Er, I actually do have one more thing to say about the subject: GDB had one project accepted in the Google Summer of Code 2009. Oguz Kayral is the student working on it, and I am his mentor. He will add support for subscribing to inferior events (e.g., signals, process and thread stops, thread creation) from Python. One use case for which this is useful was given by an IRC user at the #gdb channel:

<LimCore> how to run gdb from command line, so that it will run ./foo.bin with arguments: foo bar baz and it will run it instantly without waiting for ‘r’; And if program segfaults then it will do ‘bt’ without waiting for the command. (and if program terminates normally then it will also just quit)

LimCore will be able to write a simple and short Python script using the events API to solve his problem.

Now, moving on to other items: my team has been asked to improve GDB support for the hardware debug facilities in embedded PowerPC processors (for more info about these facilities, see Chapter 10 of Book III-E of the Power ISA v2.06). I announced this work to the GDB mailing list back in early March, and got useful insight from Joel Brobecker.

Today I posted an update on where we are with this work. We have the following ready for both native GDB and gdbserver on Linux:

• one additional hardware watchpoint (two in total),
• four hardware breakpoints,
• one ranged hardware watchpoint.

And we still have the following features ahead of us:

• support for the two DVC (Data Value Compare) registers, which enable hardware-accelerated conditions for hardware watchpoints,
• two ranged hardware breakpoints.

Last and least, I was thinking of posting monthly GDB updates on what happened in GDB in the previous month as I did back in February, but I got busy and didn’t get around to it. I still entertain the idea though, so if you think it’s worth it, I’d be glad to know.

The latest two patches committed upstream are for creating convenience functions in Python, and fo manipulating a program’s stack frames as Python objects. So, what can you do with those? I’ll borrow an example from Tom Tromey here: suppose you want to set a breakpoint which triggers only when the code is called by one specific function. You can create a convenience function like this:

import gdb
import re

class CallerIs (gdb.Function):
    """Return True if the calling function's name is equal to a string.
This function takes one or two arguments.
The first argument is the name of a function; if the calling function's
name is equal to this argument, this function returns True.
The optional second argument tells this function how many stack frames
to traverse to find the calling function.  The default is 1."""

    def __init__ (self):
        super (CallerIs, self).__init__ ("caller_is")

    def invoke (self, name, nframes = 1):
        frame = gdb.selected_frame ()
        while nframes > 0:
            frame = frame.older ()
            nframes = nframes - 1
        return frame.name () == name.string ()

CallerIs ()

And then create a conditional breakpoint using it, as in:

(gdb) break foo.c:42 if $caller_is ("some_function")

Or, to check the name of the grand-grand-caller of the code:

(gdb) break foo.c:42 if $caller_is ("some_function", 3)

Cool, huh? Now, why bother writing that big doc comment? GDB will use it as online help for the function you wrote:

(gdb) help function caller_is
Return True if the calling function's name is equal to a string.
This function takes one or two arguments.
The first argument is the name of a function; if the calling function's
name is equal to this argument, this function returns True.
The optional second argument tells this function how many stack frames
to traverse to find the calling function.  The default is 1.

One other patch which was merged in HEAD enables creating new GDB commands in Python, so you can write new commands to do some frame tricks too!

If you’d like to know more about the Python scripting work in GDB, I suggest you read the series of blog posts from Tom Tromey on this subject. It’ll give you a pretty good idea of what we have in the Python branch, and the direction we’re heading. Just keep in mind that some method names and syntax changed since he wrote that. Refer to the GDB manual in the Python branch (“make gdb.pdf” in gdb/doc/) or the example scripts (also in the Python branch, in gdb/python/lib/gdb/) for up-to-date details.

• publication of release schedule for GDB 7.0;
• merging of a handful of patches for Python scripting support (including ability to define new commands in python);
• new target supported (Ericsson’s DICOS on x86_64);
• revamp of thread support in Solaris;
• support hardware watchpoints in all i386 targets;
• ability to inspect extra signal information.

This is an incomplete list (doesn’t mention bugfixes, for instance) hacked up in the wee hours of the night, while waiting for sleep to come. If you are a GDB hacker who committed something interesting this month, I apologize in advance for not listing it here.

Anyway, this is the kind of thing we want to make easy to do in Python, so I decided to try to implement a solution using what we have in the Python branch. Tom Tromey already created the notion of convenience functions, which are functions that are implemented by GDB  that you can call in places where it accepts expressions, and he made it possible to implement such functions in Python. It follows that one can write a in_scope convenience function in Python which checks if a given variable name is valid in the current scope of the debugged program.

There were some missing pieces which I had to implement to get the in_scope function working (most notably, a method to get a string from a variable in the debugged program), and in the past few days I committed to the branch the patches which add those pieces. So now you can have the following script:

class InScope (gdb.Function):
    """Check if all the given variables or macros are in scope.
       Receives as argument a list of names separated by
       whitespace."""

    def __init__ (self):
        super (InScope, self).__init__ ("in_scope")

    def invoke (self, var):
        vars = set (var.string().split())
        found = set ()
        pc = gdb.get_selected_frame ().get_pc ()
        block = gdb.get_block_for_pc (pc)
        while block:
            for sym in block:
                if (sym.is_argument ()
                      or sym.is_constant ()
                      or sym.is_function ()
                      or sym.is_variable ()):
                    sym_name = sym.get_print_name ()
                    if sym_name in vars:
                        found.add (sym_name)
            block = block.get_superblock ()

        return vars == found

InScope ()

And use it like this:

Breakpoint 1, main (argc=1, argv=0x7fffffffe208) at /tmp/funcs.c:16
16        int i = 41;
(gdb) if $in_scope("i")
 >print "yay"
 >else
 >print "nay"
 >end
$1 = "yay"
(gdb)

If you save the script to ~/.gdb-in-scope.py, you can put the following in your ~/.gdbinit to load it automatically:

source ~/.gdb-in-scope.py

Now my goal is to cut out and submit upstream a set of patches to make the above work on regular GDB. The only problem is that I have the boring task of writing testcases and documentation for the code before I can submit it…

For now, If you want to use this script you can build a GDB from the branch, but be aware that the Python API as provided in the branch is subject to change.

First of all, I’d like to take the opportunity to encourage people interested in using this feature to experiment with what we have implemented so far. The reason is that if you still can’t do what you want with the current code in the Python branch, we’d love to hear what you miss and implement it. We are working on what is useful for ourselves, and trying to decide what other people would find useful. But it’s not possible to imagine everything that people want to use this for, or even most things. Please refer to this wiki page to learn what currently works, what we plan to implement, and how to grab the code from the Python branch.

Feel free to write to the GDB mailing list or show up in the #gdb IRC channel at Freenode to discuss this work and/or bring your use case to our attention, so that we can support it. I hope that with enough input from prospective users we can ship something that’s immediately useful for most people, and avoid having to jump through hoops later and have to shoehorn something that we forgot to cater for initially, risking breaking scripts out there or ending up with an inconsistent API.

Anyway, back to business: I have just committed the second patch in the Python series! It exports GDB’s value subsystem to Python scripts. Basically, GDB values are objects which represent data in the inferior (GDB jargon for the program being debugged), holding its address in the inferior’s addressspace, its type and so on. See the “Python API” section in the GDB manual if you want to learn more about it (yes, we are even writing documentation for the feature!).

I committed the first patch back in August, but I didn’t mention it here because it didn’t do anything the user would find useful, really. It was just groundwork for the rest (autoconf and Makefile.in changes, a ‘python’ command in GDB which basically does nothing useful, initial documentation…). Still, it was about 1500 lines long (not counting the patch’s context)! This shows how much work it is to integrate Python support in GDB. I almost regret having joined this effort.

The second patch also doesn’t allow the user to do anything useful yet, unfortunately. But it is noteworthy because it is a base upon which a lot of other Python support code depend upon. Also, it’s the first committed patch which actually exposes something from GDB to Python. It took a while to get this code ready for two reasons: one was that there was a long discussion regarding how the syntax of acessing struct/union/class elements. The other was that implementing the Value class involved playing with little-documented aspects of Python’s C interface, and it took me time to discover how to do what I needed.

Now my next step is to choose the next patch from the Python series to submit upstream, and get it ready for posting (i.e., fix FIXMEs, add testcases and documentation). This brings me to another thing I’d like to mention. Back in April when I first prepared the Python patch series, I naively thought that after cutting them out, it was just a matter of posting them, iterate through a few review/rework steps and they’d be committed. Simple enough. But here we are in mid-October and just two from nine patches went in (now it’s more like 15 patches in total)! What happened?

The problem is that we’ve been working in the branch in an experimental and exploratory way, just hacking together enough to get something useful done. This was necessary because we didn’t know exactly what we would want to expose from GDB to Python, and how we wanted to do that. As we progressed and discussed the results, things started to become clear. The problem is that now we have a lot to clean up, voids to fill, and above all documentation and testcases to write. This takes time.

At least, that was the problem with the first two patches. I noticed Tom Tromey started to write more documentation and tie more loose ends than in the beginning (me? I’ve just been working on the first two patches until they were ready. Didn’t write sexy new stuff since then…), so there’s hope that the next patches will be easier to work with. We still lack a lot of tests for the testsuite, though…

Or maybe not that much:

Yours truly also attempted to obtain the title, with what was later named the Santos Dumont look (oh well, judge for yourself):

In the end, Fred Mercury won:

Or was it Super Mario? This is still under debate.

Also Frederico, could not attend but sent this awesome picture anyway. And you can see all contestants here.

I’d like to send a big Thank You! to the people who organized this wondrous event: Alex Zanetti, André Macêdo and Paulo Vital. I had a really really good time that night!

In summary, I had a blast there! I was in last year’s summit and enjoyed it and learned a lot from it. But this time I already knew GDB people and they knew me, and I am involved in a couple of current developments and have more experience with the project, all of which made some difference. And everybody there is very friendly, of course, even if they never heard of you before. In fact, Ian Taylor in his welcome presentation urged people to be friendly to newcomers since GCC and the GNU toolchain need new blood.

There was a good number of GDB-related events: a GDB talk and two debugging information talks, a debug information BoF, an informal GDB get-together and a GDB BoF. Unfortunately I know squat about GCC internals (I intend to learn more about it, but didn’t have a chance yet) so the two debugging information were above my head, and I absorbed little. The GDB talk was interesting but since I follow the GDB mailing lists I already knew most of what was presented.

The debug information BoF was interesting, especially since the discussion didn’t focus so much on the two competing approaches to improve debug information (which was the original point of the BoF), but mostly on what should be expected from debug information generated by GCC (i.e., what a debugger should be able to do with it, especially at higher optimization levels), and how its quality can be tested in the GCC testsuite.

The most interesting events for me were of course the GDB get-together and the GDB BoF. The former was a table reserved for us at lunch one day (thanks for organizing this, Joel Brobecker!) where folks interested in GDB would get to see each other faces and talk about random stuff (GDB-related or not). It was fun, and we were able to throw some ideas around about things such as conversion of the GDB repository from CVS to Subversion, the patch review process, and even about rewriting GDB in C++ (which is a hot thread in the GDB mailing list today!). I have a picture of the event:

If you follow the link you can see the notes with the name of each person in the photo above.

The GDB BoF was very interesting, and it felt weird to be at the front (thanks for inviting me Daniel!) discussing current GDB issues with Daniel Jacobowitz, Tom Tromey, Pedro Alves (the other people at the front) and the other GDB maintainers and developers in the room.

We nailed down some pending issues that were being discussed in the mailing list at the time regarding Python scripting support (man, it’s so much easier to decide things face to face rather than by e-mail!), and also discussed a bit of reversible debugging, multithreading GDB itself, GDB scalability, what to do regarding the next release (in a nutshell: wait about a year from the last release so that all the cool stuff which is being worked on right now gets in and settle down), moving the bugs database from GNATS to bugzilla (thanks for doing this Tromey!) etc.

Also after the BoF Pedro Alves gave a very good improvised tutorial on the GDB event loop which he has been studying for the past few months. It felt like cheating, to get all that knowledge in what, half an hour? Thanks so much Pedro, it was awesome.

And of course all the interaction with the people who were there, like Joel Brobecker (playing tennis is more serious than I thought!), Gaius Mulley (Pink Floyd!), Anmol Paralkar, Ramana Radhakrishnan and many others (I don’t even try to enumerate, just a random sample).

I shared a suite in Ottawa with David Edelsohn and Kenneth Zadeck, which was an interesting thing in itself. Heading back to the hotel felt like going to an extended GCC summit. I almost learned something about GCC internals (SSA, LTO, register allocation) and also had very interesting conversations in general.

And of course my one week of backpacking in Canada after the summit, which was another blast.

Also, there will be a fair number of GDB-related events this time, and I look forward to all of those: a GDB talk and two debugging information talks, a debug information BoF and an informal GDB get-together. I look forward to the GDB-related conversations we’ll have in the last one. Even more so if we do the get-together in a bar, around some beer.

I also look forward to the closing party at Vineyards, of course. I will finally eat again that bumbleberry pie…

Update (2008/07/15): I just wrote the trip report.

Just today Carlos Seo committed a patch to GDB adding support for writing AltiVec (PowerPC’s SIMD instructions) registers to corefiles when using the gcore (or generate-core-file) command. This support will show up, then, in the next GDB release. He also had a patch committed last year, in time for GDB 6.8 release, adding support for reading AltiVec registers from core files generated by the Linux kernel, starting from version 2.6.24 (kernel patch provided by Mark Nelson).

Luis Machado posted in February a patch for ltrace which fixed a number of ABI-related bugs for PowerPC 32-bits and 64-bits. The patch was committed in March. The ltrace maintainer has been out for quite a while now, so I don’t think this will appear in an official release any time soon, but it will surely be picked up by the distros.

Going back to GDB, Luis implemented displaced instruction stepping for PowerPC, helping a bit with the non-stop multithread debugging effort carried out by CodeSourcery. The patch didn’t go in yet, it’s awaiting review. He’s also been monitoring the non-stop patches and testing them on PowerPC, having already provided some feedback and pointed out a few regressions.

But this method is inconvenient sometimes: when you want to comment on a patch posted on the list, portions of that patch (which may include long lines that shouldn’t wrap) will show up in the message body and you can have trouble. This is a problem especially when the poster also sent their patch as attachment, because you will most likely need to copy & paste the patch into your reply window and things will get mangled right there.

Turns out that there is a way to include a patch in the message body in Evolution’s compose window and it will get through unmaimed: select the Preformat option in the paragraph style dropdown, and then paste in your patch or use the Insert -> Text File… option.

Caveat: Older versions of Evolution converted tabs to spaces when pasting text, so you had to insert it from a text file to preserve whitespace. I just tested with version 2.12.3 from Debian (package version 2.12.3-1), and pasting a patch containing tabs worked fine so the bug has been fixed.

Thanks Klaus Kiwi for the tip!

This is the tale of my patch-producing efforts. I’m sorry, it is most likely boring for everyone but me. Still, I wanted to write it down so you are free to stop reading the post here.

Anyway, back to where I was: I (foolishly, perhaps?) volunteered to create the patches. One reason for me to do that is that I actually enjoy working with and learning about source code management and related issues, and this was a good opportunity for me to improve my git-fu (since I thought git could help me do the job in a sane way, which fortunately proved to be the case).

I say foolishly because I thought it wouldn’t take too much time to cut out the patches, since I knew what I would have to do… The task of course took longer than I expected, in part because of unforeseen autotools woes, but of course also because I underestimated the effort (I am an optimist).

Here is the method I used:

First of all, I wanted to update the code to the latest CVS version of GDB, because the work in the git repo was done based on a CVS version from February… Nothing to see here, actually. I just created a new branch with the latest CVS update, and rebased the commits on top of that. In hindsight, I should have merged the new CVS update into the python branch, which would have made me deal with less conflicts. I used rebase because I thought I would cherry pick the commits later, so I wanted each of them refreshed.

Then the real fun began. I started using interactive mode of git rebase to squash related commits together (to form the patches), and reorder them. I quickly realised that with this approach I would have a bit of difficulty with commits which touched different areas of the code and crossed the borders I had in my mind for the patches I wanted to generate. I would have to first split those into smaller, more behaving commits and only then squash them with other similar changes. That seemed to be more work than really necessary.

Also, the older commits did things in ways and places which were later changed, and it looked like I would have some trouble reconciling older and newer code to fit in one patch (maybe not though, maybe that would be taken care of more or less naturally). Also, I would need to take some time to familiarise myself with the commits in the branch, because I only authored some of them. The “shuffle commits around” approach wasn’t looking very promising.

I then turned to a different strategy: I generated a big patch containing all of the code in the branch, and applied it (using the plain old patch command) on top of a clean branch which contained only the CVS HEAD version I was using as a base. All I needed to do now was to selectively add to the index the changes that I wanted to include in a patch and then commit those changes together. And repeat the process for the next patch and so on.

It proved to be a good approach, especially because of the interactive mode of git-add. This mode asks you about each change inside a modified file, letting you add that change to the index or skip it, leaving it in the working directory for a future commit. git add -i streamlined the “change picking” process quite a lot, and made the patch-cutting almost mechanical. (By the way, this feature is also available in Mercurial, with the Record extension. I even believe (not sure though) that the Mercurial extension predates git add -i) (I don’t know if Bazaar has it, would be nice to know).

In this phase of the process, git rebase -i was useful. Sometimes I came accross a change in the working directory which would fit better in a patch which I had already committed. It was simply a matter of committing that change and then shuflling it back and squashing with the proper patch.

At each commit I pushed the changes to a pristine repo which I used to build GDB and guarantee that each patch included all the changes it needed.

Voilà, at the end of the process I had a git branch where each commit corresponded to one patch which I wanted to send to the mailing list.

“GDB now supports debugging C and C++ programs which use the Decimal Floating Point extension. In addition, the PowerPC target now has a set of pseudo-registers to inspect decimal float values stored in two consecutive float registers.”

The feature was actually developed by several folks. Ben Elliston started working on the feature, and then passed it on to Wu Zhou. I took up where Wu Zhou left and implemented more complete support for decimal float types. Luis Machado also helped me and posted some patches of his own. The work amounted to a total of about 12 patches.

It means that you can now use GDB (version 6.8 or later) to debug programs which make use of the proposed C extension for decimal floating-point arithmetic (i.e., the _Decimal32, _Decimal64 and _Decimal128 types). The first GCC release with support for this extension was GCC 4.2.

Why is decimal floating point useful? It avoids the unexpected surprises and rounding errors which inevitably come with binary floating point. It’s not that decimal float is more exact or precise, it’s just that it behaves just like we are used to and were thought in school in terms of representation and rounding.

GDB itself can provide an example:

(gdb) p 1.2l
$1 = 1.2000000000000000000433680868994202
(gdb) ptype 1.2l
type = long double
(gdb) p 1.2dl
$2 = 1.2
(gdb) ptype 1.2dl
type = _Decimal128

You can see that the actual value used to represent 1.2 in binary floating point is slightly larger than 1.2. Why is that? It’s because to express 0.2 in binary float you need an infinite number of digits, so what ends up being stored is the closest number possible to represent in binary. The difference is almost nothing, but this error will propagate in computations and eventually be big enough to be noticed. That’s why in some application domains (e.g., finance and civil construction), you are actually required by law to use decimal floating point to perform calculations.

More information about decimal float can be found in Mike Cowlishaw’s page.

Here’s the output I get:

(gdb) rbt
#2 0x080483bb in main at ../../src/examples/funcs.c:15
#1 0x08048391 in f1 at ../../src/examples/funcs.c:10
#0 f2 at ../../src/examples/funcs.c:5
(gdb)

A little bit more information, and the definition of the Python command which does the above can be found in my post to the mailing list. The code is on the git repo for the Python work.

I find it interesting that he chose to implement it in C++. I’m glad that the “let’s do it in C because everybody knows it” mantra that so frequently determines the programming language of open source projects doesn’t always prevail. Perhaps this will help weaken the argument?

And yes, I know about KDE. It’s the exception that proves the rule.
Besides, don’t forget that GNOME, the project created to react to it is in C, using the horrible GObject monstrosity.

The work is being done in a git repo hosted by gitorious. More details here.

This looks very promising.

Update – 2008/03/15: Sorry, I was a bit cryptic in this post. It’s not really about enabling debug of Python programs using GDB (which is a neat idea, and I hope to play with it sometime). It’s about integrating a Python interpreter into GDB and exposing its internals as an API to be used by Python scripts. This will enable people to automate and extend GDB functionality using Python. This feature has been asked many times before, and finally is being addressed. Should open many possibilities.