从 C++ 到 fortran 数组的访问冲突

Question

I am working on a C++-fortran mix compiling project. On the fortran side, I wrote an interface to segregate the C++ reference/pointers to fortran variables. After the fortran subroutine did their work, the interface will assign the value one-by-one to the C++ array. The problem occurred when it went back to C++ side. I can see the value of each array elements in the C++ debugger, but when I use std::cout<<arr[0]<<std::endl, it gives me the access error:

Exception thrown at 0x79762B8E (msvcp140d.dll) in TEST_IO.exe: 0xC0000005: Access violation reading location 0x9C6D0014

I wrote a simplified test code as:

#include <iostream>
extern "C" {void fort_interface(<typeName1> &par1,<typename2> *par2,....,<typeName> *output);}
int main()
{
..... //setup input parameters
float *arr = new float[N_arr]; //N_arr is big enough.

float check = 12321;
std::cout<<check<<std::endl;
/*the float variable check is independent from fort_interface, just to check the memory status*/

fort_interface(par1, par2,...,arr);

std::cout<<check<<std::endl;//exception occurs!
}

The same exception reported even I print out "check" that did no business with fort_interface(). At the same time, the value of "check" was 12321 in the debugger.

I wrote another toy fortran-C++ code to test how fortran play with pointers/references, everything goes well. Value can be printed, destructor works well too.

Sorry that I cannot upload the fortran-C++ interface here because it contains 100+ parameters (that's the reason why I wrote this interface to keep fortran original code intact with outside argument.) I just want to know why I can see the value in debugger but it cannot be accessed? Thank you for your help!

---

Update: The problem was solved! I passed a pointer to pointer to the fortran subroutine, which leaded the segment error. Nothing wrong about fortran-C mix compile setting. Just because of the segment error. Thank you all again!

Answer 1

Since I can't see your Fortran-C++ interface, I will make a guess that might help.

When you process arrays in fortran, they are saved as columns in your memory, while C/C++ saves them as rows. which makes the interface between these two languages tricky.

For example, if you have arr[5][5] and you call the element arr[1][0] in C, the compiler will fetch the row 1 and keep it in your cache, while fortran compiler will fetch column 0 and keep it in your cache.

I hope this helps