Application Debugging

Debugging methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware thus making it behave as expected.

Basic Steps Recognize that a bug exists Isolate the source of the bug Identify the cause of the bug Determine a fix for the bug Apply the fix and test it

Common Debug Method print statements

Preferable Debug Method Use a debugger  monitor an application program in situ  catch memory errors  can't attach print statements to a running program Graphical debuggers can provide visual aids

Available Debuggers dbx gdb PGI pgdbg Intel idb ladebug TotalView DDT Plus many others available

Step 1- Identify there is a bug If an error is severe enough to cause the program to terminate abnormally, the existence of a bug becomes obvious! if the error is minor and only causes the wrong results, it becomes much more difficult to detect that a bug exists  this is especially true if it is difficult or impossible to verify the results of the program Goal  identify the symptoms of the bug  under what conditions the problem is detected will greatly help the remaining steps to debugging the problem.

Example debug]$./dgemm_ex1 Segmentation fault

Steps to follow recompile to enable debug support often this option is '-g' check compiler documentation to be sure! all modules need to be compiled with this option re-run application

Steps to follow want failure to generate a core dump by default, core dumps are disabled on HPC machines re-enable with the command: ulimit -c unlimited

Example debug]$./dgemm_ex1 Segmentation fault (core dumped)

Core File contains the memory image of a particular process along with other information such as the values of processor registers very useful debugging tool name format is: core.PID

Using the Core File examine its contents with a debugging tool such as gdb command format is: gdb exe_file core.PID if application compiled with '-g' then odds are good you will be taken directly to the offending source line

Example debug]$ gdb./dgemm_ex1 core GNU gdb Red Hat Linux ( EL4rh) Copyright 2004 Free Software Foundation, Inc.. Core was generated by `./dgemm_ex1'. Program terminated with signal 11, Segmentation fault. Reading symbols from BLAH BLAH BLAH. Loaded symbols for /lib64/ld-linux-x86-64.so.2 #0 0x d5534d03f in _IO_vfscanf_internal () from /lib64/tls/libc.so.6 (gdb) where #0 0x d5534d03f in _IO_vfscanf_internal () from /lib64/tls/libc.so.6 #1 0x d in fscanf () from /lib64/tls/libc.so.6 #2 0x c25 in main (argc=1, argv=0x7fbffff648) at /home1/nucci/proj/debug/dgemm_ex1.c:41

Live Example and Hands-On DDT Tutorial