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posts: gdb-scripting: Grammar correction

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@ -4,9 +4,9 @@ title: Automate debugging using GDB scripting
tags: linux, gdb, gdb scripting, gstreamer
---
Recently I was working on a GStreamer
For a while, have had the pleasure of working on a GStreamer
[plugin](https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/572)
in Rust. The plugin basically rounds the corners of an incoming video,
in Rust at work. The plugin basically rounds the corners of an incoming video,
something akin to the `border-radius` property in CSS. Below is how it looks
like when running on a video.
@ -15,47 +15,44 @@ like when running on a video.
The GStreamer pipeline for the same.
```bash
gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! video/x-raw,format=I420 ! roundedcorners border-radius-px=100 ! video/x-raw,format=A420 ! videoconvert ! gtksink
gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! roundedcorners border-radius-px=100 ! videoconvert ! gtksink
```
This was my first time working on a video plugin in GStreamer. Had a lot to
This was the first time working on a video plugin in GStreamer. Had a lot to
learn on how to use the `BaseTransform` class from GStreamer, among other
things. Without getting into the GStreamer specific details here, I basically
ran into a problem for which I needed to do some debugging for figuring out
what was going on in the internals of GStreamer.
things. Without getting into the GStreamer specific details here, basically ran
into a problem for which needed some debugging for figuring out what was going
on in the internals of GStreamer.
Now, while I never had problems using GDB from the command line, but, the way I
was using it earlier was just not good enough. I would start the pipeline, then
Now, while using GDB from the command line has never been a problem, but, the
straight forward regular approach is time-consuming. Start the pipeline, then
attach gdb to a running process, place breakpoints by manually typing out the
whole thing and then start. For one off debugging sessions, where may be you
just want to quickly inspect the backtrace from a crash or may be look into a
deadlock condition where your code hanged, this could be fine. However, when
you have to repeat this multiple times, do a source code change, compile and
then check again, it becomes frustrating.
whole thing and then start. For one off debugging sessions, where perhaps you
just want to inspect the backtrace from a crash or may be look into a
deadlock condition where your code hung, this could be fine. However, when you
have to repeat this multiple times do a source code change compile and then
select again it becomes frustrating.
## GDB Dashboard
Looking for a better way, I first stumbled on
[gdb-dashboard](https://github.com/cyrus-and/gdb-dashboard).
This is quite useful since it can give the needed information without having to
type anything. Using gdb
Looking for a better way, [gdb-dashboard](https://github.com/cyrus-and/gdb-dashboard)
is what first came up as an option. This is quite useful since it can give the needed
information without having to type anything. Using gdb
[hooks](https://git.sr.ht/~sanchayanmaity/dotfiles/tree/master/item/gdb/.gdbinit.d/hooks),
the dashboard can be triggered when appropriate. See the rest of my
[gdb configuration](https://git.sr.ht/~sanchayanmaity/dotfiles/tree/master/item/gdb/.gdbinit.d)
to get an idea. I use this in scenarios like where code is stuck due to a
deadlock, I need to look at the backtrace of a crash or any such one off simple
investigation.
the dashboard can be triggered when appropriate. See the rest of the [gdb
configuration](https://git.sr.ht/~sanchayanmaity/dotfiles/tree/master/item/gdb/.gdbinit.d)
to get an idea. This is useful in scenarios like where code is stuck due to a
deadlock and one needs to look at the backtrace of a crash or any such one off
simple investigation.
## Construct breakpoint command in neovim & copy to clipboard
The next small improvement I did was more specific to my use of neovim. Am
generally navigating source code using neovim, which would be opened in one
kitty tab and gdb would be running in terminal in next tab or a split. Wanted
to be able to quickly place a breakpoint without having to type anything out on
the gdb prompt. Wrote a small piece of vimscript code which generates the gdb
command, I would have to type on the gdb prompt to enable a breakpoint,
considering the current line and file on which my cursor is at in the source
when opened in neovim.
The next small improvement is more specific to neovim. Navigating source code
with neovim opened in one kitty tab and gdb running in terminal in next tab or
a split is a preferred workflow personally. Being able to place a breakpoint
without having to type anything out on the gdb prompt would be convenient. The
vimscript code below generates the gdb command, considering the current line
and file on which the cursor is at in the source when opened in neovim.
```vimscript
function! CopyBpLocToClipboard() abort
@ -68,8 +65,8 @@ endfunction
nnoremap <silent> <Leader>yb :<C-U>call CopyBpLocToClipboard()<CR>
```
So I can hit the key binding above and a command like below will be copied to
the clipboard which I can paste on gdb prompt.
By using the preceding key binding, a command like below gets copied to the
clipboard which can be just pasted on gdb prompt.
```bash
break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:104
@ -79,19 +76,19 @@ Nifty!!!
## GDB scripting
Now imagine a scenario where may be one wants to look at multiple places in the
Now imagine a scenario where perhaps one wants to look at multiple places in the
source code and when the program is running, inspect certain variables or just
print out a back trace each time a specific code point is reached.
The dumb way to do this and the way which I was also doing it earlier, was to
load the executable in gdb or attach to a running process, place a break point,
run, inspect the local variables or print stack trace, place the next break
point and repeat this whole process. Just time consuming and a waste of time.
The manual way to do this is to load the executable in gdb or attach to a
running process, place a break point, run, inspect the local variables or print
stack trace, place the next break point and repeat this whole process. Just
time-consuming.
GDB can completely automate the above process. Let's see how.
GDB can completely automate the preceding process like below.
Below is the `.gdbinit` file I came up with for my problem. This is what is
called a command file by gdb.
Below is the `.gdbinit` file applicable for the problem facing encountered at
work. This is what's called a command file by gdb.
```bash
set confirm off
@ -156,21 +153,20 @@ disable 6
run
```
The command I was using to debug my GStreamer plugin in this pipeline with gdb.
Below is the command to debug the GStreamer plugin in this pipeline with gdb.
```bash
gdb --nx -x .gdbinit --args env RUST_BACKTRACE=1 GST_DEBUG=3,basetransform:6 GST_PLUGIN_PATH=$GST_PLUGIN_PATH:~/GitSources/gst-plugins-rs/target/debug gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! video/x-raw,format=I420 ! roundedcorners border-radius-px=100 ! video/x-raw,format=A420 ! videoconvert ! gtksink
```
In the command above, the `-x` parameter tells gdb to use the command file. The
`--nx` flag tells gdb to not read any any `.gdbinit` files in any directory, as
I did not want to use `gdb-dashboard` for this. `--args` is how I tell gdb what
to run, which is my GStreamer pipeline. You can see `gdb --help` for details on
the flags.
In the preceding command, the `-x` parameter tells gdb to use the command file.
The `--nx` flag tells gdb to not read any `.gdbinit` files in any directory, as
`gdb-dashboard` isn't intended to be used for this. `--args` is how one tells
gdb what to run, which is the GStreamer pipeline in this case. See `gdb --help`
for details on the flags.
Now, let's understand what the command file does. The ones below are just some
settings we want gdb to use. Note that we have turned on logging and pretty
printing.
Now, consider what the command file does. The ones below are just some
settings for gdb to use. Note that logging and pretty printing are enabled.
```bash
set confirm off
@ -181,8 +177,8 @@ set print pretty on
set pagination off
```
Next we specify the breakpoints. We have six breakpoints. These are the
locations which were of interest to me.
Next, specify the breakpoints. There are six breakpoints. These are the source
code locations of interest.
```bash
break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:104 if meta->n_planes == 4
@ -194,9 +190,10 @@ break subprojects/gst-plugins-base/gst-libs/gst/video/video-frame.c:136
```
Breakpoints can be enabled conditionally. The `if meta->n_planes == 4` implies
to consider this breakpoint only when we get a video frame with 4 planes.
to consider this breakpoint only when a video frame with 4 planes is received.
We can now tell gdb what should it do when each of the breakpoint above is hit.
Next gdb has to be told what should be done when each of the preceding
breakpoints is hit.
```bash
commands 1
@ -212,9 +209,9 @@ end
```
`commands 1` implies these are the commands for gdb to execute when breakpoint
1 is hit. When breakpoint 1 is hit, it will print the value of `i` and `frame`.
1 is hit. When breakpoint 1 is hit, the value of `i` and `frame` gets printed.
The other breakpoints get enabled only after the first one is hit. This is
because at the end of command file, we have
because at the end of command file, the following commands
```bash
disable 2
@ -224,38 +221,38 @@ disable 5
disable 6
```
which tells gdb to start with these breakpoints disabled. They will get enabled
only when we hit breakpoint 1. The `continue` just tells gdb to continue, as we
do not want to stop on hitting a breakpoint and only want to inspect in the end
using gdb log.
instruct gdb to start with these breakpoints off. These get enabled only
when breakpoint 1 is hit. The `continue` just tells gdb to continue, as gdb
shouldn't stop on hitting a breakpoint and logs can be inspected in the
end using gdb log.
Similarly we have for other breakpoints.
Other breakpoints are specified similarly.
The `run` at the end tells gdb to start running immediately. In normal usage
The `run` at the end tells gdb to start executing immediately. In normal usage
one would have to explicitly type `run` on the gdb prompt to make gdb start
debugging.
If it is not clear so far, basically whatever gdb commands we would have used
for debugging at the gdb prompt, we now use them in the command file.
If it's not clear so far, basically whatever gdb commands would have been used
for debugging at the gdb prompt, is what gets specified in the command file as
well.
Now, since we turned on logging after running the below on the terminal
After running the below on the terminal
```bash
gdb --nx -x .gdbinit --args env RUST_BACKTRACE=1 GST_DEBUG=3,basetransform:6 GST_PLUGIN_PATH=$GST_PLUGIN_PATH:~/GitSources/gst-plugins-rs/target/debug gst-launch-1.0 filesrc location=~/Downloads/bunny.mp4 ! decodebin ! videoconvert ! video/x-raw,format=I420 ! roundedcorners border-radius-px=100 ! video/x-raw,format=A420 ! videoconvert ! gtksink
```
gdb will run the pipeline, considering the command file it was passed and log
whatever it was asked to log when each breakpoint is encountered. And now since
we had turned on logging and pretty printing, gdb will nicely log everything in
default `gdb.txt` file. You can see the exact log text file
The pipeline gets executed by gdb, considering the command file it was passed
and log whatever it was asked to log when each breakpoint is encountered. Since
logging and pretty printing were enabled earlier, gdb logs everything in
default `gdb.txt` file. The exact log text file can be seen
[here](https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/572#note_1107146),
where I have attached the `gdbinit` and the other two log files.
with `gdbinit` and the other two log files attached.
Now, one can comfortably look at this log and see what is going on. Once the
Now, one can comfortably look at this log and see what's going on. Once the
command file is written, the whole debugging process is completely automated.
Run, sit back and then look at the logs.
Using gdb is now a breeze and hassle free experience. Being able to automate
Using gdb is now a breeze and hassle-free experience. Being able to automate
and log the debugging process like this, also means you could share your
command file and someone else can replicate this. Wish I would have learned
this sooner.
command file and someone else can replicate this.