From 98a60c1fbc611430453f0fe8af1678d2f0ae8f6c Mon Sep 17 00:00:00 2001 From: Sanchayan Maity Date: Sat, 25 Dec 2021 19:33:56 +0530 Subject: [PATCH] posts: gdb-scripting: Grammar correction --- posts/2021-11-13-gdb-scripting.md | 145 +++++++++++++++--------------- 1 file changed, 71 insertions(+), 74 deletions(-) diff --git a/posts/2021-11-13-gdb-scripting.md b/posts/2021-11-13-gdb-scripting.md index f8a1cee..9a9309d 100644 --- a/posts/2021-11-13-gdb-scripting.md +++ b/posts/2021-11-13-gdb-scripting.md @@ -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 yb :call CopyBpLocToClipboard() ``` -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.