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2008-10-17 Michael Snyder <msnyder@vmware.com> 

* gdb.texinfo: Add documentation for reverse execution.
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Michael Snyder 2008-10-17 19:44:17 +00:00
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gdb/doc/ChangeLog
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@ -1,3 +1,7 @@
2008-10-17 Michael Snyder <msnyder@vmware.com>
* gdb.texinfo: Add documentation for reverse execution.
2008-10-16 Thiago Jung Bauermann <bauerman@br.ibm.com>
Eli Zaretskii <eliz@gnu.org>

147
gdb/doc/gdb.texinfo
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@ -143,6 +143,7 @@ software in general. We will miss him.
* Commands:: @value{GDBN} commands
* Running:: Running programs under @value{GDBN}
* Stopping:: Stopping and continuing
* Reverse Execution:: Running programs backward
* Stack:: Examining the stack
* Source:: Examining source files
* Data:: Examining data
@ -4850,6 +4851,126 @@ When such an event happens, a system call in another thread may return
prematurely, even though your program does not appear to stop.
@node Reverse Execution
@chapter Running programs backward
@cindex reverse execution
@cindex running programs backward
When you are debugging a program, it is not unusual to realize that
you have gone too far, and some event of interest has already happened.
If the target environment supports it, @value{GDBN} can allow you to
``rewind'' the program by running it backward.
A target environment that supports reverse execution should be able
to ``undo'' the changes in machine state that have taken place as the
program was executing normally. Variables, registers etc.@: should
revert to their previous values. Obviously this requires a great
deal of sophistication on the part of the target environment; not
all target environments can support reverse execution.
When a program is executed in reverse, the instructions that
have most recently been executed are ``un-executed'', in reverse
order. The program counter runs backward, following the previous
thread of execution in reverse. As each instruction is ``un-executed'',
the values of memory and/or registers that were changed by that
instruction are reverted to their previous states. After executing
a piece of source code in reverse, all side effects of that code
should be ``undone'', and all variables should be returned to their
prior values@footnote{
Note that some side effects are easier to undo than others. For instance,
memory and registers are relatively easy, but device I/O is hard. Some
targets may be able undo things like device I/O, and some may not.
The contract between @value{GDBN} and the reverse executing target
requires only that the target do something reasonable when
@value{GDBN} tells it to execute backwards, and then report the
results back to @value{GDBN}. Whatever the target reports back to
@value{GDBN}, @value{GDBN} will report back to the user. @value{GDBN}
assumes that the memory and registers that the target reports are in a
consistant state, but @value{GDBN} accepts whatever it is given.
}.
If you are debugging in a target environment that supports
reverse execution, @value{GDBN} provides the following commands.
@table @code
@kindex reverse-continue
@kindex rc @r{(@code{reverse-continue})}
@item reverse-continue @r{[}@var{ignore-count}@r{]}
@itemx rc @r{[}@var{ignore-count}@r{]}
Beginning at the point where your program last stopped, start executing
in reverse. Reverse execution will stop for breakpoints and synchronous
exceptions (signals), just like normal execution. Behavior of
asynchronous signals depends on the target environment.
@kindex reverse-step
@kindex rs @r{(@code{step})}
@item reverse-step @r{[}@var{count}@r{]}
Run the program backward until control reaches the start of a
different source line; then stop it, and return control to @value{GDBN}.
Like the @code{step} command, @code{reverse-step} will only stop
at the beginning of a source line. It ``un-executes'' the previously
executed source line. If the previous source line included calls to
debuggable functions, @code{reverse-step} will step (backward) into
the called function, stopping at the beginning of the @emph{last}
statement in the called function (typically a return statement).
Also, as with the @code{step} command, if non-debuggable functions are
called, @code{reverse-step} will run thru them backward without stopping.
@kindex reverse-stepi
@kindex rsi @r{(@code{reverse-stepi})}
@item reverse-stepi @r{[}@var{count}@r{]}
Reverse-execute one machine instruction. Note that the instruction
to be reverse-executed is @emph{not} the one pointed to by the program
counter, but the instruction executed prior to that one. For instance,
if the last instruction was a jump, @code{reverse-stepi} will take you
back from the destination of the jump to the jump instruction itself.
@kindex reverse-next
@kindex rn @r{(@code{reverse-next})}
@item reverse-next @r{[}@var{count}@r{]}
Run backward to the beginning of the previous line executed in
the current (innermost) stack frame. If the line contains function
calls, they will be ``un-executed'' without stopping. Starting from
the first line of a function, @code{reverse-next} will take you back
to the caller of that function, @emph{before} the function was called,
just as the normal @code{next} command would take you from the last
line of a function back to its return to its caller
@footnote{Unles the code is too heavily optimized.}.
@kindex reverse-nexti
@kindex rni @r{(@code{reverse-nexti})}
@item reverse-nexti @r{[}@var{count}@r{]}
Like @code{nexti}, @code{reverse-nexti} executes a single instruction
in reverse, except that called functions are ``un-executed'' atomically.
That is, if the previously executed instruction was a return from
another instruction, @code{reverse-nexti} will continue to execute
in reverse until the call to that function (from the current stack
frame) is reached.
@kindex reverse-finish
@item reverse-finish
Just as the @code{finish} command takes you to the point where the
current function returns, @code{reverse-finish} takes you to the point
where it was called. Instead of ending up at the end of the current
function invocation, you end up at the beginning.
@kindex set exec-direction
@item set exec-direction
Set the direction of target execution.
@itemx set exec-direction reverse
@cindex execute forward or backward in time
@value{GDBN} will perform all execution commands in reverse, until the
exec-direction mode is changed to ``forward''. Affected commands include
@code{step, stepi, next, nexti, continue, and finish}. The @code{return}
command cannot be used in reverse mode.
@item set exec-direction forward
@value{GDBN} will perform all execution commands in the normal fashion.
This is the default.
@end table
@node Stack
@chapter Examining the Stack
@ -24565,6 +24686,22 @@ breakpoint at @var{addr}.
Don't use this packet. Use the @samp{Z} and @samp{z} packets instead
(@pxref{insert breakpoint or watchpoint packet}).
@item bc
@cindex @samp{bc} packet
Backward continue. Execute the target system in reverse. No parameter.
@xref{Reverse Execution}, for more information.
Reply:
@xref{Stop Reply Packets}, for the reply specifications.
@item bs
@cindex @samp{bs} packet
Backward single step. Execute one instruction in reverse. No parameter.
@xref{Reverse Execution}, for more information.
Reply:
@xref{Stop Reply Packets}, for the reply specifications.
@item c @r{[}@var{addr}@r{]}
@cindex @samp{c} packet
Continue. @var{addr} is address to resume. If @var{addr} is omitted,
@ -25182,6 +25319,16 @@ hex.
The packet indicates that the loaded libraries have changed.
@value{GDBN} should use @samp{qXfer:libraries:read} to fetch a new
list of loaded libraries. @var{r} is ignored.
@cindex replay log events, remote reply
@item replaylog
The packet indicates that the target cannot continue replaying
logged execution events, because it has reached the end (or the
beginning when executing backward) of the log. The value of @var{r}
will be either @samp{begin} or @samp{end}. @xref{Reverse Execution},
for more information.
@end table
@item W @var{AA}