Hi all,
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();
So, basically, in C a typical usage would be:
char *proj = NULL: OSRExportToProj4(handle, &proj); printf("%s\n", proj); CPLFree(proj);
And the question is: how is this function should be wrapped with CFFI?
Thanks, Victor
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Hello Victor,
I think you're looking for something along these lines.
(with-foreign-object (proj :pointer) (osr-export-to-proj-4 handle proj) (let ((p (mem-ref proj :pointer))) (write-line (foreign-string-to-lisp p)) (cpl-free p)))
Cheers, Luís
(painstakingly typed on a phone)
On Tue, Jul 14, 2015, 14:37 Victor bobbie@ua.fm wrote:
Hi all,
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();
So, basically, in C a typical usage would be:
char *proj = NULL: OSRExportToProj4(handle, &proj); printf("%s\n", proj); CPLFree(proj);
And the question is: how is this function should be wrapped with CFFI?
Thanks, Victor
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--
Luís
(sent from my phone)
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)
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/75b9b957b29767a55349282eb4d3389fafa...
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@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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-
Luís Oliveira -
Orivej Desh -
Victor