debugging – The Codebelay Blog

The Rails Console Makes Test Object Creation and Debugging Easy

I really like how the Rails console solves the problems of test object creation and debugging.

Usually, a web developer will push code to the webserver and hit shift-reload on the web browser. With the Rails console, you can avoid all that shift-reload madness.

If I wanted to create 10000 blog posts for testing, I could do something like this:

script/console
10000.times
{ |i| Post.create(:title => “post number: ” + i.to_s, :body => “autogen Booyah!”) }

Through the console I can also do debugging pretty easily:

>> p = Post.find 888

And get this as output:

=> #

A lot of problems in Rails are just solved by running script/console and checking the values of certain variables that are pretty hard to get at through a web browser.

There is pretty much no limit to what can be done through the Rails console. Konstantin Gredeskoul, web developer and songwriter, has found a way to load session data through the console.

Fun With GDB, Gnu’s Debugger

Here’s a pretty compact version of strcmp:

int bstrcmp(char *s1,char *s2) {
   while(*s1 == *s2++) {
      if(*s1++ == 0){ return 0; }
   }
   return (*(unsigned char *)s1 - *(unsigned char*)--s2);
}

The source that I used for compiling and calling this version of strcmp is here.

Compile that code using:
gcc -o strcmp -g strcmp.c

Fire up the debugger using:
gdb strcmp

You’ll get the gdb prompt:
(gdb)

Set a break point at the start:
b main

The debugger will echo something like:
Breakpoint 1 at 0x80483d5: file strcmp.c, line 6.
(gdb)

Then run the program:
run

The debugger will print out something like:
Starting program: /home/somedude/bin/strcmp

Breakpoint 1, main () at strcmp.c:6
6 {
(gdb)

If you type n a few times, you’ll eventually get to some variable assignments.

(gdb) n
8	char s1[] = "better";
(gdb) n
9	char s2[] = "better than"; /* than this";*/
(gdb) n
11	int i_result = 0;
(gdb) n
13	i_result = bstrcmp(s1,s2);

If you want to the values of these variables type:
p i_result

You get back:
(gdb) p i_result
$1 = 0

To step into a function, type s:

(gdb) s
bstrcmp (s1=0xbf86b469 "better", s2=0xbf86b474 "better than") at strcmp.c:26
26		while(*s1 == *s2++) { 
(gdb) n
27			if(*s1++ == 0){ return 0; } 
(gdb) n
26		while(*s1 == *s2++) { 
(gdb) n
27			if(*s1++ == 0){ return 0; } 
(gdb) n
26		while(*s1 == *s2++) { 
(gdb)

At this point you can type things like:
p s1
p *s1
p s2
p *s2

And you’ll get back the value of the pointers and what’s in memory.

Next time we’ll go over how to do this with PHP running single threaded on debug mode on Apache.