Re[2]: Using CFFI: dealing with a writeback buffer

[Victor]

Hi Orivej, Luís

thanks for your help - the code works finally now. See the method defined in the end of the file:

https://bitbucket.org/vityok/cl-gdal/src/75b9b957b29767a55349282eb4d3389fafae5387/src/ogr-spatialref.lisp?at=default

The CPL-FREE function for whatever reason is not exported by the library (contrary to what I expect based on the documentation). But I'm fine with it at the moment.

BTW, any feedback or advice on CL-GDAL library is welcome.

Thanks!
Victor



15.07.2015 04:26, Orivej Desh <c at orivej.org>
>> I don't know if it is the best place to ask this question, but I'll try it. I am trying to wrap around a following function:
> > 
> > OGRErr OSRExportToProj4(OGRSpatialReferenceH r, char **buff);
> > 
> > This function allocates the string and assigns it to the (char*) pointer passed as buff and then its contents should be copied into a C string and the obtained pointer released via CPLFree();
> 
> For the sake of example, I assume the pointer size is 8 bytes.
> 
> First you learn elsewhere that OGRSpatialReferenceH is a pointer and OGRErr is an int. [1]
> 
> Then you know that OSRExportToProj4 takes two pointers and writes to the memory addressed by the second argument a pointer (char*),
> so the second argument must point to an allocated memory region 8 bytes in size; in CFFI, you usually allocate memory using WITH-FOREIGN-OBJECT.
> 
> After OSRExportToProj4 wrote a pointer to a C-string into that region of memory, you can read it back in CFFI as a pointer using (MEM-REF ... :POINTER).
> 
> When you hold a pointer to a C-string, you can convert it into a Lisp string with FOREIGN-STRING-TO-LISP.
> 
> Overall, you need something like
> 
> (cffi:defcfun (%osr-export-to-proj-4 "OSRExportToProj4") :int
> (handle :pointer)
> (*str :pointer))
> 
> (defun osr-export-to-proj-4 (handle)
> (cffi:with-foreign-object (*str :pointer)
> (%osr-export-to-proj-4 handle *str)
> (let ((str (cffi:mem-ref *str :pointer)))
> (unwind-protect (cffi:foreign-string-to-lisp str)
> (cpl-free str)))))
> 
> 
> [1] One way to learn it is with c2ffi (https://github.com/rpav/c2ffi):
> 
> $ c2ffi -D sexp /usr/include/gdal/ogr_geometry.h 2>/dev/null | awk '$2 ~ "OGRErr|OGRSpatialReferenceH|OSRExportToProj4" {print}'
> (typedef OGRErr :int)
> (typedef OGRSpatialReferenceH (:pointer :void))
> (function "OSRExportToProj4" ((OGRSpatialReferenceH) ((:pointer (:pointer :char)))) OGRErr)


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