Fix signature of lseek on Win32. Fixes #1189.

source/vibe/core/drivers/threadedfile.d

@@ -28,8 +28,10 @@ version(Windows){
 	import std.c.windows.stat;
 
 	private {
-		extern(C){
-			alias off_t = long;
+		// TODO: use CreateFile/HANDLE instead of the Posix API on Windows
+
+		extern(C) {
+			alias off_t = sizediff_t;
 			int open(in char* name, int mode, ...);
 			int chmod(in char* name, int mode);
 			int close(int fd);
@@ -142,7 +144,11 @@ final class ThreadedFileStream : FileStream {
 
 	void seek(ulong offset)
 	{
-		enforce(.lseek(m_fileDescriptor, offset, SEEK_SET) == offset, "Failed to seek in file.");
+		version (Win32) {
+			enforce(offset <= off_t.max, "Cannot seek above 4GB on Windows x32.");
+			auto pos = .lseek(m_fileDescriptor, cast(off_t)offset, SEEK_SET);
+		} else auto pos = .lseek(m_fileDescriptor, offset, SEEK_SET);
+		enforce(pos == offset, "Failed to seek in file.");
 		m_ptr = offset;
 	}
 
@@ -208,3 +214,16 @@ final class ThreadedFileStream : FileStream {
 		flush();
 	}
 }
+
+unittest { // issue #1189
+	auto fil = new ThreadedFileStream(Path(".unittest.tmp"), FileMode.createTrunc);
+	scope (exit) {
+		fil.close();
+		removeFile(".unittest.tmp");
+	}
+	fil.write([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+	fil.seek(5);
+	ubyte[3] buf;
+	fil.read(buf);
+	assert(buf == [5, 6, 7]);
+}