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Old   April 18, 2002, 12:13
Default question on conditional breakpoint in using gdb
  #1
Wen Long
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Hi, friends:

I'm trying to use GNU gdb to detect a floating exception in my code. That exception occurs only when my time step is 2520 and iteration step is 3. So I have to wait a very long time(one day) to reach that exception point. Seems there is a trick called "conditional breakpoint" in gdb. Below is a brief fram of my code:

c-------------------------

c program my_prog

1 do it=1, it_max

2 do iter=1,iter_max

3

4 some tri-diagonal solver stuff

5

6 some convergence creterion

7 enddo

8 enddo

9 end

c-------------------------

I launch the gdb, and run the program, then I want to use the following GDB command to put a conditional breakpoint on line 1:

//--------

break 1 if it.ge.2520

//--------

it gives me back an error message saying:

//----

No symbol "it" in current context.

//----

Seems that the gdb doesn't recognize the variable "it" for me. What's the metter? I looked into some manu of gdb, saying that it's because "it" is out of the scope of something, but I don't quite understand and don't know how to get through.

Thank you very much if you can give me a hand.

wen long

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Old   April 18, 2002, 12:32
Default Re: question on conditional breakpoint in using gd
  #2
andy
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You are likely to get a much better response posting this to a gnu/gcc/cc/... newsgroup.

I do not use gdb (i.e. I do not know if your command syntax/semantics is OK) but your code looks like Fortran and the added line some C/Fortran hybrid?

If you have not built a symbol table then the debugger will not be able to do much when you refer to "it". A symbol table is not usually built by default. Have you passed the "-g" flag when you compiled?

I would suggest better options for understanding your problems are:

(a) read the core file to find where and why the exception occurred. This is usually sufficient to find the odd underflow/overflow/divide_by_zero exceptions.

(b) add exception handlers (routines to be called when an exception occurs). This is the most flexible solution enabling you to do pretty much whatever you want.

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Old   April 18, 2002, 13:59
Default Re: question on conditional breakpoint in using gd
  #3
Wen Long
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Dear Andy:

Thanks for your reply. I did use -g option to enable the gdb. My code is purely Fortran, not easy to hook some exception handler onto it. There is a core file generated, but I don't know how to open it. Can you give more directions? Thanks again,

Wen
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Old   April 18, 2002, 14:44
Default Re: question on conditional breakpoint in using gd
  #4
andy
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>> My code is purely Fortran, not easy to hook some exception handler onto it.

On the unix workstations that I have used it has always been very easy to add exception handlers to Fortran code. A quick google is a bit negative for g77 though:

http://www.delorie.com/gnu/docs/g77/g77_630.html

It looks doable but messy.

>> There is a core file generated, but I don't know how to open it. Can you give more directions?

"man gdb" says the command is "gdb -c core"

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Old   April 18, 2002, 15:56
Default Re: question on conditional breakpoint in using gd
  #5
Wen Long
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Andy, Thanks a lot. Now I got the basic idea. The gdb I'm using doesn't support variables stored in common blocks or EQUIVALENCE. While I'm using many many common blocks.

That's why when I use "p it", it doesn't show me the value of variable "it" since it's in a common block.

Wen

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