Re: [Pharo-users] UFFI C basic types passed-by-reference for output

Nov. 15, 2017

      On 15 November 2017 at 17:27, Esteban Lorenzano <estebanlm@gmail.com> wrote:
...
...
On 15 Nov 2017, at 02:05, Ben Coman <btc@openInWorld.com> wrote:
What is the recommended way for a C basic type to be passed-by-reference
to function wanting to use it for output.
For example 'width' & 'height' here in this library unction...
int FPDF_GetPageSizeByIndex(FPDF_DOCUMENT document,
                          int page_index,
                          double* width,
                          double* height);
void mytestGetPageSizeByIndex(doc) {
           double width, height;
            FPDF_GetPageSizeByIndex(doc, 0, &width, &height);
            printf("width=%f, height=%f\n", width, height);
}
gives...
   width=200.000000, height=200.000000
In Pharo I'm trying...
FFIOpaqueObject subclass: #FPDF_DOCUMENT
FPDF_GetPageSizeByIndex__document: document
                          page_index: page_index
                          width: width
                          height: height
    ^self ffiCall: #(int FPDF_GetPageSizeByIndex(
                                              FPDF_DOCUMENT *document,
                                              int page_index,
                                              FFIFloat64 * width,
                                              FFIFloat64 * height))
testGetPageSizeByIndex
      | document page_index width height result|
      PDFium FPDF_InitLibrary.
      document := PDFium FPDF_LoadDocument__file_path:  helloPdf
password: ''.
      width := 0.0.
      height := 0.0.
      page_index := 0. "Its zero based"
      result := PDFium FPDF_GetPageSizeByIndex__document: document
                       page_index: 0
                       width: width
                       height: height.
      PDFium FPDF_CloseDocument__document: document.
      PDFium FPDF_DestroyLibrary.
      self assert: document isNull not. "Document opened okay, and btw
this works for a different pageCount test"
      self assert: result > 0.  "Non-zero for success. 0 for error
(document or page not found)"
      self halt.
and at the halt the Inspector shows...
result = 1
width = 0.0
height = 0.0
no, that will not work :)
what you need to do here is to pass a âbufferâ to contain the width and
height:
testGetPageSizeByIndex
        | document page_index widthBuffer heightBuffer width height result|
        PDFium FPDF_InitLibrary.
        document := PDFium FPDF_LoadDocument__file_path:  helloPdf
password: ''.
        widthBuffer := ByteArray new: (FFIFloat64 typeSize).
        heightBuffer := ByteArray new: (FFIFloat64 typeSize).
Thanks Esteban. That worked. However I needed a minor tweak...
   widthBuffer := ByteArray new: (FFIFloat64 new typeSize).
or...
   widthBuffer := ByteArray new: (FFIFloat64 externalTypeSize).

Now it would be nice to do...
   widthBuffer := FFIFloat64 newBuffer.

which could be implemented...
   FFIExternalType class >> newBuffer.
        ^ByteArray new: (self externalTypeSize)

testGetPageSizeByIndex
        | document page_index widthBuffer heightBuffer width height result|
        PDFium FPDF_InitLibrary.
        document := PDFium FPDF_LoadDocument__file_path:  helloPdf
password: ''.
        widthBuffer := FFIFloat64 buffer.
        heightBuffer := FFIFloat64 buffer.
        page_index := 0. "Its zero based"
        result := PDFium FPDF_GetPageSizeByIndex__document: document
                         page_index: 0
                         width: widthBuffer
                         height: heightBuffer.
        width := widthBuffer doubleAt: 1.
        height := heightBuffer doubleAt: 1.

btw, a broader curiousity even though this is likely set in stone and not
even your personal choice - why "FFIFloat64" and not "FFIDouble" ?
   widthBuffer := FFIDouble newBuffer.
would be a nice match to the callout declaration.

Would it be overkill to have in Pharo an empty FFIDouble subclassed from
FFIFloat64 ?
the superclass being explicit about the format, while the subclass provides
a nice identifier for users?
...
page_index := 0. "Its zero based"
        result := PDFium FPDF_GetPageSizeByIndex__document: document
                         page_index: 0
                         width: widthBuffer
                         height: heightBuffer.
        width := widthBuffer doubleAt: 1.
        height := heightBuffer doubleAt: 1.
PDFium FPDF_CloseDocument__document: document.
...
PDFium FPDF_DestroyLibrary.
        self assert: document isNull not. "Document opened okay, and btw
this works for a different pageCount test"
        self assert: result > 0.  "Non-zero for success. 0 for error
(document or page not found)"
        self halt.
side note: I do not recommend using the UFFI type in declarations. In the
long way is worst for comprehension and maintainability. This thingy:
^self ffiCall: #(int FPDF_GetPageSizeByIndex(
                                                FPDF_DOCUMENT *document,
                                                int page_index,
                                                FFIFloat64 * width,
                                                FFIFloat64 * height))
would be better as in original version:
^self ffiCall: #(int FPDF_GetPageSizeByIndex(
                                                FPDF_DOCUMENT *document,
                                                int page_index,
                                                double * width,
                                                double * height))
Thanks for the heads up.

cheers -ben