Coding conventions for Tor
==========================

tl;dr:

   - Run configure with `--enable-fatal-warnings`
   - Document your functions
   - Write unit tests
   - Run `make check` before submitting a patch
   - Run `make distcheck` if you have made changes to build system components
   - 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-fatal-warnings`?
   - To run `make check-docs` to see whether all new options are on
     the manpage?
   - To write unit tests, as possible?
   - To run `make test-full` to test against all unit and integration tests (or
     `make test-full-online` if you have a working connection to the internet)?
   - To test that the distribution will actually work via `make distcheck`?
   - To base your code on the appropriate branch?
   - To include a file in the `changes` directory as appropriate?

If you are submitting a major patch or new feature, or want to in the future...

   - Set up Chutney and Stem, see HACKING/WritingTests.md
   - Run `make test-full` to test against all unit and integration tests.

If you have changed build system components:
   - Please run `make distcheck`
   - For example, if you have changed Makefiles, autoconf files, or anything
     else that affects the build system.

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. If you're
working on a new feature and it will take a while to implement and/or you'd
like to avoid the possibility of unrelated bugs in Tor while you're
implementing your feature, consider branching off of the latest maint- branch.
_Never_ branch off a relase- branch. Don't branch off a tag either: they come
from release branches. Doing so will likely produce a nightmare of merge
conflicts in the ChangeLog when it comes time to merge your branch into Tor.
Best advice: don't try to keep an independent branch forked for more than 6
months and expect it to merge cleanly. Try to merge pieces early and often.


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

- 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`.  This is run automatically as part of
`make check` -- if it fails, we must fix it before we release.  These
checks are implemented in `scripts/maint/lintChanges.py`.

Changes file style guide:
  * Changes files begin with "  o Header (subheading):".  The header
    should usually be "Minor/Major bugfixes/features". The subheading is a
    particular area within Tor.  See the ChangeLog for examples.

  * Make everything terse.

  * Write from the user's point of view: describe the user-visible changes
    right away.

  * Mention configuration options by name.  If they're rare or unusual,
    remind people what they're for.

  * Describe changes in the present tense and in the imperative: not past.

  * Every bugfix should have a sentence of the form "Fixes bug 1234; bugfix
    on 0.1.2.3-alpha", describing what bug was fixed and where it came from.

  * "Relays", not "servers", "nodes", or "Tor relays".

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.

What does not need a changes file?

   * Bugfixes for code that hasn't shipped in any released version of Tor

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-fatal-warnings`.  This will tell the compiler
to make all warnings into errors.

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.

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.

Also see a longer list of functions to avoid in:
https://people.torproject.org/~nickm/tor-auto/internal/this-not-that.html

Floating point math is hard
---------------------------

Floating point arithmetic as typically implemented by computers is
very counterintuitive.  Failure to adequately analyze floating point
usage can result in surprising behavior and even security
vulnerabilities!

General advice:

   - Don't use floating point.
   - If you must use floating point, document how the limits of
     floating point precision and calculation accuracy affect function
     outputs.
   - Try to do as much as possible of your calculations using integers
     (possibly acting as fixed-point numbers) and convert to floating
     point for display.
   - If you must send floating point numbers on the wire, serialize
     them in a platform-independent way.  Tor avoids exchanging
     floating-point values, but when it does, it uses ASCII numerals,
     with a decimal point (".").
   - Binary fractions behave very differently from decimal fractions.
     Make sure you understand how these differences affect your
     calculations.
   - Every floating point arithmetic operation is an opportunity to
     lose precision, overflow, underflow, or otherwise produce
     undesired results.  Addition and subtraction tend to be worse
     than multiplication and division (due to things like catastrophic
     cancellation).  Try to arrange your calculations to minimize such
     effects.
   - Changing the order of operations changes the results of many
     floating-point calculations.  Be careful when you simplify
     calculations!  If the order is significant, document it using a
     code comment.
   - Comparing most floating point values for equality is unreliable.
     Avoid using `==`, instead, use `>=` or `<=`.  If you use an
     epsilon value, make sure it's appropriate for the ranges in
     question.
   - Different environments (including compiler flags and per-thread
     state on a single platform!) can get different results from the
     same floating point calculations.  This means you can't use
     floats in anything that needs to be deterministic, like consensus
     generation.  This also makes reliable unit tests of
     floating-point outputs hard to write.

For additional useful advice (and a little bit of background), see
[What Every Programmer Should Know About Floating-Point
Arithmetic](http://floating-point-gui.de/).

A list of notable (and surprising) facts about floating point
arithmetic is at [Floating-point
complexities](https://randomascii.wordpress.com/2012/04/05/floating-point-complexities/).
Most of that [series of posts on floating
point](https://randomascii.wordpress.com/category/floating-point/) is
helpful.

For more detailed (and math-intensive) background, see [What Every
Computer Scientist Should Know About Floating-Point
Arithmetic](https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html).

Other C conventions
-------------------

The `a ? b : c` trinary operator only goes inside other expressions;
don't use it as a replacement for if. (You can ignore this inside macro
definitions when necessary.)

Assignment operators shouldn't nest inside other expressions.  (You can
ignore this inside macro definitions when necessary.)

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).

Assertions In Tor
-----------------

Assertions should be used for bug-detection only.  Don't use assertions to
detect bad user inputs, network errors, resource exhaustion, or similar
issues.

Tor is always built with assertions enabled, so try to only use
`tor_assert()` for cases where you are absolutely sure that crashing is the
least bad option.  Many bugs have been caused by use of `tor_assert()` when
another kind of check would have been safer.

If you're writing an assertion to test for a bug that you _can_ recover from,
use `tor_assert_nonfatal()` in place of `tor_assert()`.  If you'd like to
write a conditional that incorporates a nonfatal assertion, use the `BUG()`
macro, as in:

	if (BUG(ptr == NULL))
		return -1;

Allocator conventions
---------------------

By convention, any tor type with a name like `abc_t` should be allocated
by a function named `abc_new()`.  This function should never return
NULL.

Also, a type named `abc_t` should be freed by a function named `abc_free_()`.
Don't call this `abc_free_()` function directly -- instead, wrap it in a
macro called `abc_free()`, using the `FREE_AND_NULL` macro:

    void abc_free_(abc_t *obj);
    #define abc_free(obj) FREE_AND_NULL(abc_t, abc_free_, (obj))

This macro will free the underlying `abc_t` object, and will also set
the object pointer to NULL.

You should define all `abc_free_()` functions to accept NULL inputs:

    void
    abc_free_(abc_t *obj)
    {
      if (!obj)
        return;
      tor_free(obj->name);
      thing_free(obj->thing);
      tor_free(obj);
    }

If you need a free function that takes a `void *` argument (for example,
to use it as a function callback), define it with a name like
`abc_free_void()`:

    static void
    abc_free_void_(void *obj)
    {
      abc_free_(obj);
    }


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.
