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SPEAKER 1: Welcome, everyone.

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My name is Carter, and today
I'll show you my secret program.

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You can run the program by typing
./secret into your terminal,

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and if you type in the right password,
you should be able to see come on in.

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So let's try it out.

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I'll do ./secret, and maybe I don't know
the secret phrase, so I'll just say,

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let me in.

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And at which point, I
don't get any response.

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So let me try it again and do ./secret.

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Maybe password is the password here.

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But it's not.

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And I'll just reveal to you
that the password is please.

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So if I type ./secret, and I type
in please, I should see come on in.

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But I don't.

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And so what's the problem here?

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Well, maybe I should look into my code,
and maybe step through it bit by bit.

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So I'll do code secret.c, and I
can see the very top of my code,

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starting with this int
main(void) function.

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To step through my code,
remember we can use debug50

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to set a breakpoint, or a
place to pause in our code.

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So I can go to this left side
here, find a pale red circle,

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and turn it into a bright red circle as
I click it, and set that pause point,

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or that breakpoint.

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Now I can do debug50 of secret,
and then my code should boot up,

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and I should be able to pause
right at that moment I'm asking

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the user for their secret phrase.

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Of course, I have to actually
step over that line for it

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to actually run my program,
so I'll then step over,

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and I can type in my
secret phrase of please.

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OK, so that seems pretty good.

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I see that phrase becomes please,
and correct is currently false,

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but it should change to true after
I do this check phrase function.

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So I'll step over check phrase, and
I see it's actually still false.

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And so of course, if I run past
line 13, nothing will happen.

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I actually won't print "come on in."

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So at this point, debug50 seems
to be not really helping me here.

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I've walked through my
entire program, but I haven't

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seen what went wrong in my code.

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Well, debug50 has this option to
step into a function, actually dive

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in and see what's happening
underneath that hood of that function.

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So what I can do is go to debug50
again, and run debug50 with secret,

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wait for my code to go
to that breakpoint again,

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and I'll walk through that first piece
where I type in my phrase, please.

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But now, instead of stepping
over this check phrase function,

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I actually want to dive into it and
see what's going on underneath there.

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So I'll do step into in
debug50, and then see,

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I'm setting my password to be please.

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So if I step over line 22, I should
see that password becomes please,

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as it does on this left hand side.

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And now, when I check for
if phrase is password,

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I should actually be
able to return true,

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because it seems that phrase
is the same as password.

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So I'll step over that, and see--

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well actually, I get to line 29.

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And this is kind of
subtle, but in C, you

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can't compare two strings
using two equal signs.

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You actually could use a different
function called string compare,

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or strcmp for short.

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So to use strcompare, I
can use strcmp, and give it

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two arguments, phrase and password.

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And with these two inputs,
strcompare will tell

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me are they exactly equivalent or not.

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If they are equivalent, strcmp--

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or strcompare-- will give
me 0 as a value back.

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So now my code is saying, if
these strings are equivalent--

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and I know they are because
strcompare is giving me zero--

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I can go ahead and return true and
say this is the correct password.

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So with this new code here, I should
be able to exit debug50 and go back

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to the terminal.

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And now I can make secret again, run
secret, type in my password of please,

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and now there it is.

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It says, come on in.

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And with that, we've seen how debug50
can help us not only step over

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our code, but dive into functions
and see the bugs inside of them.

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