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POinteroperability between Cpython and fortran linked lists
 

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  1. POinteroperability between Cpython and fortran linked lists
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    <p>I have a fortran linked list that is roughly like </p> <pre><code>type :: node type(node), pointer :: next =&gt; null() integer :: value end type node </code></pre> <p>I would ideally like to interact with this using Cpython. I have used fortran subroutines with python a lot using the f2py program to create the shared object. However, f2py can not be used with derived types. </p> <p>My question is simply whether it possible at all to access something like a linked list in Fortran with cpython. I presume I need to follow the fortran to c to cpython route. However, I've read that for a fortran derived type to interoperable with c "Each component must have interoperable type and type parameters, must not be a pointer, and must not be allocatable." Likewise, the post <a href="https://stackoverflow.com/questions/13748276/c-fortran-interoperability-derived-types-with-pointers">c-fortran interoperability - derived types with pointers</a> seems to confirm this. </p> <p>I was wondering if anyone knows if it is definitely not possible to access a linked list in fortran from cpython. If it is possible, even if indirectly or in a round-about way, I would be grateful to hear more.</p> <p>thank you, Mark </p>
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    1. COI'm not sure that the implementation of fortran pointers is standardized between compilers -- So I'm doubtful that you'll be able to do this. If you only need 1 linked list, you could put one in a module and have a bunch of functions use that module to access it -- That's pretty hacky of course. Also note that python pretty much makes the necessity for a linked-list a non-issue ... You can always convert a python list to a numpy array and pass that around (but I understand that working with legacy code might make that difficult)...
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    2. COThat won't be C interoparable as it is. You could use `type(cptr)` as pointer to point to the next structure, but then you would have to convert inside Fortran this pointer via `c_f_pointer()` to be useable (and with `f_c_pointer()` back if needed), so rather a pain...
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      1. USBálint Aradi
    3. COBy far the easiest would probably be to create an array based on the linked list and pass that to python. But that would be a quite heavy operation. Directly accessing your existing list from C is possible, but not in a portable way (You would need to reverse engineer how the fortran pointer is represented, and how the derived type is laid out in memory. And then hope it doesn't change in the next compiler release).
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