Memory leak in dart:ffi samples

[xzfc]

Here the memory is allocated in reverse():

samples/ffi/structs/structs_library/structs.c

Line 31 in 21e809c

char *reversed_str = (char *)malloc((length + 1) * sizeof(char));

Here it used in toDartString():

samples/ffi/structs/structs.dart

Lines 67 to 68 in 21e809c

	final reversedMessage =
	reverse(backwards.toNativeUtf8(), backwards.length).toDartString();

It is not clear how the allocated memory should be freed.

Should it be done using malloc.free() call or there is a more conventional way to do it in dart? OTOH, if toDartString() actually takes ownership for this C string and there is no memory leak, then this behavior should be documented somewhere.

[dcharkes]

The snippet actually contains two memory leaks, backwards.toNativeUtf8() also leaks.

samples/ffi/structs/structs.dart

Lines 65 to 69 in 21e809c

	final reverse = dylib.lookupFunction<reverse_native, Reverse>('reverse');
	final backwards = 'backwards';
	final reversedMessage =
	reverse(backwards.toNativeUtf8(), backwards.length).toDartString();
	print('$backwards reversed is $reversedMessage');

should be

  final reverse = dylib.lookupFunction<reverse_native, Reverse>('reverse');
  final backwards = 'backwards'.toNativeUtf8();
  final reversedMessage =
      reverse(backwards, backwards.length).toDartString();
  malloc.free(backwards);
  print('$backwards reversed is $reversedMessage');
  malloc.free(reversedMessage);

However, this is slightly ugly, because now Dart is calling free on a malloc in C.

It would be cleaner to add a void free_string(char* str) to C which does the freeing and invoke that from Dart instead of malloc.free(reversedMessage). That way you are sure that malloc and free match up from the same compilation unit (which can be a problem on Windows - you cannot use free from one dll on a malloced pointer from another dll).

[xzfc]

Thanks. I think it would make sense to cover this in the sample code since memory management is a common issue with ffi code.