typenames should end with "_t".

Function names should be prefixed with a module name or object name.  (In
general, code to manipulate an object should be a module with the same name
as the object, so it's hard to tell which convention is used.)

Functions that do things should have imperative-verb names
(e.g. buffer_clear, buffer_resize); functions that return booleans should
have predicate names (e.g. buffer_is_empty, buffer_needs_resizing).

If you find that you have four or more possible return code values, it's
probably time to create an enum.  If you find that you are passing three or
more flags to a function, it's probably time to create a flags argument that
takes a bitfield.

What To Optimize
~~~~~~~~~~~~~~~~

Don't optimize anything if it's not in the critical path.  Right now, the
critical path seems to be AES, logging, and the network itself.  Feel free to
do your own profiling to determine otherwise.

Log conventions
~~~~~~~~~~~~~~~

https://www.torproject.org/docs/faq#LogLevel

No error or warning messages should be expected during normal OR or OP
operation.

If a library function is currently called such that failure always means ERR,
then the library function should log WARN and let the caller log ERR.

Every message of severity INFO or higher should either (A) be intelligible
to end-users who don't know the Tor source; or (B) somehow inform the
end-users that they aren't expected to understand the message (perhaps
with a string like "internal error"). Option (A) is to be preferred to
option (B).



Doxygen comment conventions
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Say what functions do as a series of one or more imperative sentences, as
though you were telling somebody how to be the function.  In other words, DO
NOT say:

     /** The strtol function parses a number.
      *
      * nptr -- the string to parse.  It can include whitespace.
      * endptr -- a string pointer to hold the first thing that is not part
      *    of the number, if present.
      * base -- the numeric base.
      * returns: the resulting number.
      */
     long strtol(const char *nptr, char **nptr, int base);

Instead, please DO say:

     /** Parse a number in radix <b>base</b> from the string <b>nptr</b>,
      * and return the result.  Skip all leading whitespace.  If
      * <b>endptr</b> is not NULL, set *<b>endptr</b> to the first character
      * after the number parsed.
      **/
     long strtol(const char *nptr, char **nptr, int base);

Doxygen comments are the contract in our abstraction-by-contract world: if
the functions that call your function rely on it doing something, then your
function should mention that it does that something in the documentation.  If
you rely on a function doing something beyond what is in its documentation,
then you should watch out, or it might do something else later.
