OpenBSD_malloc_Linux.c:

    The OpenBSD malloc implementation, ported to Linux.  Used only when
    --enable-openbsd-malloc is passed to the configure script.

strlcat.c
strlcpy.c

    Implementations of strlcat and strlcpy, the more sane replacements
    for strcat and strcpy.  These are nonstandard, and some libc
    implementations refuse to add them for religious reasons.

eventdns.[ch]

    A fork of Libevent's DNS implementation, used by Tor when Libevent
    2.0 or later is not available.  Once Libevent 2.0 is required, we
    should throw this away; it has diverged from evdns.[ch], and is
    no longer easily mergeable.

ht.h

    An implementation of a hash table in the style of Niels Provos's
    tree.h.  Shared with Libevent.

tinytest.[ch]
tinytest_demos.c
tinytest_macros.h

    A unit testing framework. https://github.com/nmathewson/tinytest

tor_queue.h

    A copy of sys/queue.h from OpenBSD.  We keep our own copy rather
    than using sys/queue.h, since some platforms don't have a
    sys/queue.h, and the ones that do have diverged in incompatible
    ways.  (CIRCLEQ or no CIRCLEQ? SIMPLQ or STAILQ?)  We also rename
    the identifiers with a TOR_ prefix to avoid conflicts with
    the system headers.

curve25519_donna/*.c

    A copy of Adam Langley's curve25519-donna mostly-portable
    implementations of curve25519.

csiphash.c
siphash.h

    Marek Majkowski's implementation of siphash 2-4, a secure keyed
    hash algorithm to avoid collision-based DoS attacks against hash
    tables.

trunnel/*.[ch]

    Headers and runtime code for Trunnel, a system for generating
    code to encode and decode binary formats.