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 ��buffer�� 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