Re: Bug with ISO_C_BINDING on Mac OS?

[Tobias Burnus <burnus at net-b dot de>]

Janus Weil wrote:
> attached are two small test cases (Fortran & C), which try to open a
> bz2 file for reading (they require a, possibly empty, file 'test.bz2'
> in the same dir). The Fortran version uses ISO_C_BINDING interfaces
> for the calls to 'fopen' and 'BZ2_bzReadOpen', and both work as
> expected on Linux (x86_64):

Yes, there is a bug with ISO_C_BINDING - but the bug is in your code ;-)

At least the problem below is likely to cause problems on x86-64. I 
don't know why you only see the problem on MacOS, it should also exist 
on 64bit Linux.

You declare:

>    interface
>      function fOpen (path,mode) bind (c,NAME='fopen')
>        use, intrinsic :: ISO_C_BINDING
>        character(c_char) path(*),mode(*)
>        integer(c_int) :: fOpen
>      end function
>    end interface

However, POSIX defines that fopen returns a "FILE *". On most 32 bit 
systems, it should work. However, on a 64bit system, it wont (or only by 
chance). Either you should use a proper pointer like TYPE(c_ptr). Or, if 
you want to use an integer, use: INTEGER(c_intptr_t).

>     interface
>      function bzReadOpen (bzerror,f,verbosity,small,unused,nUnused) bind (c,NAME='BZ2_bzReadOpen')
>        use, intrinsic :: ISO_C_BINDING
>        integer(c_int),value :: f,verbosity,small,nUnused
>        integer(c_int) :: bzReadOpen,bzerror
>        type(c_ptr) :: unused
>      end function
>    end interface

This one looks bogus as well. bzerror and f are both pointers (int * and 
FILE *).

For bzerror: I think you really should use "INTEGER(c_int)"  (w/o VALUE) 
- that way you get a proper error return. If you use VALUE, you will 
pass some random value which is in the actual argument, which will cause 
problems.

And for FILE *f: Here, you have to ensure that you pass all 64bits of 
the result value of "fopen". Again, I would use a "TYPE(C_PTR),VALUE" 
which reflects the black box best. But using an 
INTEGER(c_intptr_t),VALUE should also work. (Cf. above for "fopen".)


I can imagine two reasons that it works under Linux:

a) You get an error under MacOS but not under Linux, i.e. bzReadOpen 
tries to write to "bzerror".
b) FILE * returns by chance a pointer address which fits into 32bit 
under Linux but not under MacOS.

But there are surely more problems. In particular: bzReadOpen reads much 
more argument values from the stack than you put there.

Frankly, I am still surprised that it (seems to) work on a 64bit Linux.

Tobias