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17.4.4 Input Modes

This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and <RET> and <LFD> characters.

All of these flags are bits in the c_iflag member of the struct termios structure. The member is an integer, and you change flags using the operators &, | and ^. Don't try to specify the entire value for c_iflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t INPCK

If this bit is set, input parity checking is enabled. If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application.

Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see Control Modes. For example, you could clear the INPCK input mode flag and set the PARENB control mode flag to ignore parity errors on input, but still generate parity on output.

If this bit is set, what happens when a parity error is detected depends on whether the IGNPAR or PARMRK bits are set. If neither of these bits are set, a byte with a parity error is passed to the application as a '\0' character.

— Macro: tcflag_t IGNPAR

If this bit is set, any byte with a framing or parity error is ignored. This is only useful if INPCK is also set.

— Macro: tcflag_t PARMRK

If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when INPCK is set and IGNPAR is not set.

The way erroneous bytes are marked is with two preceding bytes, 377 and 0. Thus, the program actually reads three bytes for one erroneous byte received from the terminal.

If a valid byte has the value 0377, and ISTRIP (see below) is not set, the program might confuse it with the prefix that marks a parity error. So a valid byte 0377 is passed to the program as two bytes, 0377 0377, in this case.

— Macro: tcflag_t ISTRIP

If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read.

— Macro: tcflag_t IGNBRK

If this bit is set, break conditions are ignored.

A break condition is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.

— Macro: tcflag_t BRKINT

If this bit is set and IGNBRK is not set, a break condition clears the terminal input and output queues and raises a SIGINT signal for the foreground process group associated with the terminal.

If neither BRKINT nor IGNBRK are set, a break condition is passed to the application as a single '\0' character if PARMRK is not set, or otherwise as a three-character sequence '\377', '\0', '\0'.

— Macro: tcflag_t IGNCR

If this bit is set, carriage return characters ('\r') are discarded on input. Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the <RET> key.

— Macro: tcflag_t ICRNL

If this bit is set and IGNCR is not set, carriage return characters ('\r') received as input are passed to the application as newline characters ('\n').

— Macro: tcflag_t INLCR

If this bit is set, newline characters ('\n') received as input are passed to the application as carriage return characters ('\r').

— Macro: tcflag_t IXOFF

If this bit is set, start/stop control on input is enabled. In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it. The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission. See Start/Stop Characters.

— Macro: tcflag_t IXON

If this bit is set, start/stop control on output is enabled. In other words, if the computer receives a STOP character, it suspends output until a START character is received. In this case, the STOP and START characters are never passed to the application program. If this bit is not set, then START and STOP can be read as ordinary characters. See Start/Stop Characters.

— Macro: tcflag_t IXANY

If this bit is set, any input character restarts output when output has been suspended with the STOP character. Otherwise, only the START character restarts output.

This is a BSD extension; it exists only on BSD systems and GNU/Linux and GNU/Hurd systems.

— Macro: tcflag_t IMAXBEL

If this bit is set, then filling up the terminal input buffer sends a BEL character (code 007) to the terminal to ring the bell.

This is a BSD extension.