I have a project that is half in C and half in Fortran 77. [No, not Fortran 90 or 03, Fortran 77.] The code would be much cleaner if I could pass pointers generated on the C side back to Fortran, which would then pass them back as necessary for handling in other C functions. As it is, the C code is filled with global variables that shouldn't be global, and is otherwise on the verge of becoming an unstructured mess. So are there any reasonably reliable ways to pass an opaque pointer between C and Fortran?
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A:
If you are on a 32-bit platform, consider casting the pointers to integers and passing those integers to the Fortran code. When the Fortran passes them back, reconvert the integer back into a pointer, cross-fingers, and use.
From what I remember (from 25+ years ago), Fortran 77 tends to pass everything to C by pointer anyway - and character strings get passed with a length, and arrays get passed with their dimensions.
If you're on a 64-bit platform, you'll have to work out whether the Fortran 77 compiler provides any 8-byte integers (INTEGER*8?) - my suspicion is that it won't (largely confirmed by looking at the GNU documentation; if you were using Fortran 2003, you'd be in better shape, it seems). If it does, the same trick works. If it does not, you are into much dodgier territory.
You could try - against recommendations - using a union of a double and a pointer. In the C, you'd set the pointer in the union from your C code pointer, then copy the double out of the union into a Fortran REAL*8, and as long as no-one touches that except to copy it or pass it back, maybe you will be OK if the gods smile favourably upon your endeavours. Most likely though, the whole thing will explode - this sort of union has an incredible ability to detect when the customer will be most annoyed if something doesn't work and will then proceed to explode at exactly the right moment - part way through the demo, or fifteen minutes after the program goes live.
An alternative to consider (still with gritted teeth) is a union of a 64-bit pointer and an array of two 32-bit integers, and then requiring the Fortran code to pass an array of two integers when you need to return a (64-bit) pointer. Clearly, an array of one integer(s) would work to 32-bit code; maybe just require the calling code to pass an array of two integers in all cases, zeroing the unused integer value in the 32-bit pointer case? That gives you forward migratability.
Jonathan Leffler
2009-10-16 21:18:41