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# System Services Descriptions

BEA eLink for Mainframe TCP for IMS (hereafter referenced as eLink TCP for IMS) uses the following system services:

- IMS

- TCP/IP for OS/390

- LE/370

- OS/390

  ## IMS

  BEA eLink TCP for IMS BMP uses the following IMS services by issuing the appropriate API calls.

  IMS Service | Description
  --- | ---
  CHKP (Checkpoint) | Retrieve the first (or only) segment of a message from the IMS message queue and commit processing against recoverable IMS resources
  CHNG (Change) | Set the destination of a modifiable, alternate PCB
  GN (Get Next) | Retrieve subsequent segments (other than the first) of a message from the IMS message queue
  GU (Get Unique) | Retrieve the first (or only) segment of a message from the IMS message queue
  ISRT (Insert) | Insert a message segment into the IMS message queue
  PURG (Purge) | Inform IMS that all segments related to a particular message have been inserted and that the message is complete

  ## TCP/IP for OS/390

  BEA eLink TCP for IMS makes use of the following TCP/IP for OS/390 services by issuing the appropriate API calls.

  TCP/IP for OS/390 Service | Description
  --- | ---
  accept | Accept a TCP/IP connection request
  bind | Bind a socket to an IP address and port
  close | Close a socket and free associated resources
  connect | Initiate a TCP/IP connection with a specified IP address and port
  fcntl | Set the operational characteristics of a socket
  gethostbyaddr | Resolve an IP address into a domain name
  gethostbyname | Resolve a hostname into a domain name
  htonl | Convert a long integer from host to network byte order
  htons | Convert a short integer from host to network byte order
  inet_network | Convert an IP address from character format to internal format
  inet_ntoa | Convert an IP address from internal format to character format
  listen | Initiate listening on a socket bound to an IP address and port
  read | Read data on a TCP/IP connection
  selectex | Wait on one or more socket FDs to be posted, a timeout to occur, or an ECB to be posted
  setsockopt | Set socket-related processing options
  socket | Obtain (allocate) a socket descriptor for a TCP/IP connection
  shutdown | Shut down a TCP/IP connection
  write | Write data to a TCP/IP connection

  ## LE/370

  BEA eLink TCP for IMS makes use of the following LE/370 services by issuing the appropriate API calls.

  LE/370 Service | Description
  --- | ---
  CEE3ABD | Issue an abend

  ## OS/390

  BEA eLink TCP for IMS makes use of the following OS/390 services by issuing the appropriate API calls.

  OS/390 Service | Description
  --- | ---
  WTO | Issue a write-to-operator (WTO) message
  WTOR | Issue a write-to-operator-with-reply (WTOR) message
  XCF | Uses cross-system coupling facility to communicate to OTMA
  Latch Manager Services | Locks storage resources to prevent simultaneous updates of shared memory

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