Fortran 77 handling C++ memory allocations - c++

Question

I'm trying to write a C++ program that utilizes a few tens of thousands of lines of Fortran 77 code, but running into some strange errors. I'm passing three coordinates (x,y,z) and the address of three vectors from C++ into fortran, then having fortran run some computations on the initial points and return results in the three vectors.
I do this a few hundred times in a C++ function, leave that function, and then come back to do it again. It works perfectly the first time through, but the second time through it stops returning useful results (returns nan) for points with a positive x component.
Initially it seems like an algorithm problem, except for three things:
It works perfectly the first 200 times I run it
It works if I call it from fortran and eliminate C++ altogether (not viable for the final program)
I've tried adding print statements to fortran to debug where it goes wrong, but turns out if I add print statments to a specific subroutine (even something as simple as PRINT *,'Here'), the program starts returning NaNs even on the first run.
This is why I think it's something to do with how memory is being allocated and deallocated between C and fortran function/subroutine calls. The basic setup looks like this:
C++:
void GetPoints(void);
extern"C"
{
void getfield_(float*,float*,float*,float[],float[],float[],int*,int*);
}
int main(void)
{
GetPoints(); //Works
GetPoints(); //Doesn't
}
void GetPoints(void)
{
float x,y,z;
int i,n,l;
l=50;
n=1;
x=y=z=0.0;
float xx[l],yy[l],zz[l]
for(i=0;i<l;i++)
getfield_(&x,&y,&z,xx,yy,zz,&n,&l);
//Store current xx,yy,zz in large global array
}
Fortran:
SUBROUTINE GETFIELD(XI,YI,ZI,XX,YY,ZZ,IIN,NP)
DIMENSION XX(NP),YY(NP),ZZ(NP)
EXTERNAL T89c
T89c(XI,YI,ZI,XX,YY,ZZ)
RETURN
END
!In T89c.f
SUBROUTINE T89c(XI,YI,ZI,XX,YY,ZZ)
COMMON /STUFF/ ARRAY(100)
!Lots of calculations
!Calling ~20 other subroutines
RETURN
END
Do any of you see any glaring memory issues that I'm creating? Perhaps common blocks that fortran thinks exist but are really deallocated by C++? Without the ability to debug using print statements, nor the time to try to understand the few thousand lines of someone else's Fortran 77 code, I'm open to trying just about anything you all can suggest or think of.
I'm using g++ 4.5.1 for compiling the C++ code and final linking, and gfortran 4.5.1 for compiling the fortran code.
Thanks
**Edit:**
I've tracked the error down to some obscure piece of the code that was written before I was even born. It appears it's looking for some common variable that got removed in the updates over the years. I don't know why it only affected one dimension, nor why the bug was replicatable by adding a print statement, but I've eliminated it nonetheless. Thank you all for the help.

Answer 1

You may be running into the "off-by-one" error. Fortran arrays are 1-based, while C arrays are 0-based. Make sure the array sizes you pass into Fortran are not 1 less than they should be.
Edit:
I guess it looks right... Still, I would try allocating 51 elements in the C++ function, just to see what happens.
By the way float xx[l]; is not standard. This is a gcc feature. Normally you should be allocating memory with new here, or you should be using std::vector.
Also, I am confused by the call to getfield_ in the loop. Shouldn't you be passing i to getfield_?

Answer 2

You should declare XX, YY and ZZ as arrays also in the subroutine T89c as follows:
REAL*4 XX(*)
REAL*4 YY(*)
REAL*4 ZZ(*)
C/C++ should in general never deallocate any Fortran common blocks. These are like structs in C (i.e. memory is reserved at compile time, not at runtime).
For some reason, gfortran seems to accept the following in T89c even without the above declarations:
print *,XX(1)
during compilation but when executing it I get a segmentation fault.