Calling a global string variable from Fortran in C causes segmentation fault

Question

I'm trying to call a global string variable which is defined in a Fortran subroutine, in C. the C code is Cfile.c:

#include <stdio.h>

typedef struct {
        int length;
        char* string;
} fstring;

extern fstring stringf_;
void fortfunc_();

int main() {
        fstring stringC = stringf_;
        stringC.string[stringC.length-1] = '\0';
        printf("%s \n",stringC.string);
        return 0;
}

and FORTRAN code is Ffile.f:

subroutine fortfunc()
  
        character*30 string
        common/stringF/ string
        string = 'this is a string in FROTRAN77'

return
end

it compiles with:

gcc -c Cfile.c
gfortran -c -std=legacy Ffile.f
gfortran -c file.out -std=legacy Cfile.o Ffile.o

but when running I get the segmentation fault. I do not understand when I'm violating the memory boundaries though.

My operating system is:

Linux ubuntu 4.15.0-39-generic #42-Ubuntu SMP Tue Oct 23 15:48:01 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

and my compilers are:

GNU Fortran (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

I would appreciate if you could help me know where is my mistake and how I can solve it? other solutions to define a global variable in Fortran and then calling it in C are also welcomed.

Answer 1

Based on the comments I got here and on Reddit, I now have a code which works. The C code:

#include <stdio.h>

typedef struct {
    char s[30];
} fstring;

extern fstring stringf_;

int main() {
    fstring stringc = stringf_;
    stringc.s[29] = '\0';
    printf("%s\n",stringc.s);
    return 0;
}

and FORTRAN code:

        BLOCK DATA

                CHARACTER*30 S
                COMMON /STRINGF/ S
                DATA S /'this is a string in FROTRAN77'/

        end

The segfault was happening because the passed stringC.length value is zero. It means unlike the example I was following here when calling a string from the FORTRAN side it does not pass the length as an integer!