MSL2008. Debugging

Master on Free Software

DebuggingJuan A. Suárez Romero

<[email protected]>


What is a bug?

● Basically, an error in a program● When a program performs as it shouldn't

perform ● All software have bugs

– Programming is complex– Human task -> errors


Type of bugs● Resource-accessing bugs● Semantic bugs

– Wrong results– Unfullfil specifications

● Performance bugs● Memory bugs

– Overflows => crashes– Leaks

● Concurrent bugs


Bugfixing

● Find a bug– By developers– By users

● Report a bug– Bugzilla

● Locate the problem– Debugging tools

● Fix it


Finding bugs

● By developers

– Compile-time errors are not bugs!

– Running time

● By chance● Running tests● Using tools (see debugging)

● By users

– They don't look for, but bugs turn up!

– Reported to developers

● By testers

– Mix between developers and users

– Use more sophisticated techniques


Reporting bugs

● Crash-report applications● Bugtrackers: issue tracker system

– Open a ticket (bug)– Add comments and/or other stuff– Tickets have states (life cycle)– Close ticket (bug)– Examples

● Bugzilla (from Mozilla Project)● Mantis Bug Tracker


Finding the problem

● To find something, we need to know about it– Develop small tests to reproduce the problem– Use debugging tools

● Debuggers● Profilers● Memory checkers● ...


Fixing bugs

● Using input from the last point● Verify tests perform fine

– Bug is fixed– Not introduce new bugs

● Document and commit


Bugzilla

● Developed by Mozilla Foundation (1998)● Web-based (LAMP)● Widely used● Highly configurable


Bugzilla


Bugzilla

Where is the bug?


Bugzilla

Severity of bug Blocker Critical Major Normal Minor


Bugzilla

Bug description Overview Preconditions Steps to reproduce Actual results Expected results Frequency Other relevant information


Bugzilla

● Priority: How urgent is to fix the bug?● Differences between severity and priority

– Severity is a technical matter– Priority is a business matter– Severity is absolute– Priority is relative– Priority can be calculated from severity


Bugzilla


Triagging

● Medical term: choose which patient would receive medical cares

● Useful when there are a lot of bugs, but few developers– Share bugs among developers– Use bug features and developers habilities to

assign the bugs


Bugzilla


Bugzilla

From Bugzilla Documentation


Some tools

● Printf ;-)● GDB (GNU Debugger)● Valgrind● Electric Fence● OProfile● G_DEBUG (fatal_warnings, fatal_criticals)


● Profile all running code

OProfile


Electric Fence

● Find overflows / underflows● Easy to use: preload/link against efence

library● Mark with “red zones” buffers

– Crash inmediately when touching them– Thus, errors don't turn up in a future time


G_DEBUG

● Useful to debug problems in Glib● Environment variable

– fatal_criticals: g_critical generates a core dump

– fatal_warnings: g_warning generates a core dump


Valgrind

● Tool suite for debugging and profiling● Simulates a x86 processor

– Memcheck: memory problems● Memory accesses not allowed● Memory leaks● Bad frees

– Cachegrind: cache profiler– Massif: heap profiler– Helgrind: thread debugger


GDB

● GNU Debugger● Text mode

– Other frontends: IDEs, emacs, DDD, XXGDB

● Trace the code– See what is doing– Inspect the data


GDB

● Requirements to debug an application– Add debugging symbols -> not stripped– Remove optimizations (-O0)


GDB

● How to use– Run process with gdb– Inspect core dumps– Attach to running process


Core dumps

● Dump of information of a process when it crashes– See while it crashed– Inspect data

● ulimit -c <size of core>● ./gdb <program> <core>


GDB● Run: runs the program from start

● Continue: continues running a stopped program

● Backtrace: show the stack

● Break: add a breakpoint

● Delete: delete a breakpoint

● Next: runs the current line to next

● Step: same as Next, but if current is a function, enter inside

● Print: print the content of a variable

● Display: same as Print, but more permanent


References

● http://www.google.es● http://www.wikipedia.org● http://www.bugzilla.org● http://www.mantisbt.org● http://library.gnome.org/devel/glib/unstable/glib-running.html

● http://sourceware.org/gdb/● http://oprofile.sourceforge.net● http://valgrind.org

http://www.google.es/

http://www.wikipedia.org/

http://www.bugzilla.org/

http://www.mantisbt.org/

http://library.gnome.org/devel/glib/unstable/glib-running.html

http://sourceware.org/gdb/

http://oprofile.sourceforge.net/

http://valgrind.org/

MSL2008. Debugging

Education

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