Loading and saving recordings¶

When suitably licensed, UDB can load and save LiveRecorder recordings.

Loading recordings¶

Loading a LiveRecorder recording loads the execution history of the program loaded from the recording file, including any debug symbols, so that it is ready to be replayed. It also loads bookmarks created by the same user when previously replaying the recording.

Terminal Visual Studio Code

Load a recording using the uload command in UDB, or by specifying the filename on the UDB command line.

Loading a recording from the command line¶

You can specify a LiveRecorder recording when starting UDB. When a recording is passed as a command-line argument to udb, it is automatically loaded. For example:

$ udb recording.undo

The debugged program is at the beginning of recorded history. Start debugging
from here or, to proceed towards the end, use:
    continue - to replay from the beginning
    ugo end  - to jump straight to the end of history

No more reverse-execution history.
start 1> continue
Continuing.

Program received signal SIGABRT, Aborted.
Have reached end of recorded history.

The recorded program has exited.
You can use UDB reverse commands to go backwards; see "help udb" for details.

__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
end 225,575> backtrace
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007fca9fdd4859 in __GI_abort () at abort.c:79
#2  0x000055821417d3ca in main () at cache.c:85

uload [-e|-goto-end] filename¶

Load a LiveRecorder recording.

The filename argument names the recording file.

For example:

$ udb
not running> uload recording.undo

The debugged program is at the beginning of recorded history. Start debugging
from here or, to proceed towards the end, use:
    continue - to replay from the beginning
    ugo end  - to jump straight to the end of history

No more reverse-execution history.
0x000055821417d0c0 in _start ()
start 1 at-start> continue
Continuing.

Program received signal SIGABRT, Aborted.
Have reached end of recorded history.

The recorded program has exited.
You can use UDB reverse commands to go backwards; see "help udb" for details.

__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.

-e, -goto-end¶

Jump to the end of execution history immediately after loading. This has the same effect as the ugo end command, but can be substantially faster.

$ udb
not running> uload -e recording.undo

No more reverse-execution history.
0x000055821417d0c0 in _start ()

Imported 2 bookmarks from the previous session. Use "info bookmarks" to list
them.


Performing "ugo end" to jump to the end of the recording. Press ^C to
interrupt this and go back to the start.

Have reached end of recorded history.

info recording-version¶

The Undo version used to create the loaded LiveRecorder recording.

For example:
end 225,575 at-end> info recording-version
6.9.0-dev.g678fcb52d7ca

Going to the end of execution history after loading¶

By default, UDB starts replaying the recording from the beginning of execution history, but when investigating a crash it is often more convenient to start at the end.

Use the udb --load-goto-end command-line option to start debugging the loaded recording from the end of execution history. This is significantly faster than loading the recording and issuing the ugo end command as separate steps. For example:

$ udb --load-goto-end recording.undo

No more reverse-execution history.


Performing "ugo end" to jump to the end of the recording. Press ^C to
interrupt this and go back to the start.

Have reached end of recorded history.
end 225,575 at-end> backtrace
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007fca9fdd4859 in __GI_abort () at abort.c:79
#2  0x000055821417d3ca in main () at cache.c:85

This can be configured for all UDB sessions by setting UNDO_load_goto_end=true in the environment.

Replaying on a different CPU¶

Replaying a recording that was created on a different CPU requires that the replay CPU supports all the instructions used by the program when it was recorded. UDB refuses to load a recording if these requirements cannot be satisfied, although a warning may be shown instead when loading recordings made with older versions of LiveRecorder.

The info replayability command shows the CPU features required to replay the loaded recording.

info replayability¶

The CPU features required to replay the loaded LiveRecorder recording.

The features are reported in the same style as they appear in the flags fields of /proc/cpuinfo. A feature is listed if the program executed an instruction requiring that feature when it was originally recorded. Any hardware which supports all of the listed features can be used to replay this recording.

For example:
end 225,575 at-end> info replayability
The CPU features required to replay 'recording.undo' are: avx avx2 bmi1 lm sse
sse2 xsave.

If you have not yet added a launch configuration to your workspace for replaying a LiveRecorder recording, then from the Run menu, select Add Configuration… and then C/C++: (UDB) Replay a LiveRecorder recording.

From the View menu, select Run (Ctrl+Shift+D) and from the drop-down menu at the top of the side bar, select (UDB) Replay a LiveRecorder recording.

Visual Studio Code pops up a file selection dialog. Select the LiveRecorder recording to replay. Note that the recording must have a filename with the extension .undo.

Going to the start of execution history after loading¶

By default, UDB starts replaying the recording from the end of execution history. At the end of execution history, a program that called a function such as exit() or abort() will be stopped in library code, typically with source code unavailable.

In this case, use the Reverse Step Out button in the toolbar (Ctrl+Shift+F11) to execute backward out of the library functions, until you reach a stack frame for which you have source code.

The end of execution history is usually the most convenient place to start debugging when investigating a failure, but if not, set the "stopAtEntry" field to true in the “(UDB) Replay a LiveRecorder recording” configuration. This causes UDB to start replaying from the beginning of execution history, stopping at the beginning of main() if this was captured in the recording.

Saving recordings¶

Terminal Visual Studio Code

usave [filename]¶

Save execution history as a LiveRecorder recording.

The optional filename argument names the recording file. If omitted, a suitable filename is automatically generated.

The recording can be loaded into UDB using the uload command.

Recording File Locking¶

Recordings are locked while they are being written in order to prevent other Undo processes from accessing them before they are completed.

Attempts to save a recording to a file path that is already being saved to will fail:

$ live-record -o recording.undo true
live-record: Maximum event log size is 1G
live-record: Saving to 'recording.undo'...
live-record: Failed to save recording recording.undo: Recording is being written in another process.
live-record: Detaching...

Attempts to load a recording that is being saved to will also fail:

$ udb recording.undo
ERROR: Failed to load LiveRecorder recording
       because: Recording is being written in another process.

New in version 6.5.