(libc.info.gz) Line Speed

 
 17.4.8 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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