Peter M. Groen / libmodbus

- change return argument from void to int
- update documentation

Thanks to Petr Gladkiy

Thanks to oldfaber and endrelovas.

It will better to code something such as:
https://github.com/crossroads-io/libxs/blob/master/src/err.cpp

I don't intend to rewrite glib neither :(

Thanks to oldfaber

Thanks to Thomas Stalder and Torello Querci for the report.

Original patch for new connect function by Thomas Stalder.

- export new symbols to create specific behavior (RTU)
- the flag can't only be set by slaves
- new tests

Warning, modbus_receive returns 0 when a query is ignored.

- function removed from TCP and RTU code and from backend structure
- updated documentation
- updated examples

- remove static variable in the library

libabc is a really good source of inspiration!
https://git.kernel.org/?p=linux/kernel/git/kay/libabc.git

Thanks to Pavel Mazniker for the report.

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

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+

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

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.