Setting up termios flags

Some hints on setting up the termios flags when using a serial device
that you've opened yourself (as opposed to using your existing control
tty):

3.6.2.1 c_iflag

You probably want to set all the bits in c_iflag to 0,
unless you want to use software flow control (ick) in which case you set
IXON and IXOFF.

Most of the bits of c_oflag are hacks of one sort or another to
make output to slow terminals work, and as such some newer systems have
dropped almost all of them as obsolete (especially all the gory
output-padding options). As with c_iflag, setting everything to 0
is reasonable for most applications.

3.6.2.3 c_cflag

When setting the character size, remember to mask using CSIZE
first; e.g. to set 8-bit characters, use:



    attr.c_cflag &= ~CSIZE;


    attr.c_cflag |= CS8;

Other important flags found in c_cflag that you probably want to
turn on and CREAD and HUPCL.

If you need to generate even parity, then set PARENB and clear
PARODD; if you need to generate odd parity then set both
PARENB and PARODD. If you don't want parity at all, then
make sure PARENB is clear.

Clear CSTOPB unless you actually need to generate two stop bits.

Flags for enabling hardware flow control may also be found in
c_cflag, but they aren't standardised (pity).

3.6.2.4 c_lflag

Most applications will probably want to turn off ICANON
(canonical, i.e. line-based, input processing), ECHO, and
ISIG.

IEXTEN is a more complex issue. If you don't turn it off, the
implementation is allowed to do nonstandard things (like define
additional control characters in c_cc) that might cause
unexpected results, but you might need to leave IEXTEN enabled
on some systems to get useful features like hardware flow control.

3.6.2.5 c_cc

This is an array of characters that have special meanings on input.
These characters are given names like VINTR, VSTOP etc.;
the names are indexes into the array.

(Two of these "characters" are not really characters at all, but control
the behaviour of read() when ICANON is disabled; these are
VMIN and VTIME.)

The indexes are often referred to as though they were actual variables,
e.g. "set VMIN to 1" actually means "set c_cc[VMIN] to 1". The shorthand
is useful and only occasionally confusing.

Many of the slots in c_cc are only used if some other combination
of flags is set:

Used only if ICANON is set: VEOF, VEOL, VERASE, VKILL (and also
VEOL2, VSTATUS and VWERASE if defined and
IEXTEN is set)
Used only if ISIG is set: VINTR, VQUIT, VSUSP (and also VDSUSP if
defined and IEXTEN is set)
Used only if IXON or IXOFF is set: VSTOP, VSTART
Used only if ICANON is not set: VMIN, VTIME

Implementations may define additional entries in c_cc. It may
be prudent to initialise all the entries to _POSIX_VDISABLE
(the constant NCCS gives the array size) before setting the
specific values you wish to use.

VMIN and VTIME (which may share slots with VEOF and
VEOL respectively, depending on the implementation) have the
following meaning. The value of VTIME is (if not 0) always
interpreted as a timer in tenths of seconds.

c_cc[VMIN] > 0, c_cc[VTIME] > 0: read() will return when either VMIN bytes of input are available,
or if at least one character has been read and VTIME has expired between
characters, or if interrupted by a signal.
c_cc[VMIN] > 0, c_cc[VTIME] == 0: read() will return when VMIN bytes of input are available, or if
interrupted. Otherwise it will wait indefinitely.
c_cc[VMIN] == 0, c_cc[VTIME] > 0: read() will return as soon as any input is available; if VTIME
expires with no data arriving, it will return with no characters read.
(This conflicts slightly with the end-of-file indication received in
the event of modem hangup; using 1 for VMIN and either alarm()
or select() for a timeout avoids this particular problem.)
c_cc[VMIN] == 0, c_cc[VTIME] == 0: read() will always return immediately; if no data is available
it will return with no characters read (with the same problem as above).

Home

FAQ

ASGAUR