• Welcome To Guide Dogs For The Blind | Guide Dogs For The Blind
• Further Information
• Gdb Online
• What Is The Difference Between LLDB And GDB - Pediaa.Com
• Tutorial: Debugging Embedded Devices Using GDB - Chris Simmonds, 2net Ltd
• RMS's Gdb Tutorial: How Do I Use Breakpoints?

If you experience low-level problems such as crashes or deadlocks(e.g. when tinkering with parts of CPython which are written in C),it can be convenient to use a low-level debugger such as gdb inorder to diagnose and fix the issue. By default, however, gdb (or anyof its front-ends) doesn’t know about high-level information specific to theCPython interpreter, such as which Python function is currently executing,or what type or value has a given Python object represented by a standardPyObject* pointer. We hereafter present two ways to overcome thislimitation.

Gdb is in the gnu package on CEC machines. If you don't have this package loaded then type pkgadd gnu at a shell prompt. If you can run g, then you will be able to run gdb.

gdb 7 and later¶

Sep 28, 2018 Miscellaneous gdb commands. L command: Use gdb command l or list to print the source code in the debug mode. Use l line-number to view a specific line number (or) l function to view a specific function. Bt: backtrack – Print backtrace of all stack frames, or innermost COUNT frames. Help – View help for a particular gdb topic — help TOPICNAME. Normally, GDB knows in advance which syscalls are valid for each OS, so you can use the GDB command-line completion facilities (see command completion) to list the available choices. You may also specify the system call numerically. A syscall’s number is the value passed to the OS’s syscall dispatcher to identify the requested service. % gdb broken This only starts the debugger; it does not start running the program in the debugger. Look at the source code and set a breakpoint at line 43 (gdb) b 43 which is double seriesValue = ComputeSeriesValue(x, n); Now, we start to run the program in the debugger. GDB: The GNU Project Debugger bugs GDB Maintainers contributing current git documentation download home irc links mailing lists news schedule song wiki.

In gdb 7, support for extending gdb with Python wasadded. When CPython is built you will notice a python-gdb.py file in theroot directory of your checkout. Read the module docstring for details on howto use the file to enhance gdb for easier debugging of a CPython process.

To activate support, you must add the directory containing python-gdb.pyto GDB’s “auto-load-safe-path”. Put this in your ~/.gdbinit file:

You can also add multiple paths, separated by :.

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This is what a backtrace looks like (truncated) when this extension isenabled:

(Notice how the dictionary argument to PyDict_GetItemString is displayedas its repr(), rather than an opaque PyObject* pointer.)

The extension works by supplying a custom printing routine for values of typePyObject*. If you need to access lower-level details of an object, thencast the value to a pointer of the appropriate type. For example:

The pretty-printers try to closely match the repr() implementation of theunderlying implementation of Python, and thus vary somewhat between Python 2and Python 3.

An area that can be confusing is that the custom printer for some types look alot like gdb’s built-in printer for standard types. For example, thepretty-printer for a Python 3 int gives a repr() that is notdistinguishable from a printing of a regular machine-level integer:

A similar confusion can arise with the str type, where the output looks alot like gdb’s built-in printer for char*:

The pretty-printer for str instances defaults to using single-quotes (asdoes Python’s repr for strings) whereas the standard printer for char*values uses double-quotes and contains a hexadecimal address:

Here’s how to see the implementation details of a str instance (for Python3, where a str is a PyUnicodeObject*):

As well as adding pretty-printing support for PyObject*,the extension adds a number of commands to gdb:

List the Python source code (if any) for the current frame in the selectedthread. The current line is marked with a “>”:

Use py-listSTART to list at a different line number within the pythonsource, and py-listSTART,END to list a specific range of lines withinthe python source.

The py-up and py-down commands are analogous to gdb’s regular upand down commands, but try to move at the level of CPython frames, ratherthan C frames.

gdb is not always able to read the relevant frame information, depending onthe optimization level with which CPython was compiled. Internally, thecommands look for C frames that are executing PyEval_EvalFrameEx (whichimplements the core bytecode interpreter loop within CPython) and look upthe value of the related PyFrameObject*.

They emit the frame number (at the C level) within the thread.

For example:

so we’re at the top of the python stack. Going back down:

and we’re at the bottom of the python stack.

Further Information

The py-bt command attempts to display a Python-level backtrace of thecurrent thread.

For example:

The frame numbers correspond to those displayed by gdb’s standardbacktrace command.

Gdb Online

The py-print command looks up a Python name and tries to print it.It looks in locals within the current thread, then globals, then finallybuiltins:

The py-locals command looks up all Python locals within the currentPython frame in the selected thread, and prints their representations:

You can of course use other gdb commands. For example, the frame commandtakes you directly to a particular frame within the selected thread.We can use it to go a specific frame shown by py-bt like this:

What Is The Difference Between LLDB And GDB - Pediaa.Com

The infothreads command will give you a list of the threads within theprocess, and you can use the thread command to select a different one:

You can use threadapplyallCOMMAND or (taaCOMMAND for short) to runa command on all threads. You can use this with py-bt to see what everythread is doing at the Python level:

Tutorial: Debugging Embedded Devices Using GDB - Chris Simmonds, 2net Ltd

Note

RMS's Gdb Tutorial: How Do I Use Breakpoints?

This is only available for Python 2.7, 3.2 and higher.

gdb 6 and earlier¶

The file at Misc/gdbinit contains a gdb configuration file which providesextra commands when working with a CPython process. To register these commandspermanently, either copy the commands to your personal gdb configuration fileor symlink ~/.gdbinit to Misc/gdbinit. To use these commands froma single gdb session without registering them, type sourceMisc/gdbinitfrom your gdb session.

Updating auto-load-safe-path to allow test_gdb to run¶

test_gdb attempts to automatically load additional Python specifichooks into gdb in order to test them. Unfortunately, the command lineoptions it uses to do this aren’t always supported correctly.

If test_gdb is being skipped with an “auto-loading has been declined”message, then it is necessary to identify any Python build directories asauto-load safe. One way to achieve this is to add a line like the followingto ~/.gdbinit (edit the specific list of paths as appropriate):