GDB print to file instead of stdout

Question

I am running GDB and want to examine one of those unfortunate god objects. It takes many pages (and I have a 24" monitor turned sideways!) to see the whole thing.

For ease of use, I'd like GDB to print the object to a file instead of the screen so that I can open it in vi and move around with ease.

With all of GDB's versatility, there must be a way to do this, right?

score 201 · Accepted Answer · edited Dec 14 '21 at 19:19

201

You need to enable logging:

(gdb) set logging on

Now GDB will log to ./gdb.txt. You can tell it which file to use:

(gdb) set logging file my_god_object.log

And you can examine the current logging configuration:

(gdb) show logging

edited Dec 14 '21 at 19:19

Matthias Braun

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answered May 09 '11 at 19:04

Tadeusz A. Kadłubowski

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And if you want the output to go _only_ to the log file, use `set logging redirect on`. – Ben C Sep 05 '16 at 08:30
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Shouldn't we set `set logging file my_god_object.log` before `set logging on`? – Herpes Free Engineer Jan 02 '19 at 13:33
This plus `tail -f` and `awk` was really helpful today. Thanks! – remcycles Apr 05 '19 at 15:49

score 22 · Answer 2 · answered Apr 24 '16 at 09:54

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Extending on @qubodup's answer

gdb core.3599 -ex bt -ex quit |& tee backtrace.log

the -ex switch runs a gdb command. So the above loads the core file, runs bt command, then quit command. Output is written to backtrace.log and also on the screen.

Another useful gdb invocation (giving stacktrace with local variables from all threads) is

gdb core.3599 -ex 'thread apply all bt full' -ex quit

answered Apr 24 '16 at 09:54

user7610

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On some systems, you need to type `gdb -c core.3599 ...` – user7610 Jun 10 '16 at 08:31
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> gdb core.3599 -ex 'thread apply all bt full' -ex quit Add --batch to run without a prompt -eg sudo gdb --batch core.3599 -ex 'thread apply all bt full' -ex quit > output.log – David Skelly Dec 08 '19 at 20:27

score 21 · Answer 3 · answered Oct 31 '13 at 19:31

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I've found that you can redirect the output from gdb to a file via the run command:

(gdb) run > outfile

answered Oct 31 '13 at 19:31

Lucas Gabriel Sánchez

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That would be the output from the program being debugged, not the output from gdb itself. OP wanted to log gdb's own output. – the paul May 06 '15 at 15:08
see also: http://stackoverflow.com/questions/2388561/block-output-of-debugged-program-gdb – Ciro Santilli OurBigBook.com Feb 17 '16 at 15:25
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@thepaul but this is in fact very useful as I'm debugging a QT program that just trashes gdb's stdin with it's QDebug garbage – rostamn739 Jun 16 '16 at 14:47
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@rostamn739 oops, didn't help at all – rostamn739 Jun 16 '16 at 14:50
```run &> outfile``` is sometimes needed to capture the complete output. Noticed while using GoogleTest that all lines weren't getting added (unless the stderr was also redirected with &>) – Ambareesh Dec 09 '21 at 03:24

score 16 · Answer 4 · edited Apr 13 '19 at 09:14

From https://sourceware.org/gdb/onlinedocs/gdb/Logging-Output.html:

You may want to save the output of gdb commands to a file. There are several commands to control gdb's logging.
set logging on
Enable logging.
set logging off
Disable logging.
set logging file file
Change the name of the current logfile. The default logfile is gdb.txt.
set logging overwrite [on|off]
By default, gdb will append to the logfile. Set overwrite if you want set logging on to overwrite the logfile instead.
set logging redirect [on|off]
By default, gdb output will go to both the terminal and the logfile. Set redirect if you want output to go only to the log file.
show logging
Show the current values of the logging settings.

Copy paste of https://sourceware.org/gdb/onlinedocs/gdb/Logging-Output.html without citation. — Herpes Free Engineer, Jan 02 '19 at 13:34

score 9 · Answer 5 · answered Dec 11 '15 at 04:09

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A simple method to log gdb to a file while still seeing the output (which eases writing commands) is to use tee:

gdb command |& tee gdb.log

answered Dec 11 '15 at 04:09

qubodup

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Note that one cannot use `tui enable` or `layout` commands in this mode. `gdb` says `Cannot enable the TUI when output is not a terminal` – codeman48 Feb 24 '22 at 08:39

Niko · Answer 6 · 2019-02-13T10:29:37.583

Although there are many good answers here, I still have to post the only thing that worked for me:

[niko@my-laptop]# gdb MyBinary 2>&1 log.txt

This was the only way to get gdb and binary output into the same log.txt file, while also seeing it on the console.

EDIT:

Caution: Output seems to be partially not synced among the gdb output and the binary output. Can someone confirm? You might want to check whether your telnet/ssh client has a function to log the output that you see in your console.

score 2 · Answer 7 · answered Mar 19 '20 at 10:55

You've had multiple answers here. They are corrects. I just want to add a command that will help you to collect all the output at once. This really helpful when your collecting a huge backtrace. Before doing any logging configuration, do this:

(gdb)set height 0

I found it on this article: https://askaralikhan.blogspot.com/2016/05/gdb-all-threads-bt-to-file.html?showComment=1584614942454#c4584028195226351332

score 2 · Answer 8 · answered Sep 15 '20 at 12:06

I had a backtrace that was so long (over 100k lines) that holding down the enter key was taking too long. I found a solution to that:

Andreas Schneider's bt command writes a backtrace to file without any user interaction – just prefix your command with bt .

Here, I've turned it into a script:

#!/usr/bin/env bash
ex=(
    -ex "run"
    -ex "set logging overwrite on" 
    -ex "set logging file gdb.bt" 
    -ex "set logging on" 
    -ex "set pagination off"
    -ex "handle SIG33 pass nostop noprint"
    -ex "echo backtrace:\n"
    -ex "backtrace full"
    -ex "echo \n\nregisters:\n"
    -ex "info registers"
    -ex "echo \n\ncurrent instructions:\n"
    -ex "x/16i \$pc"
    -ex "echo \n\nthreads backtrace:\n"
    -ex "thread apply all backtrace"
    -ex "set logging off"
    -ex "quit"
)
echo 0 | gdb -batch-silent "${ex[@]}" --args "$@"

MiniMax · Answer 9 · 2022-07-22T19:01:27.707

Redirection of a gdb's command output to a file can be made by using Python (my version of gdb is 8.1.1):

(gdb) python
>with open("/path/to/file/bt.txt", "w") as outfile:
> outfile.write(gdb.execute("bt", to_string=True))
>end

For convenience this functionality can be added as a command to gdb by adding into .gdbinit:

python
import argparse

class RedirOutput(gdb.Command):
    def __init__(self):
        super().__init__("rdr", gdb.COMMAND_USER, gdb.COMPLETE_COMMAND)

    def invoke(self, argstr, from_tty):
        parser = argparse.ArgumentParser()
        parser.add_argument('command', nargs='+')
        parser.add_argument('-o', "--outfile", required=True, help="output file")

        nm = parser.parse_args(argstr.split())

        with open(nm.outfile, "w") as output_file:
            try:
                output_file.write(gdb.execute(' '.join(nm.command), to_string=True))
            except Exception as e:
                print(str(e))

RedirOutput()
end

Variants of usage:

### Inside gdb
(gdb) rdr info -o /absolute/path/info.txt
(gdb) rdr info registers eax -o ./eax.txt
(gdb) rdr bt -o ./bt.txt

Note: if saving in a file is needed for subsequent opening in vim or less for more convenient viewing and paging, then the "file step" is redundant, because gdb's output can be redirected right to these programs, see redirection to less.

GDB print to file instead of stdout

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