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wuwuxin

How to "correctly" pass an array to a C function

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Posted (edited)

I have a C DLL function,

void FillArray(int* data, int num_element);

 

from Delphi side,  is the following caller code correct? 

 

var data: TArray<Integer>;
SetLength(data, 100);
FillArray(data, 100);

 Also, what would be the correct Delphi translation of the C function? Is the following correct?  Should I use "const" or not?

procedure FillArray(const data: TArray<Integer>; num_element: Integer);

 

Edited by wuwuxin

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Posted (edited)

You need to specify the calling convention, probably cdecl. And I'd declare it as PInteger and pass Pointer(data). 

Edited by David Heffernan

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1 hour ago, wuwuxin said:

FillArray(data, 100);

Should be

FillArray(data, 100 * SizeOf(Integer));

And you can use const or var with the same result in this case, As David pointed to declare the function with PInteger and pass it by PInteger(@data[0]).

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1 hour ago, Kas Ob. said:

As David pointed to declare the function with PInteger and pass it by PInteger(@data[0]).

The reason I prefer Pointer(data) is that it doesn't trigger range check error if the array has length 0.

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Thank you very much for the advice.

 

1 minute ago, David Heffernan said:

The reason I prefer Pointer(data) is that it doesn't trigger range check error if the array has length 0.

 

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8 minutes ago, David Heffernan said:

The reason I prefer Pointer(data) is that it doesn't trigger range check error if the array has length 0.

You are totally right there, but i prefer sometimes to trigger it explicitly (in development) to make sure (in similar case to this as with an external API) that the called function is well equipped to handle such case, in other words to make sure i am not leaving it unhandled and unchecked, just as safe measure.

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1 hour ago, Kas Ob. said:

You are totally right there, but i prefer sometimes to trigger it explicitly (in development) to make sure (in similar case to this as with an external API) that the called function is well equipped to handle such case, in other words to make sure i am not leaving it unhandled and unchecked, just as safe measure.

Your way makes it impossible to pass an empty array. If an empty array is never valid, then fine. If an empty array is supported then the Pointer cast is preferable. 

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33 minutes ago, David Heffernan said:

Your way makes it impossible to pass an empty array. If an empty array is never valid, then fine. If an empty array is supported then the Pointer cast is preferable. 

Right, but i am talking about this case in general, see, it is very rare to design your own code where you need to pass TArray (or any other defined indexed type) to a function with "pointer to array" with element length, most likely the destination belongs to another realm or simply put belongs to a code by different coder (or library), and on same side in cases other than the above example where length established locally, the length might be 0, in this case i prefer to have a nice overflow exception instead of unpredicted behaviour in case i missed explicitly a check against 0 length.

 

It is just personal point of view, in many cases i prefer the compiler checks, unless performance is priority to consider.

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23 hours ago, Kas Ob. said:

Should be


FillArray(data, 100 * SizeOf(Integer));

Why?  The C code clearly names the parameter "num_elements", so it stands to reason that it expects an "element count", not a "byte count".

23 hours ago, Kas Ob. said:

As David pointed to declare the function with PInteger and pass it by PInteger(@data[0]).

A dynamic array is already a pointer, so you don't need to index into the array, PInteger(data) will suffice.

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14 minutes ago, Remy Lebeau said:

Why?  The C code clearly names the parameter "num_elements", so it stands to reason that it expects an "element count", not a "byte count".

I missed that with FillChar, you are right there.

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