tor/doc/HACKING

0. Useful tools.

0.0 The buildbot.

  http://tor-buildbot.freehaven.net:8010/

  - Down because nickm isn't running services at home any more. ioerror says
    he will resurrect it.

0.1. Useful command-lines that are non-trivial to reproduce but can
help with tracking bugs or leaks.

dmalloc -l ~/dmalloc.log
(run the commands it tells you)
./configure --with-dmalloc

valgrind --leak-check=yes --error-limit=no --show-reachable=yes src/or/tor

0.2. Running gcov for unit test coverage

  make clean
  make CFLAGS='-g -fprofile-arcs -ftest-coverage'
  ./src/or/test
  cd src/common; gcov *.[ch]
  cd ../or; gcov *.[ch]

  Then, look at the .gcov files.  '-' before a line means that the
  compiler generated  no code for that line.  '######' means that the
  line was never reached.  Lines with numbers were called that number
  of times.

1. Coding conventions

1.0. 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.

1.0.1. Getting emacs to edit Tor source properly.

  Hi, folks!  Nick here.  I like to put the following snippet in my .emacs
  file:
    (add-hook 'c-mode-hook
          (lambda ()
            (font-lock-mode 1)
            (set-variable 'show-trailing-whitespace t)

            (let ((fname (expand-file-name (buffer-file-name))))
              (cond
               ((string-match "^/home/nickm/src/libevent" fname)
                (set-variable 'indent-tabs-mode t)
                (set-variable 'c-basic-offset 4)
                (set-variable 'tab-width 4))
               ((string-match "^/home/nickm/src/tor" fname)
                (set-variable 'indent-tabs-mode nil)
                (set-variable 'c-basic-offset 2))
               ((string-match "^/home/nickm/src/openssl" fname)
                (set-variable 'indent-tabs-mode t)
                (set-variable 'c-basic-offset 8)
                (set-variable 'tab-width 8))
            ))))

  You'll note that it defaults to showing all trailing whitespace.  The
  "cond" test detects whether the file is one of a few C free software
  projects that I often edit, and sets up the indentation level and tab
  preferences to match what they want.

  If you want to try this out, you'll need to change the filename regex
  patterns to match where you keep your Tor files.

  If you *only* use emacs to edit Tor, you could always just say:

    (add-hook 'c-mode-hook
          (lambda ()
            (font-lock-mode 1)
            (set-variable 'show-trailing-whitespace t)
            (set-variable 'indent-tabs-mode nil)
            (set-variable 'c-basic-offset 2)))

  There is probably a better way to do this.  No, we are probably not going
  to clutter the files with emacs stuff.

1.1. Details

  Use tor_malloc, tor_free, tor_strdup, and tor_gettimeofday 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', so that we work properly on Windows.

1.2. 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.

1.3. 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.

1.4. Log conventions

  http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#LogLevels

  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.

  [XXX Proposed convention: 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). -NM]

1.5. Doxygen

  We use the 'doxygen' utility to generate documentation from our
  source code. Here's how to use it:

  1. Begin every file that should be documented with
         /**
          * \file filename.c
          * \brief Short description of the file.
          **/

     (Doxygen will recognize any comment beginning with /** as special.)

  2. Before any function, structure, #define, or variable you want to
     document, add a comment of the form:

        /** Describe the function's actions in imperative sentences.
         *
         * Use blank lines for paragraph breaks
         *   - and
         *   - hyphens
         *   - for
         *   - lists.
         *
         * Write <b>argument_names</b> in boldface.
         *
         * \code
         *     place_example_code();
         *     between_code_and_endcode_commands();
         * \endcode
         */

  3. Make sure to escape the characters "<", ">", "\", "%" and "#" as "\<",
     "\>", "\\", "\%", and "\#".

  4. To document structure members, you can use two forms:

       struct foo {
         /** You can put the comment before an element; */
         int a;
         int b; /**< Or use the less-than symbol to put the comment
                 * after the element. */
       };

  5. To generate documentation from the Tor source code, type:

     $ doxygen -g

     To generate a file called 'Doxyfile'.  Edit that file and run
     'doxygen' to generate the API documentation.

  6. See the Doxygen manual for more information; this summary just
     scratches the surface.

1.5.1. 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.

2. Code notes

2.1. Dataflows

2.1.1. How Incoming data is handled

There are two paths for data arriving at Tor over the network: regular
TCP data, and DNS.

2.1.1.1. TCP.

When Tor takes information over the network, it uses the functions
read_to_buf() and read_to_buf_tls() in buffers.c.  These read from a
socket or an SSL* into a buffer_t, which is an mbuf-style linkedlist
of memory chunks.

read_to_buf() and read_to_buf_tls() are called only from
connection_read_to_buf() in connection.c.  It takes a connection_t
pointer, and reads data into it over the network, up to the
connection's current bandwidth limits.  It places that data into the
"inbuf" field of the connection, and then:
  - Adjusts the connection's want-to-read/want-to-write status as
    appropriate.
  - Increments the read and written counts for the connection as
    appropriate.
  - Adjusts bandwidth buckets as appropriate.

connection_read_to_buf() is called only from connection_handle_read().
The connection_handle_read() function is called whenever libevent
decides (based on select, poll, epoll, kqueue, etc) that there is data
to read from a connection.  If any data is read,
connection_handle_read() calls connection_process_inbuf() to see if
any of the data can be processed.  If the connection was closed,
connection_handle_read() calls connection_reached_eof().

Connection_process_inbuf() and connection_reached_eof() both dispatch
based on the connection type to determine what to do with the data
that's just arrived on the connection's inbuf field.  Each type of
connection has its own version of these functions.  For example,
directory connections process incoming data in
connection_dir_process_inbuf(), while OR connections process incoming
data in connection_or_process_inbuf().  These
connection_*_process_inbuf() functions extract data from the
connection's inbuf field (a buffer_t), using functions from buffers.c.
Some of these accessor functions are straightforward data extractors
(like fetch_from_buf()); others do protocol-specific parsing.


2.1.1.2. DNS

Tor launches (and optionally accepts) DNS requests using the code in
eventdns.c, which is a copy of libevent's evdns.c.  (We don't use
libevent's version because it is not yet in the versions of libevent
all our users have.)  DNS replies are read in nameserver_read();
DNS queries are read in server_port_read().