Continue an iteration in C++ until a specific iteration number in gdb

Question

I am using gdb-7.0.1 and I think I have detected a bug in a certain section of my code, which has a for loop. The for loop looks like

for (int i=0 ; i< end ; ++i )
{
   //Code here.
}

Here end is a very large integer. The code does not crash at the first iteration, and seems to crash somewhere at iteration number end/2.

Since I want to understand the behaviour of the code at iteration number end/2 , just stepping and nexting from i=0 till I reach this iteration point, is unfeasible.

Is there a way to tell gdb to continue through a for loop till i gets the value end/2 and then wait for the user to manually step through iteration number end/2?

I am using gcc-4.5.2 on Ubuntu Linux

http://blog.vinceliu.com/2009/07/gdbs-conditional-breakpoints.html — Karoly Horvath, Apr 16 '12 at 18:19

score 6 · Answer 1 · answered Apr 16 '12 at 18:19

6

Here's a tutorial on conditional breakpoints with gdb.

I'm guessing you didn't know the term for this, otherwise it would have been easy to google.

answered Apr 16 '12 at 18:19

Luchian Grigore

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Another alternative is to `(gdb) ignore end/2-1` – Employed Russian Apr 16 '12 at 22:26

score 5 · Accepted Answer · answered Apr 16 '12 at 18:19

5

When you set the breakpoint it'll give you a breakpoint number (for the moment, let's assume it's 1). You'll then make that breakpoint conditional, something like:

condition 1 i==end/2

answered Apr 16 '12 at 18:19

Jerry Coffin

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score 5 · Answer 3 · edited May 23 '17 at 11:54

5

You have to use conditional breakpoint. Here is more about it: http://www.cs.cmu.edu/~gilpin/tutorial/#3.4

And on SO: How do I set a conditional breakpoint in gdb, when char* x points to a string whose value equals "hello"?

In your case (not tested):

break <line_number> if i==end/2

edited May 23 '17 at 11:54

Community

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answered Apr 16 '12 at 18:20

psur

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score 1 · Answer 4 · answered Apr 16 '12 at 18:19

1

You should be able to place an if (i == (end/2 -1)) { Foo; } in there then set a breakpoint at Foo, which would allow you to continue stepping from there.

answered Apr 16 '12 at 18:19

Michael Wilson

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+1 as this used to be my method until I found conditional breakpoints as @LuchianGrigore suggests. – Ash Burlaczenko Apr 16 '12 at 18:25
Thanks yeah, I'm not sufficiently fluent in gdb to have picked up on it. I figured it was a sufficiently generic solution to leave it up even in the face of the more poignant ones. – Michael Wilson Apr 16 '12 at 18:28

score 1 · Answer 5 · answered Apr 16 '12 at 19:26

If end is big (in the tens of thousands), then the conditional breakpoint solution can be very slow - gdb has to evaluate the condition each time round the loop. If this is a problem for you, then you can use this trick:

for (int i=0 ; i< end ; ++i )
{
   if (i == end/2)
      i %= end ; // This has no effect, but lets you set a breakpoint here
   //Code here.
}

I do this all the time :-)

Another solution is to set a skip-count on the breakpoint. I use gdb in a Qt environment, so I can't give you the gdb syntax. But it's faster than setting a condition.

Continue an iteration in C++ until a specific iteration number in gdb

5 Answers5