Class 11: Understanding your code

Held:

We consider ways to approach debugging and other issues with your programs

Preliminaries

Overview

• The joy of segfaults
• Other memory problems
• Tracing errors with gdb
• Checking memory usage with valgrind

Related Pages

• Eboard 11 (Source)

Updates

Debuggers

• Believe it or not, but many people write incorrect code.
• Testing sometimes helps you identify that code is incorrect.
  • Unit testing helps you narrow done a possible location of the problem - if the unit tests for a procedure fail, it’s likely that the procedure is incorrect.
  • Clear box testing (or white box testing) helps you narrow it even further.
• In C, segmentation faults also help you identify that code is incorrect.
• Once you know that it is incorrect, you need tools that help you identify why it is incorrect.
• What tools are available?
  • The most important tool is yourself - you should read through the code to see if there are things that don’t make sense.
  • You’ll often find yourself saying “I’m pretty sure that the value is this at this point.” How do you verify?
    • Use assert.
    • Use printf statements. (Not a great idea.)
    • Use a debugger - a program that lets you trace the execution of your program and inspect variables
• I can’t do much about about the ways you look at code, and I don’t want you to use printf, so we’ll look at using a debugger.
• Detour: Why not use printf? Because you spend a lot of time inserting and removing print statements. A debugger is usually much faster.

gdb

• gdb is the standard debugging tool for C.
• It’s a command-line tool.
  • There are GUIs available, but my experience is that people are often faster without them.
• It’s fast.
• It lets you do all sorts of creative things, like attach to a running process.
• It has a huge array of options. We’ll focus on just a few.

Tracing Crashes

We’ll start with one typical use of gdb - figuring out what line in your program caused a segfault.

We’ll use boom.c, a program that I designed to crash.

• You should compile with the -g flag. This enables debugging information.
• Start the debugger

$ gdb boom

• Run the program

(gdb) run

• Observe the output

Program received signal SIGSEGV, Segmentation fault.
0x08048440 in baz (i=0) at boom.c:64
64          *ip = i;

• Check the stack

(gdb) bt
#0  0x08048440 in baz (i=0) at boom.c:64
#1  0x080483f5 in qux (i=1) at boom.c:55
#2  0x0804840c in qux (i=2) at boom.c:57
...
#12 0x0804844f in baz (i=67) at boom.c:66
#13 0x08048466 in main (argc=1, argv=0xbffffa54) at boom.c:77

• Analyze
  • Clearly, 13 recursive calls is bad luck.
  • Hmmm … the problem seems to be in the assignment
• Look at variables

(gdb) p i
$1 = 0
(gdb) p ip
$2 = (int *) 0x0

• Fix.

Some other operations

l *LINE_NUMBER*

• List code starting with that line

l *PROCEDURE*

• List code starting with that procedure

b *LINE_NUMBER*

• Stop when execution hits that line number

run

• Run the program until the first breakpoint

n

• Execute the next line

s

• Step into a procedure call (if the next line does not involve a procedure call, does the same thing as n* )

c

• Continue execution

p *EXPRESSION*

• Print the value of an expression

watch *variable*

• Stop when the variable has changed.

Another Example

• We’ll explore maplike.c, a program with a common (and perhaps subtle) bug.