Exchange/SNA Manual

Programming Considerations
Using the Exchange/SNA Programmatic Interface
4–2 104700 Tandem Computers Incorporated
Programming
Considerations
This subsection discusses those programming considerations that apply particularly to
writing programs that use the Exchange/SNA programmatic interface. For a general
discussion of programming on Tandem NonStop systems, see the Guardian
Programmer’s Guide.
Wait and Nowait I/O
Operations
As with any file, you can perform I/O operations on the subdevices supported by an
Exchange/SNA line server either as wait I/O operations or as nowait I/O operations.
You select the type you want when you open the subdevice. After you have opened
the subdevice, the same procedure calls are used to perform either wait or nowait I/O
operations, depending on how you opened the line server.
Wait I/O Operations
The following steps describe how wait I/O operations are performed.
1. Your program makes a call to a procedure that performs an I/O operation, such as
READ.
2. The execution of your program is suspended, and the operating system sends a
message initiating the I/O operation to the appropriate process. This process is
the Exchange/SNA line server for I/O operations using the Exchange/SNA
programmatic interface.
3. The process performs the I/O operation.
4. When the process has finished the I/O operation, it passes a message to the
operating system indicating the completion of the I/O operation.
5. The operating system sets the condition code indicating the success or failure of
the I/O operation and starts the execution of your program again.
Nowait I/O Operations
The following steps describe how nowait I/O operations are performed.
1. Your program makes a call to a procedure that performs an I/O operation, such as
READ.
2. The execution of your program is suspended, and the operating system sends a
message initiating the I/O operation to the appropriate process. This process is
the Exchange/SNA line server for I/O operations using the Exchange/SNA
programmatic interface.
3. After the message initiating the I/O operation has been sent, the operating system
starts the execution of your program again.
4. The process performs the I/O operation. Concurrently, your program continues
executing.
5. When the process has finished the I/O operation, it passes a message to the
operating system indicating the completion of the I/O operation. This step may
happen after Step 6.