Advanced Programming in the UNIX® Environment

ptg10805159

692 Te rminal I/O Chapter 18

Option names preceded by a hyphen aredisabled. The last four lines display the

current settings for each of the terminal special characters (Section 18.3). The first line

displays the number of rows and columns for the current terminal window; we discuss

the terminal window size in Section 18.12.

Thesttycommand uses its standardinput to get and set the terminal option flags. Although

some older implementations used standardoutput, POSIX.1 requires that the standardinput

be used. All four implementations discussed in this text provide versions ofsttythat operate

on the standardinput. This means that we can type

stty -a </dev/tty1a

if we areinterested in discovering the settings on the terminal namedtty1a.

18.7 Baud Rate Functions

The termbaud rateis a historical term that should be referred to today as ‘‘bits per

second.’’Although most terminal devices use the same baud rate for both input and

output, the capability exists to set the two rates to different values, if the hardware

allows this.

#include <termios.h>

speed_t cfgetispeed(const struct termios *termptr);

speed_t cfgetospeed(const struct termios *termptr);

Both return: baud rate value

int cfsetispeed(struct termios *termptr,speed_tspeed);

int cfsetospeed(struct termios *termptr,speed_tspeed);

Both return: 0 if OK,−1 on error

The return value from the twocfgetfunctions and thespeedargument to the two

cfsetfunctions areone of the following constants:B50,B75,B110,B134,B150,B200,

B300,B600,B1200,B1800,B2400,B4800,B9600,B19200,orB38400.The constant

B0means ‘‘hang up.’’IfB0is specified as the output baud rate whentcsetattris

called, the modem control lines are no longer asserted.

Most systems define additional baud rate values, such asB57600andB115200.

To use these functions, we must realize that the input and output baud rates are

stored in the device’stermiosstructure, as shown in Figure18.8. Beforecalling either

of thecfgetfunctions, we first have to obtain the device’stermiosstructureusing

tcgetattr.Similarly,after calling either of the twocfsetfunctions, all we’ve done is

set the baud rate in atermiosstructure. For this change to affect the device, we have

to calltcsetattr.Ifthere is an error in either of the baud rates that we set, we may

not find out about the error until we calltcsetattr.