[SOLVED] segmentation fault problem??

s_hy · 03-04-2012, 08:44 PM

dear all,

i have a code with line segmentation fault error. please anyone advice me how to debug this code. thank you...

##################################################################

Program Main

IMPLICIT NONE

call fd1d

return
END PROGRAM Main

! 1d scalar wave equation with fd method and stability condition = 1
SUBROUTINE fd1d
implicit none

real,dimension(1) :: x
real :: x_delta
real :: t_delta
real :: c
real :: xlast
real :: xinit
real :: tinit
real :: tlast
real :: k
real :: x0
real,dimension(:,

,allocatable :: u
real :: alpha
integer :: iinit
integer :: ilast
integer :: ninit
integer :: nlast
integer :: i
integer :: n
real, parameter :: pi = 3.141592654

allocate (u(0:200,0:600))

!initialisation
c = 300
x_delta = 0.005
t_delta = x_delta/c
alpha = c*t_delta/x_delta
xlast = 1.0
xinit = 0
iinit = 0
ilast = ((xlast-xinit)/x_delta)
tlast = 0.01
tinit = 0
ninit = 0
nlast = ((tlast-tinit)/t_delta)
x(iinit) = xinit
x(ilast) = xlast

!initial profile
k = 1000.0
x0 = 0.3
do i = iinit,ilast
u(i,0) = 0.0
u(i,-1) = 0.0
end do

open (unit=100,file='n0.dat',action='write')
do i = iinit, ilast
x(i) = x(iinit) + i*x_delta
u(i,0) = exp (-k*(x(i)-x0)**2)
u(i,-1) = exp (-k*(x(i)-x0)**2)
Print *, u(i,0)
write (*,*) u(i,0)
end do
close (unit = 100)

open (unit=110,file='n1.dat',action='write')
do n = 0,nlast
do i = iinit,ilast
u(i,n+1) = 2*(1-(alpha**2))*u(i,n)-u(i,n-1)+(alpha**2)*(u(i+1,n)+u(i-1,n))
end do !i

!boundary condition
u(0,n+1) = 0
u(ilast,n+1) = 0
!end of boundary condition
!export data
print *, u(i,n+1)
write (110,*) u(i,n+1)
end do !n
close (unit = 110)

return
end SUBROUTINE fd1d

bigearsbilly · 03-05-2012, 05:28 PM

gdb?

does it do assembler.

use the 'where' command.

ta0kira · 03-05-2012, 05:51 PM

Quote:

Originally Posted by bigearsbilly

gdb?

does it do assembler.

use the 'where' command.

I believe this is Fortran, which you can debug with gdb.
Kevin Barry

s_hy · 03-05-2012, 08:29 PM

yes, this is fortran90. can anyone teach me how to use gdb??

ta0kira · 03-05-2012, 08:35 PM

It will be a lot easier to learn from an intro online than for me to try to explain it. It's very simple if you're just trying to find a segfault, as long as you compile with the -g flag (I'm assuming you're running some sort of *nix with gcc for this; if not, this probably doesn't help.)
Kevin Barry

s_hy · 03-05-2012, 08:39 PM

i use makefile to compile this(main.f90). the online version of gdb just show how to run it with .out, but my output are main.o and subroutines.o. Have no idea how to use gdb with this kind of file. pls advice.

firstfire · 03-05-2012, 10:54 PM

Hi.

To use gdb, compile your program with the `-g' option, like this

Code:

$ gfortran -g wave.f90

or add the option to makefile. To learn GDB read any online tutorial, this for example. Here is a sample gdb session with your program:

Code:

$ gfortran -g wave.f90 
$ gdb ./a.out
GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /tmp/a.out...done.
(gdb) run
Starting program: /tmp/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x0804897c in fd1d () at wave.f90:58
58            u(i,-1) = 0.0
(gdb) l     # list -- type the text of the program in the vicinity of where it is presently stopped.
53            !initial profile
54            k = 1000.0
55            x0 = 0.3
56            do i = iinit,ilast
57            u(i,0) = 0.0
58            u(i,-1) = 0.0
59            end do
60            
61            open (unit=100,file='n0.dat',action='write')
62            do i = iinit, ilast
(gdb) p u    # p = print
$1 = (( 0) )
(gdb) p i
$2 = 0
(gdb) p u(0,-1)
no such vector element
(gdb) quit
A debugging session is active.

        Inferior 1 [process 20884] will be killed.

Quit anyway? (y or n) y

We see that the bug reveal itself in the line where you are trying to assign 'u(0,-1)'. You should either rethink this line or change the definition of u:

Code:

allocate (u(0:200,-1:600))

Next, when we made this change, we similarly reveal next bug in the line

Code:

(gdb) run
Starting program: /tmp/a.out 
  8.19395264E-40
  8.19395264E-40
  1.60516147E-38
  1.60516147E-38

Program received signal SIGSEGV, Segmentation fault.
0x08048a46 in fd1d () at wave.f90:64
64            u(i,0) = exp (-k*(x(i)-x0)**2)
(gdb) p i
$1 = 2
(gdb)

This time we access element 2 of array x, which has only one element... So we change definition of x to

Code:

real,dimension(200) :: x

and voila, the program works.

Hope that helps.

s_hy · 03-05-2012, 10:59 PM

wow...it's work!! great...thank you so much