Peter M. Groen / libmodbus

The function name was confusing because the write operation is performed
before the read. Take care to swap the arguments in the migration process.

Avoid incompatibility with GPLv2 program. This change has been approved by
Tobias Doerffel, Florian octo Forster and Hannu Vuolasaho.

The two modes are complementary, MODBUS_ERROR_RECOVERY_LINK handles
errors at data link level (bad file descriptor, timeout, etc) and
MODBUS_ERROR_RECOVERY_PROTOCOL checks Modbus error (eg. invalid
function code or trame length).

This change introduces the use of the Sleep function for Windows.
Some duplicated code has been moved from backends to modbus core.
A new debug message is now available when a flush occurs.

The unit tests are now based on this error recovery code.

The change for serial on Windows is temporary. If you're interested of
improving the situation for this, please have a look at the FIXME.

The following functions have been renamed:
- modbus_get_timeout_begin -> modbus_get_response_timeout
- modbus_set_timeout_begin -> modbus_set_response_timeout
- modbus_get_timeout_end -> modbus_get_byte_timeout
- modbus_set_timeout_end -> modbus_set_byte_timeout

The meaning of these timeout intervals is now clearer. The documentation
has been updated.

These new functions seem clearer than the recent modbus_receive_from
function IHMO. This change fixes the bandwidth-server-many-up program.

Split the original modbus_receive function in two functions to avoid
the strange -1 value to ignore the sockfd argument.

The loop test ought to iterate again to end the loop but this is
only required if the socket contained more data than read.

The arguments are surrounded by parentheses to be evaluated first.

The _modbus_receive_confirmation()_ function shall receive a
request via the socket of the context 'ctx'. This function must be
used for debugging purposes because the received response isn't
checked against the initial request. This function can be
convenient to receive request not handled by the library.

The _modbus_send_raw_request()_ function shall send a request via
the socket of the context 'ctx'. This function must be used for
debugging purposes because you have to take care to make a valid
request by hand. The function only adds to the message, the header
or CRC of the selected backend, so 'raw_req' must start and contain
at least a slave/unit identifier and a function code. This function
can be used to send request not handled by the library.

The error reported by #9 was already fixed in master but this change
scrupulously respects the definition of DWORD.

Each function is decribed in one txt file. The file libmodbus.txt
offers an overview of the library.

It's possible to generate HTML and man outputs from the txt file.
The documentation can be generated only if you have the required
tools (asciidoc and xmlto).

Better results are obtained with AsciiDoc v8.6+

- updated comments
- renamed internal constants (eg. _TIME_OUT_BEGIN_OF_TRAME in
  _TIME_OUT_BEGIN_OF_MESSAGE)

To avoid to duplicate the documentation between source and
documentation. The information already available in documentation
is removed from source.

Reported by Tobias Doerffel.

Reported by Allan Cornet.

- removed ENOTCONN errno not supported
- set WINVER to Windows XP (and above) and add ws2_32.dll. So older
  Windows versions aren't supported.

Reported by Patsy Kaye.
Removed useless asignment at the same time.

Inspired by the branch ff/ipv6 of Florian Forster but this version uses
a specific PI backend to isolate the IPv4 and IPv6 implementations.

The existing TCP/IPv4 implementation has been kept intact (not rebased
on the PI one) because its data backend requires fewer bytes than the PI.

New functions modbus_new_tcp_pi, modbus_tcp_pi_listen and
modbus_tcp_pi_accept and new backend called modbus_tcp_pi_t.

In modbus-tcp.c:
- Needed to include <signal.h>
- Added Jay's fix for __OpenBSD__
- SIGPIPE ignore handler needs to return a pointer instead of an integer.

Patch provided bu Barry Grumbine and Jason Oster.

Reported by Marco Sala.

Page 38 in the document Modbus_Application_Protocol_V1_1b.pdf:
6.17 23 (0x17) Read/Write Multiple registers

This function code performs a combination of one read operation
and one write operation in a single MODBUS transaction. The write
operation is performed before the read.

The unit test has been updated.

Original patch by Jason Oster