Raw input mode

In raw mode, io-char performs no editing on received characters. This reduces the processing done on each character to a minimum and provides the highest performance interface for reading data.

Fullscreen programs and serial communications programs are examples of applications that use a character device in raw mode.

In raw mode, each character is received into the raw input buffer by the interrupt handler. When an application requests data from the device, it can specify under what conditions an input request is to be satisfied. Until the conditions are satisfied, the interrupt handler won't signal the driver to run, and the driver won't return any data to the application. The normal case of a simple read by an application would block until at least one character was available.

The following diagram shows the full set of available conditions:

Figure 1. Conditions for satisfying an input request.

In the case where multiple conditions are specified, the read will be satisfied when any one of them is satisfied.

The qualifier MIN is useful when an application has knowledge of the number of characters it expects to receive.

Any protocol that knows the character count for a frame of data can use MIN to wait for the entire frame to arrive. This significantly reduces IPC and process scheduling. MIN is often used in conjunction with TIME or TIMEOUT. MIN is part of the POSIX standard.

The qualifier TIME is useful when an application is receiving streaming data and wishes to be notified when the data stops or pauses. The pause time is specified in 1/10ths of a second. TIME is part of the POSIX standard.
The qualifier TIMEOUT is useful when an application has knowledge of how long it should wait for data before timing out. The timeout is specified in 1/10ths of a second.

Any protocol that knows the character count for a frame of data it expects to receive can use TIMEOUT. This in combination with the baud rate allows a reasonable guess to be made when data should be available. It acts as a deadman timer to detect dropped characters. It can also be used in interactive programs with user input to time out a read if no response is available within a given time.

TIMEOUT is a QNX Neutrino extension and is not part of the POSIX standard.

The qualifier FORWARD is useful when a protocol is delimited by a special framing character. For example, the PPP protocol used for TCP/IP over a serial link starts and ends its packets with a framing character. When used in conjunction with TIMEOUT, the FORWARD character can greatly improve the efficiency of a protocol implementation. The protocol process will receive complete frames, rather than character by character. In the case of a dropped framing character, TIMEOUT or TIME can be used to quickly recover.

This greatly minimizes the amount of IPC work for the OS and results in a much lower processor utilization for a given TCP/IP data rate. It's interesting to note that PPP doesn't contain a character count for its frames. Without the data-forwarding character, an implementation might be forced to read the data one character at a time.

FORWARD is a QNX Neutrino extension and is not part of the POSIX standard.

The ability to “push” the processing for application notification into the service-providing components of the OS reduces the frequency with which user-level processing must occur. This minimizes the IPC work to be done in the system and frees CPU cycles for application processing. In addition, if the application implementing the protocol is executing on a different network node than the communications port, the number of network transactions is also minimized.

For intelligent, multiport serial cards, the data-forwarding character recognition can also be implemented within the intelligent serial card itself, thereby significantly reducing the number of times the card must interrupt the host processor for interrupt servicing.