How to pass a list of strings to C In dart ffi

Issue

I’m learning how to use dart ffi.

I don’t have much C experience or knowledge.

I’m trying to call a function from a C library(tiny file dialogs).

I can call other functions however I can’t call this one

char * tinyfd_saveFileDialog(
        char const * aTitle , /* NULL or "" */
        char const * aDefaultPathAndFile , /* NULL or "" */
        int aNumOfFilterPatterns , /* 0 */
        char const * const * aFilterPatterns , /* NULL or {"*.jpg","*.png"} */
        char const * aSingleFilterDescription ) /* NULL or "image files" */
{

the main issue is that I don’t understand what

char const* const * a FilterProperties, / *Null or {"*.jpg", "*.png"} */

means.

As far as I was able to get from this issue the C function is asking for an array of strings. The arg aNumberOfFilterPatterns defines the length of the array.

If that’s the case, how can I prepare the array in dart to pass it to C?

Solution

I was able to do it by making a pointer to pointer to char

like this

/// Define the type like this
typedef SaveFileDialogD = Pointer<Utf8> Function(
 
  Pointer<Pointer<Utf8>> listOfStrings,

);

Look up the function in the dynamic library

SaveFileDialogD saveFileDialog = dylib
    .lookupFunction<SaveFileDialogC, SaveFileDialogD>('dylib');

make a function to convert Dart list of string to something more C friendly

/// Don't forget to run malloc.free with result!
Pointer<Pointer<Utf8>> strListToPointer(List<String> strings) {
  List<Pointer<Utf8>> utf8PointerList =
      strings.map((str) => str.toNativeUtf8()).toList();

  final Pointer<Pointer<Utf8>> pointerPointer =
      malloc.allocate(utf8PointerList.length);

  strings.asMap().forEach((index, utf) {
    pointerPointer[index] = utf8PointerList[index];
  });

  return pointerPointer;
}

Finally call the C function like this

Pointer<Utf8> result;
    final strListPointer = strListToPointer(["one", "two", "three"]);

    result = saveFileDialog(
       strListPointer;
    );

Answered By – xerotolerant

Answer Checked By – Timothy Miller (FlutterFixes Admin)