Special characters

The special characters described below are assigned at task initialization to program keys by a precursor task. They are associated with certain special functions on input and/or output. Cases where the association between a character and function cannot be changed are indicated by enclosing the relevant character in parentheses:

INTR	Special character on input, which is recognized if the ISIG flag is set. It generates a SIGINT signal (interrupt) which is sent to all processes in the foreground process group associated with the terminal. If the ISIG flag is set, the INTR character is discarded when processed. Under normal circumstances, this results in the termination of all these processes; however, arrangements may be made to ignore the signal or initiate a jump to a previously defined address location (see sigaction() and signal()).
QUIT	Special character on input, which is recognized if the ISIG flag is set. It generates a SIGQUIT signal (quit) which is sent to all processes in the foreground process group associated with the terminal. If ISIG is set, the QUIT character is discarded when processed. Its treatment is almost identical to the interrupt signal SIGINT, except that, if a receiving process has not made other arrangements, the process will not only be terminated, but a core dump will be generated (see sigaction()).
ERASE	Special character on input, which is recognized if the ICANON flag is set. It erases the preceding character, but not beyond the start of a line, i.e. an NL, EOF or EOL character (see also the section “Canonical mode inputprocessing”). If ICANON is set, the ERASE character is discarded when processed.

KILL	Special character on input, which is recognized if the ICANON flag is set. It deletes the entire line as of the last NL, EOF or EOL character. If ICANON is set, the KILL character is discarded when processed. This character is not supported on BS2000 block terminals.
EOF	Special character on input, which is recognized if the ICANON flag is set. When EOF is received, all the bytes waiting to be read are immediately passed to the program without waiting for an NL character, and the EOF is discarded. If no bytes are present, i.e. the EOF is at the beginning of a line, read() returns a value of 0. A return value of 0 for a read operation is the default end-of-file identifier. If ICANON is set, the EOF character is discarded when processed.
NL	Special character on input, which is recognized if the ICANON flag is set. NL is the standard line delimiter \n and cannot be changed.
EOL	Special character on input, which is recognized if the ICANON flag is set. EOL is an additional line delimiter and serves the same function as NL. It is normally not used.
SUSP	If job control is supported (section “Special control characters”) on an X/Open-compatible system, the SUSP special character is recognized on input. If the ISIG flag is set, receipt of the SUSP character causes a SIGTSTP signal to be sent to all processes in the foreground process group associated with the terminal. After it is processed, the SUSP character is discarded as well. This character has no effect in the POSIX subsystem, since job control is not supported in its current implementation.
STOP	Special character on both input and output, which is recognized if either the IXON (for input) or IXOFF (for output) flag is set. STOP can be used to suspend output temporarily. It is useful with CRT terminals to prevent output from disappearing before it can be read. If IXON is set, the STOP character is discarded when processed. So long as the output is suspended, additional STOP characters are ignored and not read. The STOP character can be neither changed nor escaped. This character is not supported on BS2000 block terminals.
START	Special character on both input and output, which is recognized if either the IXON (for input) or IXOFF (for output) flag is set. The START character is used to resume output that has been suspended by a STOP character. So long as output continues, subsequent START characters are ignored and not read. If IXON is set, the START character is discarded when processed. The START character can be neither changed nor escaped. This character is not supported on BS2000 block terminals.

CR	Special character on input, which is recognized if the ICANON flag is set. This character corresponds to the character \r. If ICANON and ICRNL are set, and IGNCR is not, this character is translated into an NL, and has the same effect as an NL character. The character CR cannot be changed.

The values for INTR, QUIT, ERASE, KILL, EOF, EOL and SUSP (job control only) can be changed by the user.

If two or more special characters have the same value, the function performed when that character is received is undefined.

The ERASE, KILL and EOF characters can be escaped by a preceding \ (escape character), in which case the function associated with it is not executed.

The user may overwrite key assignments at any time. In such cases, the default XPG4 Version 2-conformant key assignments on the BS2000 command level can be restored with/RESTORE-CONTROL-KEYS.