GNU Info
(libc.info)Line Speed
Line Speed
----------
The terminal line speed tells the computer how fast to read and write
data on the terminal.
If the terminal is connected to a real serial line, the terminal
speed you specify actually controls the line--if it doesn't match the
terminal's own idea of the speed, communication does not work. Real
serial ports accept only certain standard speeds. Also, particular
hardware may not support even all the standard speeds. Specifying a
speed of zero hangs up a dialup connection and turns off modem control
signals.
If the terminal is not a real serial line (for example, if it is a
network connection), then the line speed won't really affect data
transmission speed, but some programs will use it to determine the
amount of padding needed. It's best to specify a line speed value that
matches the actual speed of the actual terminal, but you can safely
experiment with different values to vary the amount of padding.
There are actually two line speeds for each terminal, one for input
and one for output. You can set them independently, but most often
terminals use the same speed for both directions.
The speed values are stored in the `struct termios' structure, but
don't try to access them in the `struct termios' structure directly.
Instead, you should use the following functions to read and store them:
- Function: speed_t cfgetospeed (const struct termios *TERMIOS-P)
This function returns the output line speed stored in the structure
`*TERMIOS-P'.
- Function: speed_t cfgetispeed (const struct termios *TERMIOS-P)
This function returns the input line speed stored in the structure
`*TERMIOS-P'.
- Function: int cfsetospeed (struct termios *TERMIOS-P, speed_t SPEED)
This function stores SPEED in `*TERMIOS-P' as the output speed.
The normal return value is 0; a value of -1 indicates an error.
If SPEED is not a speed, `cfsetospeed' returns -1.
- Function: int cfsetispeed (struct termios *TERMIOS-P, speed_t SPEED)
This function stores SPEED in `*TERMIOS-P' as the input speed.
The normal return value is 0; a value of -1 indicates an error.
If SPEED is not a speed, `cfsetospeed' returns -1.
- Function: int cfsetspeed (struct termios *TERMIOS-P, speed_t SPEED)
This function stores SPEED in `*TERMIOS-P' as both the input and
output speeds. The normal return value is 0; a value of -1
indicates an error. If SPEED is not a speed, `cfsetspeed' returns
-1. This function is an extension in 4.4 BSD.
- Data Type: speed_t
The `speed_t' type is an unsigned integer data type used to
represent line speeds.
The functions `cfsetospeed' and `cfsetispeed' report errors only for
speed values that the system simply cannot handle. If you specify a
speed value that is basically acceptable, then those functions will
succeed. But they do not check that a particular hardware device can
actually support the specified speeds--in fact, they don't know which
device you plan to set the speed for. If you use `tcsetattr' to set
the speed of a particular device to a value that it cannot handle,
`tcsetattr' returns -1.
*Portability note:* In the GNU library, the functions above accept
speeds measured in bits per second as input, and return speed values
measured in bits per second. Other libraries require speeds to be
indicated by special codes. For POSIX.1 portability, you must use one
of the following symbols to represent the speed; their precise numeric
values are system-dependent, but each name has a fixed meaning: `B110'
stands for 110 bps, `B300' for 300 bps, and so on. There is no
portable way to represent any speed but these, but these are the only
speeds that typical serial lines can support.
B0 B50 B75 B110 B134 B150 B200
B300 B600 B1200 B1800 B2400 B4800
B9600 B19200 B38400 B57600 B115200
B230400 B460800
BSD defines two additional speed symbols as aliases: `EXTA' is an
alias for `B19200' and `EXTB' is an alias for `B38400'. These aliases
are obsolete.
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