6.5 Pseudoterminal Functions
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This section discusses the following pseudoterminal functions:
To read data from the pseudoterminal, the control program uses the PTD$READ routine. When a PTD$READ routine is called, the operating system queues a read operation. The read operation completes when the pseudoterminal has characters to output. The read request queries TTDRIVER whether there is data found to be returned. If so, the resulting string of characters is returned. If a read request is issued and no data is available, the read request is queued and then completed at a later time. In this case, the routine always returns at least one character. The read request may complete even when there are no characters available to output. In this rare case when TTDRIVER indicates that there is no more data to be output and there is really no data, the read operation completes with zero bytes of data.
An application that issues an asynchronous pseudoterminal read can use the $SYNCH system service to find out when the read completed. The efn argument for the $SYNCH service must be the same as the efn specified in the original PTD$READ call, and the iosb for the $SYNCH service must match the readbuf of the PTD$READ call.
To write data to the pseudoterminal, the control program uses the PTD$WRITE routine. The write request allows you to specify a buffer to receive any output that the write request generates; you do not need to issue a separate read request to read this data. When you use an echo buffer, the control application can significantly reduce the number of I/O requests required.
An application can issue only one write request at a time. Once the write request completes, the application must check the write buffer status longword to see whether all the data supplied was written. If not, the application must issue additional write requests until all the data has been accepted.
If a read request is pending when a write-with-echo request is issued, the echo data is placed in the echo buffer. If more data is echoed than can fit in the echo buffer, the remaining data is placed in the pending read requests buffer. If no pending read exists, the data is held by the driver until another request that can take the data is issued. Both the read and the write with echo must use completion ASTs to allow the driver to report request completions to the application in the correct order.
If an application is not using the write-with-echo capability, the application should avoid using completion ASTs if possible. Unnecessary use of completion ASTs significantly increases the number of instructions needed to complete a read or write operation.
When using write with echo, both the wrtbuf and echobuf arguments contain I/O status information. An application must check both of these status longwords if the PTD$WRITE completes successfully. If a write operation wrote no characters, characters might still be in the echo buffer. If no data was echoed, the status in the echobuf is SS$_NORMAL with zero bytes transferred.
By default, the driver attempts to notify the control program of data overrun or loss. The pseudoterminal sends an XOFF AST when the type-ahead buffer is getting full. Once the pseudoterminal delivers an XOFF AST, the pseudoterminal also returns a status of SS$_DATAOVERUN with the actual number of characters input. This prevents a single request from flooding the type-ahead buffer. If a control program makes repeated attempts to insert data after receiving the SS$_DATAOVERUN message, it can flood the terminal type-ahead buffer. When the type-ahead buffer has filled, the pseudoterminal returns the status of SS$_DATALOST.
If the control program is writing to the terminal or terminal driver, it should let the terminal and terminal driver handle flow control. To do this, the application should enable all three input flow control notification ASTs. The control program should write a DC1 to the terminal if an XON AST is delivered. It should write a DC3 to a terminal if an XOFF AST is delivered, and write a BELL character to the terminal if the BELL AST is delivered. These signals allow the terminal to decide what to do with the flow control data. The application should ignore the SS$_DATAOVERUN and SS$_DATALOST return status and continue writing data to the pseudoterminal.
This section describes how the pseudoterminal driver provides notification of important driver events.
The driver provides three ways to indicate when the class driver wants to stop input and one way to signal when it is safe to resume output:
The driver returns a status of SS$_DATAOVERUN and the number of characters input for the control program write.
The control program can enable a BELL attention AST to be delivered when the class driver calls the PTD$SET_TERMINAL_NOTIFICATION routine. This AST is delivered if the pseudoterminal does not have the HOSTSYNC attribute set. If only a BELL or only an XOFF AST event is enabled and an XOFF or a BELL AST needs to be delivered, the AST that is available is delivered.
The control program can enable an XOFF attention AST to be delivered when the class driver calls the PTD$SET_TERMINAL_NOTIFICATION routine. This AST is delivered if the pseudoterminal has the HOSTSYNC attribute set.
The control program can enable an XON attention AST to be delivered when the class driver calls the PTD$SET_TERMINAL_NOTIFICATION routine. This AST is delivered only if the pseudoterminal has the HOSTSYNC attribute set.
The Output Stop AST tells the control program that the terminal driver is stopping output. This keeps the control program from having to determine whether an XOFF written to the control side is being treated by the terminal driver as flow control or data.
The Output Resume AST tells the control program that the terminal driver wants to resume output. This AST can be delivered at any time, even if output is active or has previously been stopped. The control program should always restart output processing when it receives this AST.
The Characteristics Changed AST tells the control program that the terminal driver has called the pseudoterminal CHANGE CHARACTERISTICS routine. This routine is called whenever the terminal driver has changed the device characteristics. The control program should then read the pseudoterminal characteristics to determine what has changed.
The Output Abort AST tells the control program that the terminal driver has called the pseudoterminal ABORT OUTPUT routine. This routine is called when the terminal driver wants to flush any outstanding output data. The control program should flush any internally buffered data when this AST is received.
Three special event types notify the control program when a terminal read request starts and finishes. By default, the pseudoterminal does not deliver the read notification ASTs associated with these events. The PTD$SET_EVENT_NOTIFICATION routine must be used explicitly to enable or disable their delivery.
Start Read—Tells the control program that the terminal driver is starting a read request. Some applications require this in order to know when to start inputting a logged session script. The special event types are:
Middle Read—Tells the control program that the terminal driver has finished writing the prompt string if one was supplied.
End Read—Tells the control program that the terminal driver has finished a read request.
Once an event notification AST is enabled, it continues to be delivered until it is canceled, or until the device is deleted. This characteristic allows the control program to enable the AST once, which greatly reduces the risk of missing multiple rapid occurrences of an event. If the driver cannot get sufficient resources to deliver the notification AST, that report is lost. Only one AST per event is allowed, and attempts to specify multiple ASTs result in use of the last one specified.
To enable or disable event notification, the control program uses the PTD$SET_EVENT_NOTIFICATION routine, which is described in Appendix D.