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Proposed Backend Interface

typedef union _PDCLIB_fd_t {
    uintptr_t u;
    intptr_t  i;
    void     *p;
} _PDCLIB_fd_t;

struct _PDCLIB_file_vtbl {

size_t (*write) (_PDCLIB_fd_t fd, const char    *restrict buf, size_t len, fpos_t* restrict pos);
size_t (*read)  (_PDCLIB_fd_t fd,       char    *restrict buf, size_t len, fpos_t* restrict pos); 

size_t (*wwrite)(_PDCLIB_fd_t fd, const wchar_t *restrict buf, size_t len, fpos_t* restrict pos);
size_t (*wread) (_PDCLIB_fd_t fd,       wchar_t *restrict buf, size_t len, fpos_t* restrict pos);

// Other I/O functions - seek/close/etc 

}; 

All the functions take the following parameters:

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