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# 10.3 Terminal I/O

As with signals, terminal I/O control has three different implementations under SVR4, BSD, and POSIX.1.

SVR4 uses the `termio` structure, and various ioctl calls (such as `TCSETA`, `TCGETA`, and so forth) on a terminal device to obtain and set parameters with the `termio` structure. This structure looks like:

      struct termio {
        unsigned short c_iflag;  /* Input modes */
        unsigned short c_oflag;  /* Output modes */
        unsigned short c_cflag;  /* Control modes */
        unsigned short c_lflag;  /* Line discipline modes */
        char c_line;             /* Line discipline */
        unsigned char c_cc[NCC]; /* Control characters */
      };

Under BSD, the `sgtty` structure is used with various ioctl calls, such as `TIOCGETP`, `TIOCSETP`, and so forth.

Under POSIX, the `termios` struct is used, along with various functions defined by POSIX.1, such as tcsetattr and `tcgetattr`. The `termios` structure is identical to `struct termio` used by SVR4, but the types are renamed (such as `tcflag_t` instead of `unsigned short`), and `NCCS` is used for the size of the `c_cc` array.

Under Linux, both POSIX.1 termios and SVR4 termio are supported directly by the kernel. This means that if your program uses either of these methods for accessing terminal I/O, it should compile directly under Linux. If you're ever in doubt, it's easy to modify code using `termio` to use `termios`, using a bit of knowledge of both methods. Hopefully, this shouldn't ever be necessary. But, do pay attention if a program attempts to use the `c_line` field in the `termio` structure. For nearly all applications, this should be `N_TTY`, and if the program assumes that some other line discipline is available you might have trouble.

If your program uses the BSD sgtty implementation, you can link against `libbsd.a` as described above. This will provide a replacement for ioctl which will resubmit the terminal I/O requests in terms of the POSIX `termios` calls used by the kernel. When compiling such a program, if symbols such as `TIOCGETP` are undefined, you will need to link against `libbsd`.