Modbus TCP client shm

Modbus tcp client that stores its data (registers) in shared memory objects.

Dependencies

• cxxopts by jarro2783 (https://github.com/jarro2783/cxxopts) (only required for building the application)
• libmodbus by Stéphane Raimbault (https://github.com/stephane/libmodbus)
• cxxshm (https://github.com/NikolasK-source/cxxshm)
• cxxsemaphore (https://github.com/NikolasK-source/cxxsemaphore)

On Arch linux they are available via the official repositories and the AUR:

• https://archlinux.org/packages/extra/any/cxxopts/
• https://aur.archlinux.org/packages/libmodbus
• https://aur.archlinux.org/packages/cxxshm
• https://aur.archlinux.org/packages/cxxsemaphore

Build

cmake -B build -DCMAKE_CXX_COMPILER=$(which clang++) -DCMAKE_BUILD_TYPE=Release -DCLANG_FORMAT=OFF -DCLANG_TIDY=OFF -DCOMPILER_WARNINGS=OFF -DBUILD_DOC=OFF
cmake --build .

Use

modbus-tcp-client-shm [OPTION...]

 network options:
  -i, --host arg         host to listen for incoming connections (default: any)
  -p, --service arg      service or port to listen for incoming connections (default: 502)
  -c, --connections arg  number of allowed simultaneous Modbus Server connections. (default: 1)
  -r, --reconnect        do not terminate if no Modbus Server is connected anymore.
  -t, --tcp-timeout arg  tcp timeout in seconds. Set to 0 to use the system defaults (not recommended). (default: 5)

 shared memory options:
  -n, --name-prefix arg  shared memory name prefix (default: modbus_)
      --force            Force the use of the shared memory even if it already exists. Do not use this option per default! It should only be used if the shared memory of an improperly terminated instance continues to exist as an orphan 
                         and is no longer used.
  -s, --separate arg     Use a separate shared memory for requests with the specified client id. The client id (as hex value) is appended to the shared memory prefix (e.g. modbus_fc_DO). You can specify multiple client ids by 
                         separating them with ','. Use --separate-all to generate separate shared memories for all possible client ids.
      --separate-all     like --separate, but for all client ids (creates 1028 shared memory files! check/set 'ulimit -n' before using this option.)
      --semaphore arg    protect the shared memory with a named semaphore against simultaneous access
      --semaphore-force  Force the use of the semaphore even if it already exists. Do not use this option per default! It should only be used if the semaphore of an improperly terminated instance continues to exist as an orphan and is 
                         no longer used.
  -b, --permissions arg  permission bits that are applied when creating a shared memory. (default: 0640)

 modbus options:
      --do-registers arg      number of digital output registers (default: 65536)
      --di-registers arg      number of digital input registers (default: 65536)
      --ao-registers arg      number of analog output registers (default: 65536)
      --ai-registers arg      number of analog input registers (default: 65536)
  -m, --monitor               output all incoming and outgoing packets to stdout
      --byte-timeout arg      timeout interval in seconds between two consecutive bytes of the same message. In most cases it is sufficient to set the response timeout. Fractional values are possible.
      --response-timeout arg  set the timeout interval in seconds used to wait for a response. When a byte timeout is set, if the elapsed time for the first byte of response is longer than the given timeout, a timeout is detected. When 
                              byte timeout is disabled, the full confirmation response must be received before expiration of the response timeout. Fractional values are possible.

 other options:
  -h, --help          print usage
      --license       show licences (short)
      --license-full  show licences (full license text)

 version information options:
      --version       print version and exit
      --longversion   print version (including compiler and system info) and exit
      --shortversion  print version (only version string) and exit
      --git-hash      print git hash


The modbus registers are mapped to shared memory objects:
    type | name                      | mb-server-access | shm name
    -----|---------------------------|------------------|----------------
    DO   | Discrete Output Coils     | read-write       | <name-prefix>DO
    DI   | Discrete Input Coils      | read-only        | <name-prefix>DI
    AO   | Discrete Output Registers | read-write       | <name-prefix>AO
    AI   | Discrete Input Registers  | read-only        | <name-prefix>AI

Use privileged ports

The standard modbus port (502) can be used only by the root user under linux by default. To circumvent this, you can create an entry in the iptables that redirects packets on the standard modbus port to a higher port. The following example redirects packets from port 502 (standard modbus port) to port 5020

iptables -A PREROUTING -t nat -p tcp --dport 502 -j REDIRECT --to-port 5020

The modbus client must be called with the option -p 5020