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

18.3. Special Input Characters

POSIX.1 defines 11 characters that are handled specially on input. Implementations define additional special characters. Figure 18.9 summarizes these special characters.

Figure 18.9. Summary of special terminal input characters

Of the 11 POSIX.1 special characters, we can change 9 of them to almost any value that we like. The exceptions are the newline and carriage return characters (\n and \r, respectively) and perhaps the STOP and START characters (depends on the implementation). To do this, we modify the appropriate entry in the c_cc array of the termios structure. The elements in this array are referred to by name, with each name beginning with a V (the third column in Figure 18.9).

POSIX.1 allows us to disable these characters. If we set the value of an entry in the c_cc array to the value of _POSIX_VDISABLE, then we disable the corresponding special character.

In older versions of the Single UNIX Specification, support for _POSIX_VDISABLE was optional. It is now required.

All four platforms discussed in this text support this feature. Linux 2.4.22 and Solaris 9 define _POSIX_VDISABLE as 0; FreeBSD 5.2.1 and Mac OS X 10.3 define it as 0xff.

Some earlier UNIX systems disabled a feature if the corresponding special input character was 0.

Example

Section 18.9) to check this.

Figure 18.10. Disable interrupt character and change end-of-file character

Figure 9.7). But the delayed-suspend character generates a signal only when a process reads from the controlling terminal, not when the character is typed. This character is discarded when processed (i.e., it is not passed to the process).

EOF

The end-of-file character. This character is recognized on input in canonical mode (ICANON). When we type this character, all bytes waiting to be read are immediately passed to the reading process. If no bytes are waiting to be read, a count of 0 is returned. Entering an EOF character at the beginning of the line is the normal way to indicate an end of file to a program. This character is discarded when processed in canonical mode (i.e., it is not passed to the process).

EOL

The additional line delimiter character, like NL. This character is recognized on input in canonical mode (ICANON) and is returned to the reading process; however, this character is not normally used.

EOL2

Another line delimiter character, like NL. This character is treated identically to the EOL character.

ERASE

The erase character (backspace). This character is recognized on input in canonical mode (ICANON) and erases the previous character in the line, not erasing beyond the beginning of the line. This character is discarded when processed in canonical mode (i.e., it is not passed to the process).

ERASE2

The alternate erase character (backspace). This character is treated exactly like the erase character (ERASE).

INTR

The interrupt character. This character is recognized on input if the ISIG flag is set and generates the SIGINT signal that is sent to all processes in the foreground process group (refer to Figure 9.7). This character is discarded when processed (i.e., it is not passed to the process).

KILL

The kill character. (The name "kill" is overused; recall the kill function used to send a signal to a process. This character should be called the line-erase character; it has nothing to do with signals.) It is recognized on input in canonical mode (ICANON). It erases the entire line and is discarded when processed (i.e., it is not passed to the process).

LNEXT

The literal-next character. This character is recognized on input in extended mode (IEXTEN) and causes any special meaning of the next character to be ignored. This works for all special characters listed in this section. We can use this character to type any character to a program. The LNEXT character is discarded when processed, but the next character entered is passed to the process.

NL

The newline character, which is also called the line delimiter. We cannot change this character. This character is recognized on input in canonical mode (ICANON). This character is returned to the reading process.

QUIT

The quit character. This character is recognized on input if the ISIG flag is set. The quit character generates the SIGQUIT signal, which is sent to all processes in the foreground process group (refer to Figure 9.7). This character is discarded when processed (i.e., it is not passed to the process).

Recall from Figure 10.1 that the difference between INTR and QUIT is that the QUIT character not only terminates the process by default, but also generates a core file.

REPRINT

The reprint character. This character is recognized on input in extended, canonical mode (both IEXTEN and ICANON flags set) and causes all unread input to be output (reechoed). This character is discarded when processed (i.e., it is not passed to the process).

START

The start character. This character is recognized on input if the IXON flag is set and is automatically generated as output if the IXOFF flag is set. A received START character with IXON set causes stopped output (from a previously entered STOP character) to restart. In this case, the START character is discarded when processed (i.e., it is not passed to the process).

When IXOFF is set, the terminal driver automatically generates a START character to resume input that it had previously stopped, when the new input will not overflow the input buffer.

Figure 9.7). Additionally, if the NOKERNINFO flag is not set, status information on the foreground process group is also displayed on the terminal. This character is discarded when processed (i.e., it is not passed to the process).

STOP

The stop character. This character is recognized on input if the IXON flag is set and is automatically generated as output if the IXOFF flag is set. A received STOP character with IXON set stops the output. In this case, the STOP character is discarded when processed (i.e., it is not passed to the process). The stopped output is restarted when a START character is entered.

When IXOFF is set, the terminal driver automatically generates a STOP character to prevent the input buffer from overflowing.

SUSP

The suspend job-control character. This character is recognized on input if job control is supported and if the ISIG flag is set. The suspend character generates the SIGTSTP signal, which is sent to all processes in the foreground process group (refer to Figure 9.7). This character is discarded when processed (i.e., it is not passed to the process).

WERASE

The word-erase character. This character is recognized on input in extended, canonical mode (both IEXTEN and ICANON flags set) and causes the previous word to be erased. First, it skips backward over any white space (spaces or tabs), then backward over the previous token, leaving the cursor positioned where the first character of the previous token was located. Normally, the previous token ends when a white space character is encountered. We can change this, however, by setting the ALTWERASE flag. This flag causes the previous token to end when the first nonalphanumeric character is encountered. The word-erase character is discarded when processed (i.e., it is not passed to the process).