MSL2008. Debugging

Debugging Juan A. Suárez Romero <jasuarez@igalia.com> Master on Free Software

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 Master on Free Software

Type of bugs ● Resource-accessing bugs ● Semantic bugs – Wrong results – Unfullfil specifications ● Performance bugs ● Memory bugs – Overflows => crashes – Leaks ● Concurrent bugs Master on Free Software

Bugfixing ● Find a bug – By developers – By users ● Report a bug – Bugzilla ● Locate the problem – Debugging tools ● Fix it Master on Free Software

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 Master on Free Software

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 Master on Free Software

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 ● ... Master on Free Software

Fixing bugs ● Using input from the last point ● Verify tests perform fine – Bug is fixed – Not introduce new bugs ● Document and commit Master on Free Software

Bugzilla ● Developed by Mozilla Foundation (1998) ● Web-based (LAMP) ● Widely used ● Highly configurable Master on Free Software

Bugzilla Master on Free Software

Bugzilla Where is the bug? Master on Free Software

Bugzilla Severity of bug Blocker Critical Major Normal Minor Master on Free Software

Bugzilla Bug description Overview Preconditions Steps to reproduce Actual results Expected results Frequency Other relevant information Master on Free Software

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 Master on Free Software

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 Master on Free Software

Bugzilla From Bugzilla Documentation Master on Free Software

Some tools ● Printf ;-) ● GDB (GNU Debugger) ● Valgrind ● Electric Fence ● OProfile ● G_DEBUG (fatal_warnings, fatal_criticals) Master on Free Software

OProfile ● Profile all running code Master on Free Software

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 Master on Free Software

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 Master on Free Software

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 Master on Free Software

GDB ● GNU Debugger ● Text mode – Other frontends: IDEs, emacs, DDD, XXGDB ● Trace the code – See what is doing – Inspect the data Master on Free Software

GDB ● Requirements to debug an application – Add debugging symbols -> not stripped – Remove optimizations (-O0) Master on Free Software

GDB ● How to use – Run process with gdb – Inspect core dumps – Attach to running process Master on Free Software

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> Master on Free Software

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 Master on Free Software

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 Master on Free Software

MSL2008. Debugging

by Juan A. Suárez Romero on Feb 11, 2010

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MSL2008. Debugging — Presentation Transcript