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libmodbus - Man Page

a fast and portable Modbus library


#include <modbus.h>

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


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 http://www.modbus.org site provides documentation about the protocol at http://www.modbus.org/specs.php.

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 Ethernet 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 establish a connection with other 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, http://en.wikipedia.org/wiki/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 before sending a message, you must set the slave (receiver) with modbus_set_slave(3). If you’re running a slave, its slave number will be used to filter received messages.

The libmodbus implementation of RTU isn’t time based as stated in original Modbus specification, instead all bytes are sent as fast as possible and a response or an indication is considered complete when all expected characters have been received. This implementation offers very fast communication but you must take care to set a response timeout of slaves less than response timeout of master (ortherwise other slaves may ignore master requests when one of the slave is not responding).

Create a Modbus RTU context

Set the serial mode

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 same.

Create a Modbus TCP context

TCP PI (IPv4 and IPv6) Context

The TCP PI (Protocol Independent) 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
Set slave ID
Enable debug mode
Timeout settings
Error recovery mode
Setter/getter of internal socket
Information about header

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_INT64_FROM_INT16(tab_int16, index), builds an int64 from the four first int16 starting at tab_int16[index]
  • 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]
  • MODBUS_SET_INT32_TO_INT16(tab_int16, index, value), set an int32 value into the two first int16 starting at tab_int16[index]
  • MODBUS_SET_INT64_TO_INT16(tab_int16, index, value), set an int64 value into the four first int16 starting at tab_int16[index]

Handling of bits and bytes

Set or get float numbers


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

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 requests:

Read data

Write data

Write and read data

Raw requests

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

Error Handling

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 <stephane.raimbault@gmail.com>


Main web site: http://www.libmodbus.org/

Report bugs on the issue tracker at http://github.com/stephane/libmodbus/issues.


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

Referenced By

modbus_get_header_length(3), modbus_reply(3), modbus_reply_exception(3).

01/25/2024 libmodbus v3.1.7 libmodbus Manual