Provided by: libmodbus-dev_3.0.1-2_amd64 bug


       libmodbus - fast and portable Modbus library


       #include <modbus.h>

       cc `pkg-config --cflags --libs libmodbus` files


       libmodbus is a library to send/receive data with a device which respects the Modbus
       protocol. This library contains various backends to communicate over different networks
       (eg. serial in RTU mode or Ethernet in TCP/IPv6). The site provides
       documentation about the protocol at

       libmodbus provides an abstraction of the lower communication layers and offers the same
       API on all supported platforms.

       This documentation presents an overview of libmodbus concepts, describes how libmodbus
       abstracts Modbus communication with different hardware and platforms and provides a
       reference manual for the functions provided by the libmodbus library.

       The Modbus protocol contains many variants (eg. serial RTU or Ehternet TCP), to ease the
       implementation of a variant, the library was designed to use a backend for each variant.
       The backends are also a convenient way to fulfill other requirements (eg. real-time
       operations). Each backend offers a specific function to create a new modbus_t context. The
       modbus_t context is an opaque structure containing all necessary information to etablish a
       connection with others Modbus devices according to the selected variant.

       You can choose the best context for your needs among:

       RTU Context
           The RTU backend (Remote Terminal Unit) is used in serial communication and makes use
           of a compact, binary representation of the data for protocol communication. The RTU
           format follows the commands/data with a cyclic redundancy check checksum as an error
           check mechanism to ensure the reliability of data. Modbus RTU is the most common
           implementation available for Modbus. A Modbus RTU message must be transmitted
           continuously without inter-character hesitations (extract from Wikipedia, Modbus,
  (as of Mar. 13, 2011, 20:51 GMT).

           The Modbus RTU framing calls a slave, a device/service which handle Modbus requests,
           and a master, a client which send requests. The communication is always initiated by
           the master.

           Many Modbus devices can be connected together on the same physical link so you need to
           define which slave is concerned by the message with modbus_set_slave(3). If you’re
           running a slave, the slave number is used to filter messages.

           Create a Modbus RTU context


           Set the serial mode

               modbus_rtu_get_serial_mode(3) modbus_rtu_set_serial_mode(3)

       TCP (IPv4) Context
           The TCP backend implements a Modbus variant used for communications over TCP/IPv4
           networks. It does not require a checksum calculation as lower layer takes care of the

           Create a Modbus TCP context


       TCP PI (IPv4 and IPv6) Context
           The TCP PI (Protocol Indepedent) backend implements a Modbus variant used for
           communications over TCP IPv4 and IPv6 networks. It does not require a checksum
           calculation as lower layer takes care of the same.

           Contrary to the TCP IPv4 only backend, the TCP PI backend offers hostname resolution
           but it consumes about 1Kb of additional memory.

           Create a Modbus TCP context


           Before using any libmodbus functions, the caller must allocate and initialize a
           modbus_t context with functions explained above, then the following functions are
           provided to modify and free a context:

           Free libmodbus context


           Context setters and getters

               modbus_get_byte_timeout(3) modbus_set_byte_timeout(3) modbus_set_debug(3)
               modbus_set_error_recovery(3) modbus_get_header_length(3)
               modbus_get_response_timeout(3) modbus_set_response_timeout(3) modbus_set_slave(3)
               modbus_set_socket(3) modbus_get_socket(3)

           A libmodbus context is thread safe and may be shared among as many application threads
           as necessary, without any additional locking required on the part of the caller.

           Macros for data manipulation
               MODBUS_GET_HIGH_BYTE(data), extracts the high byte from a byte
               MODBUS_GET_LOW_BYTE(data), extracts the low byte from a byte
               MODBUS_GET_INT32_FROM_INT16(tab_int16, index), builds an int32 from the two first
               int16 starting at tab_int16[index] MODBUS_GET_INT16_FROM_INT8(tab_int8, index),
               builds an int16 from the two first int8 starting at tab_int8[index]
               MODBUS_SET_INT16_TO_INT8(tab_int8, index, value), set an int16 value into the two
               first bytes starting at tab_int8[index]

           Functions for data manipulation

               modbus_set_bits_from_byte(3) modbus_set_bits_from_bytes(3)
               modbus_get_byte_from_bits(3) modbus_get_float(3) modbus_set_float(3)

       The following functions are provided to establish and close a connection with Modbus

       Establish a connection


       Close a connection


       Flush a connection


       The Modbus protocol defines different data types and functions to read and write them
       from/to remote devices. The following functions are used by the clients to send Modbus

       Read data

           modbus_read_bits(3) modbus_read_input_bits(3) modbus_read_registers(3)
           modbus_read_input_registers(3) libkmb:modbus_report_slave_id[3]

       Write data

           modbus_write_bit(3) modbus_write_register(3) modbus_write_bits(3)

       Write and read data


       Raw requests

           modbus_send_raw_request(3) modbus_receive_confirmation(3)

       Reply an exception


       The server is waiting for request from clients and must answer when it is concerned by the
       request. The libmodbus offers the following functions to handle requests:

       Data mapping: modbus_mapping_new(3) modbus_mapping_free(3)




           modbus_reply(3) modbus_reply_exception(3)


       The libmodbus functions handle errors using the standard conventions found on POSIX
       systems. Generally, this means that upon failure a libmodbus function shall return either
       a NULL value (if returning a pointer) or a negative value (if returning an integer), and
       the actual error code shall be stored in the errno variable.

       The modbus_strerror() function is provided to translate libmodbus-specific error codes
       into error message strings; for details refer to modbus_strerror(3).


       The LIBMODBUS_VERSION_STRING constant indicates the libmodbus version the program has been
       compiled against. The variables libmodbus_version_major, libmodbus_version_minor,
       libmodbus_version_micro give the version the program is linked against.


       The libmodbus documentation was written by Stéphane Raimbault


       Main web site:

       Report bugs on the issue tracker at


       Free use of this software is granted under the terms of the GNU Lesser General Public
       License (LGPL v2.1+). For details see the files COPYING and COPYING.LESSER included with
       the libmodbus distribution.