Chapter 4Interaction
This chapter describes the MDB interactive command-line
editing and history functions, the output pager, and debugger signal handling.
Command Re-entry
The text of the last HISTSIZE (default 128) commands entered from a terminal device
are saved in memory. The in-line editing facility, described next, provides
key mappings for searching and fetching elements from the history list.
In-line Editing
If standard input is a terminal device, MDB provides some simple emacs-style
facilities for editing
the command line. The search, previous,
and next commands in edit mode provide access to the history
list. Only strings, not patterns, are matched when searching. In the list
below, the notation for control characters is caret (^) followed by a character
shown in upper case. The notation for escape sequences is M- followed by a
character. For example, M-f (pronounced meta- eff) is entered by depressing
<ESC> followed by 'f', or by depressing Meta followed by 'f' on keyboards
that support a Meta key. A command line is committed and executed using RETURN
or NEWLINE. The edit commands are:
| ^F | Move cursor forward (right) one
character.
| | M-f | Move cursor forward one word.
| | ^B | Move cursor backward (left) one
character.
| | M-b | Move cursor backward one word.
| | ^A | Move cursor to start of line.
| | ^E | Move cursor to end of line.
| | ^D | Delete current character, if
the current line is not empty. If the current line is empty, ^D denotes EOF
and the debugger will exit.
| | M-^H | (Meta-backspace) Delete previous
word.
| | ^K | Delete from the cursor to the
end of the line.
| | ^L | Reprint the current line.
| | ^T | Transpose current character with
next character.
| | ^N | Fetch the next command from the
history. Each time ^N is entered, the next command forward in time is retrieved.
| | ^P | Fetch the previous command from
the history. Each time ^P is entered, the next command backward in time is
retrieved.
| | ^R[string] | Search backward in the
history for a previous command line containing string. The string should be
terminated by a RETURN or NEWLINE. If string is omitted, the previous history
element containing the most recent string is retrieved.
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The editing mode also interprets the following user-defined
sequences as editing commands. User defined sequences can be read or
modified using the stty(1) command.
| erase | User defined erase character
(usually ^H or ^?). Delete previous character.
| | intr | User defined interrupt character
(usually ^C). Abort the current command and print a new prompt.
| | kill | User defined kill character
(usually ^U). Kill the entire current command line.
| | quit | User defined quit character
(usually ^\). Quit the debugger.
| | suspend | User defined suspend character
(usually ^Z). Suspend the debugger.
| | werase | User defined word erase character
(usually ^W). Erase the preceding word.
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On keyboards that support an extended keypad with arrow keys, mdb will interpret these keystrokes as editing commands:
| up-arrow | Fetch the previous command
from the history (same as ^P).
| | down-arrow | Fetch the next command
from the history (same as ^N).
| | left-arrow | Move cursor backward
one character (same as ^B).
| | right-arrow | Move cursor forward
one character (same as ^F)
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Output Pager
mdb provides a built-in output pager. The output pager is enabled if the
debugger's standard output is a terminal device. Each time a command is executed, mdb will pause after one screenful of output is produced and will
display a pager prompt:
>> More [<space>, <cr>, q, n, c, a] ?
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The following key sequences are recognized by the pager:
| SPACE | Display the next screenful
of output.
| | a, A | Abort the current top-level
command and return to the prompt.
| | c, C | Continue displaying output
without pausing at each screenful until the current top-level command is complete.
| | n, N, NEWLINE, RETURN | Display the
next line of output.
| | q, Q, ^C, ^\ | Quit (abort) the current dcmd only.
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Signal Handling
The debugger ignores the PIPE and QUIT signals. The INT signal aborts the command that is currently
executing. The debugger intercepts and provides special handling for the ILL,
TRAP, EMT, FPE, BUS, and SEGV signals. If any of these signals are generated
asynchronously (that is, delivered from another process using kill(2)), mdb will restore the signal to its default disposition and
dump core. However, if any of these signals are generated synchronously by
the debugger process itself and a dcmd from an externally loaded dmod is currently executing, and standard input is a terminal, mdb will provide a menu of choices allowing the user to force a
core dump, quit without producing a core dump, stop for attach by a debugger,
or attempt to resume. The resume option will abort all active commands and
unload the dmod whose dcmd was active at the time the fault
occurred. It can then be subsequently re-loaded by the user. The resume option
provides limited protection against buggy dcmds. Refer to Warnings,
Use of the Error Recovery Mechanism, for information about the risks associated
with the resume option.
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