Advanced Programming in the UNIX Environment: Second Edition [Electronic resources] نسخه متنی

Figure 18.1. Logical picture of input and output queues for a terminal device

Figure 2.11), is finite. When the input queue for a particular device fills, the system behavior is implementation dependent. Most UNIX systems echo the bell character when this happens.

Most UNIX systems implement all the canonical processing in a module called the

terminal line discipline . We can think of this module as a box that sits between the kernel's generic read and write functions and the actual device driver (see Figure 18.2).

Note the similarity of this picture and the diagram of a stream shown in Figure 14.14. We'll return to this picture in Chapter 19, when we discuss pseudo terminals.

All the terminal device characteristics that we can examine and change are contained in a termios structure. This structure is defined in the header <termios.h>, which we use throughout this chapter:

struct termios {
tcflag_t c_iflag; /* input flags */
tcflag_t c_oflag; /* output flags */
tcflag_t c_cflag; /* control flags */
tcflag_t c_lflag; /* local flags */
cc_t c_cc[NCCS]; /* control characters */
};

Roughly speaking, the input flags control the input of characters by the terminal device driver (strip eighth bit on input, enable input parity checking, etc.), the output flags control the driver output (perform output processing, map newline to CR/LF, etc.), the control flags affect the RS-232 serial lines (ignore modem status lines, one or two stop bits per character, etc.), and the local flags affect the interface between the driver and the user (echo on or off, visually erase characters, enable terminal-generated signals, job control stop signal for background output, etc.).

The type tcflag_t is big enough to hold each of the flag values and is often defined as an unsigned int or an unsigned long. The c_cc array contains all the special characters that we can change. NCCS is the number of elements in this array and is typically between 15 and 20 (since most implementations of the UNIX System support more than the 11 POSIX-defined special characters). The cc_t type is large enough to hold each special character and is typically an unsigned char.

Versions of System V that predated the POSIX standard had a header named <termio.h> and a structure named termio. POSIX.1 added an s to the names, to differentiate them from their predecessors.

Section 18.5.

Section 9.7.)

Figure 18.7. Summary of terminal I/O functions

Note that the Single UNIX Specification doesn't use the classic ioctl on terminal devices. Instead, it uses the 13 functions shown in Figure 18.7. The reason is that the ioctl function for terminal devices uses a different data type for its final argument, which depends on the action being performed. This makes type checking of the arguments impossible.

Although only 13 functions operate on terminal devices, the first two functions in Figure 18.7 (tcgetattr and tcsetattr) manipulate almost 70 different flags (see Figures 18.3 through 18.6). The handling of terminal devices is complicated by the large number of options available for terminal devices and trying to determine which options are required for a particular device (be it a terminal, modem, printer, or whatever).

The relationships among the 13 functions shown in Figure 18.7 are shown in Figure 18.8.