Section 18.4. Getting and Setting Terminal Attributes

18.4. Getting and Setting Terminal Attributes

To get and set a termios structure, we call two functions: tcgetattr and tcsetattr. This is how we examine and modify the various option flags and special characters to make the terminal operate the way we want it to.

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 #include <termios.h> int tcgetattr(int filedes, struct termios *termptr); int tcsetattr(int filedes, int opt, const struct  termios *termptr); 

Both return: 0 if OK, 1 on error

Both functions take a pointer to a termios structure and either return the current terminal attributes or set the terminal's attributes. Since these two functions operate only on terminal devices, errno is set to ENOTTY and 1 is returned if filedes does not refer to a terminal device.

The argument opt for tcsetattr lets us specify when we want the new terminal attributes to take effect. This argument is specified as one of the following constants.

TCSANOW	The change occurs immediately.
TCSADRAIN	The change occurs after all output has been transmitted. This option should be used if we are changing the output parameters.
TCSAFLUSH	The change occurs after all output has been transmitted. Furthermore, when the change takes place, all input data that has not been read is discarded (flushed).

The return status of tcsetattr confuses the programming. This function returns OK if it was able to perform any of the requested actions, even if it couldn't perform all the requested actions. If the function returns OK, it is our responsibility to see whether all the requested actions were performed. This means that after we call tcsetattr to set the desired attributes, we need to call tcgetattr and compare the actual terminal's attributes to the desired attributes to detect any differences.