Calling C function from Fortran with pointers

Question

I am attempting to create a function in C that is called from Fortran. First things first. The fortran code is being compiled with f77, and the c is compiled with gcc. Both are compiled into .so libraries. The c function is going to read the memory on a device at an address, and size, specified by the fortran. I am able to see the address, and size being passed to the c, but I am having trouble filling the data in the c function and returning it to the fortran. See the relevant code below. My assumption is that there is something wrong in the memory allocation or pointer syntax for the data variable.

C

void copymemory_( uint32_t *addr, int *size, uint8_t *data )
{

    int i;
    printf("addr %x \n", *addr);
    printf("size %i \n", *size);

    for ( i = 0; i<*size; i++)
    {
        *data[i] = i;
        printf("memory %i \n",*data[i]);
    }

}

Fortran

integer memory(4)
call copymemory(z'dead', 4, memory)

DO i = 1,memsize
   call printf(memory(i))
END DO

Why do you think something is wrong? What does happen? On the Fortran side, what are memsize and printf and what is that z'dead' doing where it is? On the C side, why uint8_t? Why can't you use standard Fortran-C interoperability? — francescalus
– francescalus, Commented Jan 6, 2017 at 18:06
Well mate, this is pretty much dependent on calling conventions... I've never written fortran code throughout my entire life, but i'll try now — DrPrItay
– DrPrItay, Commented Jan 6, 2017 at 18:09
any error messages? Which ones? Wrong results printed? How do they look like? — Vladimir F Героям слава
– Vladimir F Героям слава, Commented Jan 6, 2017 at 18:19
printf, is a function which just prints to the screen. memsize is an integer equal to 4 after printing the values in the array in the c function the executable will crash, the actual error is that the process id died due to signal 11, and left a core dump. — cwheat
– cwheat, Commented Jan 6, 2017 at 18:36
Don't bother with these descriptions. Edit the question and copy here what you have, it is much clearer. — Vladimir F Героям слава
– Vladimir F Героям слава, Commented Jan 7, 2017 at 11:14

Vladimir F Героям слава · Accepted Answer · 2017-01-07 11:33:50Z

2

I have several points to your code.

Please, supply compilable code and describe the output of that exact code!

That includes the #include for standard headers, if you want people to debug your code, make it easy for them so that they don't have to search which lines did you omit because they seemed "obvious" to you. Just paste everything. The code should compile when copied from here!

Even the executable part of your code does not compile in my compiler. I had to change the *data to data. Are you sure you copied your actual code?

cfun.c: In function ‘copymemory_’:
cfun.c:13:9: error: invalid type argument of unary ‘*’ (have ‘int’)
         *data[i] = i;
         ^
cfun.c:14:31: error: invalid type argument of unary ‘*’ (have ‘int’)
         printf("memory %i \n",*data[i]);
                           ^

Your Fortran code contains a call to some printf subroutine. Where is this defined? Is it present in your actual code? I doubt so. Please copy to StackOverflow always a complete and compilable code.

So after fixing the obvious your present code is:

#include <stdio.h>
#include <stdint.h>

void copymemory_( uint32_t *addr, int *size, uint8_t *data )
{

    int i;
    printf("addr %x \n", *addr);
    printf("size %i \n", *size);

    for ( i = 0; i<*size; i++)
    {
        data[i] = i;
        printf("memory %i \n",data[i]);
    }

}


implicit none

integer :: memsize = 4

integer memory(4)
integer i

call copymemory(z'dead', 4, memory)

DO i = 1,memsize
   print *, (memory(i))
END DO

END

It does not crash but memory in Fortran contains garbage. It has to, because it is integer and you are treating it as int8_t in C.

So either treat it as an array of four integers in C or copy it byte by byte, but then you must pass the correct number of bytes to copy. From your description it is not clear which one is your intention so I will show just one possibility:

void copymemory_( uint32_t *addr, int *size, uint32_t *data )

The output is correct then:

> gfortran-4.10 -fsanitize=address cfun.c ffun.f90 
> ./a.out 
addr dead 
size 4 
memory 0 
memory 1 
memory 2 
memory 3 
           0
           1
           2
           3

answered Jan 7, 2017 at 11:33

Vladimir F Героям слава

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2 Comments

cwheat Over a year ago

After much more research, I found that there is actually no solution for my purposes. The solution you provided was correct, and would work if I were not using the f77 compiler. f77 does not allow passing from c to fortran, and using the code you provided, the fortran array always had garbage in it. If I could use f90 the solution you provided would work though.

Vladimir F Героям слава Over a year ago

Every compiler has its own calling convention Basically all F77 compilers allowed this, but there is no "the F77 compiler", there are many different compilers and you must study the manual to find out how to do that properly. This is not F77 vs F90, this is one compiler vs. different compiler. My solution is not standard F90, it is gfortran specific. There is no standard F90 solution, standard C-Fortran interoperability is F2003.

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