18.7. Baud Rate Functions

The term baud rate is 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 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
[View full width] int cfsetispeed(struct termios *termptr, speed_t speed); int cfsetospeed(struct termios *termptr, speed_t speed);
Both return: 0 if OK, 1 on error

The return value from the two cfget functions and the speed argument to the two cfset functions are one of the following constants: B50, B75, B110, B134, B150, B200, B300, B600, B1200, B1800, B2400, B4800, B9600, B19200, or B38400. The constant B0 means "hang up." When B0 is specified as the output baud rate when tcsetattr is called, the modem control lines are no longer asserted.

Most systems define additional baud rate values, such as B57600 and B115200.

To use these functions, we must realize that the input and output baud rates are stored in the device's termios structure, as shown in Figure 18.8. Before calling either of the cfget functions, we first have to obtain the device's termios structure using tcgetattr. Similarly, after calling either of the two cfset functions, all we've done is set the baud rate in a termios structure. For this change to affect the device, we have to call tcsetattr. If there is an error in either of the baud rates that we set, we may not find out about the error until we call tcsetattr.

The four baud rate functions exist to insulate applications from differences in the way that implementations represent baud rates in the termios structure. BSD-derived platforms tend to store baud rates as numeric values equal to the rates (i.e., 9,600 baud is stored as the value 9,600), whereas Linux and System Vderived platforms tend to encode the baud rate in a bitmask. The speed values we get from the cfget functions and pass to the cfset functions are untranslated from their representation as they are stored in the termios structure.