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This file should help you to add new address types and address options to
socat.

NOTE:
socat will in future releases be split into a library "libxio" containing all
the address stuff, useful also for many other purposes, and the socat main()
and data shuffler. If you intend to perform major changes to the xio part and
to publish them, please contact me before!

ADDING A NEW ADDRESS TYPE:

* Create new files xio-newaddr.c and xio-newaddr.h

* Create a new record of struct addrdesc in xio-newaddr.c, with declaration in xio-newaddr.h.

* Make a new entry to addressnames[] in xioopen.c with the addresses main name
and maybe with alias names. Keep this array ASCII sorted, without uppercase
chars.

* config.h.in: #undef WITH_NEWADDR

* configure.in: Copy the disable part of, e.g., WITH_SOCKS4 and adapt it to
NEWADDR

* In socat.c, add to socat_version

* Write a function xioopen_newaddr() in xio-newaddr.c, declaration in
xio-newaddr.h
Do not forget the following option processing calls:
All groups: _xio_openlate()
Group FD: applyopts_cloexec()
Group NAMED: applyopts_file() for phases PREOPEN, OPEN, and FD

* Describe a tested example in file EXAMPLES, and maybe in the socat manpage
source.

* Try to define a test for this address type in test.sh

* Update file CHANGES

ADDING A NEW ADDRESS OPTION:

xioopen.c:

* If this option depends on a #define that is probably not available on all
platforms, make all new code for this option dependent on the existence of this
C header define:
#ifdef PREFIX_NEWOPTION
...
#endif

* Add an OPT_NEWOPTION to enum e_optcode in xioopts.h, preferably keeping
alphabetic order

* Add a struct optdesc opt_newoption record in xio-newaddr.c and its
declaration in xio-newaddr.h. The complete structure definition must be in one
line without breaks for automatic docu extraction.
Build the record from the following components:
. A canonical default name (e.g. "newoption")
. A short, preferable name (e.g. "newopt") or NULL
. OPT_NEWOPTION (from enum e_optcode, see above)
. A group membership that restricts appliance of the new option to matching
address types (e.g., one of GROUP_ANY, GROUP_IP_TCP, GROUP_EXEC)
. A phase specification that positions this option within address processing.
Note that the function code can override this value.
. A representation type for option arguments (e.g., TYPE_INT, TYPE_STRING etc.;
use TYPE_BOOL if this option just triggers an action)
. A function or action definition for applying this option. If it does not use
one of the standard functions (open(), ioctl(), setsockopt()...), then use
OFUNC_SPEC (specific).

* For the canonical name and all its aliases and abbreviations, add entries to
the array optionnames in xioopts.c. KEEP STRICT ALPHABETIC (ASCII) ORDER!
The entries must be embedded in an IF_... macro of their group for conditional
compiling.

* For options using some predefined action (see OFUNC above), this might be
enough - test the option and document it in xio.help!
For OFUNC_SPEC, it might suffice to add another "case" to the OFUNC_SPEC branch
in applyopts() in xioopts.c. If you need more special handling, you should try
to understand the address specific functions and add your code there.

* If you use system or low level C library calls or library calls that might
hang or induce problems, please invoke them with capitalized name; if no such
name is defined, add an appropriate debug function to sycls.c, and a header
entry and a wrapper "define" to sycls.h

* Update file CHANGES

INFO ABOUT ADDRESS PHASES:

Each option entry has a field specifying a default phase for its application.
Of course, the code that analyses and applies an address may override this
default phase.

Depending on the type of address there are several major phase sequences:

OPEN addresses:

PH_INIT		retrieving info from original state
PH_EARLY	before any other processing
PH_PREOPEN	before file creation/opening (not UNIX sockets)
PH_OPEN		during file creation/opening (not UNIX sockets)
PH_PASTOPEN	past file creation/opening (not UNIX sockets)
PH_FD		soon after FD creation or identification
PH_LATE		FD is ready, before start of data loop
PH_LATE2	FD is ready, dropping privileges

SOCKET addresses:

PH_INIT		retrieving info from original state
PH_EARLY	before any other processing
PH_PRESOCKET	before socket call
PH_SOCKET	for socket call
PH_PASTSOCKET	after socket call
PH_FD		soon after FD creation or identification
PH_PREBIND	before socket bind()
PH_BIND		during socket bind()
PH_PASTBIND	past socket bind()
PH_PRECONNECT	before connect()
PH_CONNECT	during connect()
PH_PASTCONNECT	after connect()
PH_CONNECTED	phase common with listen
PH_LATE		FD is ready, before start of data loop
PH_LATE2	FD is ready, dropping privileges

SOCKET with LISTEN and ACCEPT:

PH_INIT		retrieving info from original state
PH_EARLY	before any other processing
PH_PRESOCKET	before socket call
PH_SOCKET	for socket call
PH_PREBIND	before socket bind()
PH_BIND		during socket bind()
PH_PASTBIND	past socket bind()
PH_PRELISTEN	before listen()
PH_LISTEN	during listen()
PH_PASTLISTEN	after listen()
PH_PREACCEPT	before accept()
PH_ACCEPT	during accept()
PH_PASTACCEPT	after accept()
# and the following on the new FD:
PH_FD		soon after FD creation or identification
PH_PASTSOCKET	after socket call
PH_CONNECTED	phase common with connect
PH_PREFORK	before forking
PH_FORK		during fork()
PH_PASTFORK	after fork()
PH_LATE		FD is ready, before start of data loop
PH_LATE2	FD is ready, dropping privileges

FD addresses:

PH_INIT		retrieving info from original state
PH_EARLY	before any other processing
PH_FD		soon after FD identification
PH_LATE		FD is ready, before start of data loop
PH_LATE2	FD is ready, dropping privileges

EXEC addresses:

PH_INIT		retrieving info from original state
PH_EARLY	before any other processing
PH_PREBIGEN	before socketpair() pipe() openpty()
PH_BIGEN	during socketpair() pipe() openpty()
PH_PASTBIGEN	past socketpair() pipe() openpty()
PH_PASTSOCKET	for socketpair()
PH_FD		soon after FD creation or identification
PH_PREFORK	before forking
PH_FORK		during fork()
PH_PASTFORK	after fork()
PH_LATE		FD is ready, before start of data loop
PH_LATE2	FD is ready, dropping privileges
PH_PREEXEC	before exec() or system()
PH_EXEC		during exec() or system()

There are lots of semantic relations between group, phase, and func fields of
an option.

There exists something like an overall phase sequence:
PH_INIT						# su-d.1
PH_EARLY					# chroot-early
PH_PREOPEN,	PH_OPEN,	PH_PASTOPEN	# (chroot before/after?)
PH_PRESOCKET,	PH_SOCKET,	PH_PASTSOCKET	# (su after (root for raw)?)
PH_PREBIGEN,	PH_BIGEN,	PH_PASTBIGEN	# (chroot before/after (/dev..)?)
PH_FD
PH_PREBIND,	PH_BIND,	PH_PASTBIND	# (su after(before?))
PH_PRELISTEN,	PH_LISTEN,	PH_PASTLISTEN
PH_PRECONNECT,	PH_CONNECT,	PH_PASTCONNECT	# (chroot before/after (AF_UNIX)?)
PH_PREACCEPT,	PH_ACCEPT,	PH_PASTACCEPT
PH_CONNECTED
PH_PREFORK,	PH_FORK,	PH_PASTFORK	# (all before/after?)
PH_LATE						# chroot
PH_LATE2					# su, su-d.2
PH_PREEXEC,	PH_EXEC				# (all before)

===============================================================================
// Up to 1.7.2.4 socat used non async signal safe system and library calls in signal handlers, mostly for logging purposes. This problem was fixed in release 1.7.3.0 with the following concepts:

Signal handlers set on entry and unset on return the diag_in_handler global variable. The logging system, when this variable is set, queues the text message together with errno and exit info in a UNIX datagram socket. When invoked with unset diag_in_handler it first checks if there are messages in that queue and prints them first.

A async signal safe but minimal version of vsnprintf, named vsnprintf_r, was written so no value arguments need to be queued.

Because strerror is not async signal safe a new function snprinterr was written that replaces the (glibc compatible) %m format with strerror output. The original errno is passed in the message queue, snprinterr is called when dequeuing messages outside of signal handler.

// List of signal handlers in socat
socat.c:socat_signal (generic, just logs and maybe exits)
xioshutdown.c:signal_kill_pid (SIGALRM, kill child process)
xiosigchld.c:childdied (SIGCHLD: get info, log; possibly close channel)
xiosignal.c:socatsignalpass: cascades signal to channel child processes; w/ options sighup,sigint,sigquit
xio-socket.c:xiosigaction_hasread: SIGUSR1,SIGCHLD, tells parent that datagram has been consumed

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