Fortran Debugging

Question

I am trying to create a Navier Stokes solver with a bunch of subroutines and multiple iterations which also uses some LAPACK subroutines on MAC through the accelerate Framework.

The program compiles correctly on the first iteration but collapses somewhere in the middle saying this:

a.out(28015,0x7fff74e78300) malloc: *** error for object 
0x7f81e902a008: incorrect checksum for freed object - 
object was probably modified after being freed.
 *** set a breakpoint in malloc_error_break to debug

I am looking for some advice on how to try and debug this. A simpler version of the program runs smoothly but when I try to add the following subroutine to modify a variable I get the error. I am reproducing the subroutine here too. The subroutine modifies ui and vi variables:

   subroutine interpforce(ui,vi,ub,vb,
 1       xb,yb,x,y,m,n,uz,h,dt)
   implicit double precision (a-h,o-z)
   integer uz,k,q,IPIV,INFO
   dimension ub(0:uz-1),vb(0:uz-1),xb(0:uz),yb(0:uz)
 1     ,x(-1:m+1),y(-1:n+1),a(0:uz-1,0:uz-1),
 2      fkx(0:uz-1),fky(0:uz-1),dels(0:uz-1),
 3     bx(0:uz-1),by(0:uz-1),IPIV(uz),
 4     fex(0:m,0:n),fey(0:m,0:n),ui(0:m,0:n),vi(0:m,0:n)

   do i=0,m
       do j=0,m

       fex(i,j)=0.0d0
       fey(i,j)=0.0d0

       enddo
   enddo


   do k=0,uz-1
      fkx(k)=0.0d0
      fky(k)=0.0d0
      dels(k)=sqrt((xb(k+1)-xb(k))**2 +  (yb(k+1)-yb(k))**2)

        do q=0,uz-1
          a(k,q)=0.0d0
        enddo
   enddo

   do k=0,uz-1
       do q=0,uz-1

           do i=0,m
              do j=0,n

              a(k,q)=a(k,q)+(umdirac(x(i)-xb(q),h)*
 1            umdirac(y(j)-yb(q),h)*dels(q)*h*
 2            umdirac(x(i)-xb(k),h)*umdirac(y(j)-yb(k),h))

              enddo
            enddo
         enddo
   enddo



   do k=0,uz-1
       bx(k)=-ub(k)/dt
       by(k)=-vb(k)/dt
   enddo

   call DGESV(uz,1,a,uz,IPIV,bx,uz,INFO)
   call DGESV(uz,1,a,uz,IPIV,by,uz,INFO)

   do k=0,uz-1
       fkx(k)=bx(k)
       fky(k)=by(k)
   enddo

   do i=0,m
     do j=0,n
       do k=0,uz-1
          fex(i,j)=fex(i,j)+fkx(k)*(umdirac(x(i)-xb(k),h))
 1                 *(umdirac(y(j)-yb(k),h))*dels(k)

          fey(i,j)=fey(i,j)+fky(k)*(umdirac(x(i)-xb(k),h))
 1                 *(umdirac(y(j)-yb(k),h)*dels(k))
       enddo
     enddo
   enddo

   do i=0,m
       do j=0,n
          ui(i,j)=ui(i,j)+dt*fex(i,j)
          vi(i,j)=vi(i,j)+dt*fey(i,j)
       enddo
   enddo

   return
   end

c ************************************ UMDIRAC FUNCTION          ***************************************

   function umdirac(x,h)
   implicit real*8 (a-h,o-z)

   if (abs(x).lt.h) then
      umdirac=(h-abs(x))/h**2
   else
      umdirac=0.0d0
   endif

   return
   end

Answer 1

This kind of error is generally the result of memory corruption from a stray write operation.

Assuming you're using gfortran, try adding the compiler option -fcheck=bounds and running again.

If that doesn't do it, try running your program under valgrind to see if it can spot your error.