Correctly write data into sink's internal buffer

core/apple/src/BuffersApple.kt

@@ -22,7 +22,7 @@ internal fun Buffer.write(source: CPointer<uint8_tVar>, maxLength: Int) {
 
         val toCopy = minOf(maxLength - currentOffset, Segment.SIZE - tail.limit)
         tail.data.usePinned {
-            memcpy(it.addressOf(tail.pos), source + currentOffset, toCopy.convert())
+            memcpy(it.addressOf(tail.limit), source + currentOffset, toCopy.convert())
         }
 
         currentOffset += toCopy

core/apple/test/SinkNSOutputStreamTest.kt

@@ -18,8 +18,35 @@ import platform.darwin.NSUInteger
 import platform.posix.uint8_tVar
 import kotlin.test.*
 
+private fun NSOutputStream.write(vararg strings: String) {
+    for (str in strings) {
+        str.encodeToByteArray().apply {
+            assertEquals(size, this.write(this@write))
+        }
+    }
+}
+
 @OptIn(UnsafeNumber::class)
 class SinkNSOutputStreamTest {
+
+    @Test
+    fun multipleWrites() {
+        val buffer = Buffer()
+        buffer.asNSOutputStream().apply {
+            open()
+            write("hello", " ", "world")
+            close()
+        }
+        assertEquals("hello world", buffer.readString())
+
+        RealSink(buffer).asNSOutputStream().apply {
+            open()
+            write("hello", " ", "real", " sink")
+            close()
+        }
+        assertEquals("hello real sink", buffer.readString())
+    }
+
     @Test
     fun bufferOutputStream() {
         testOutputStream(Buffer(), "abc")

core/apple/test/utilApple.kt

@@ -3,7 +3,7 @@
  * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENCE file.
  */
 
-@file:OptIn(UnsafeNumber::class, ExperimentalForeignApi::class, ExperimentalForeignApi::class)
+@file:OptIn(UnsafeNumber::class, ExperimentalForeignApi::class)
 
 package kotlinx.io
 
@@ -71,3 +71,9 @@ fun NSStreamEvent.asString(): String {
         else -> "Unknown event $this"
     }
 }
+
+fun ByteArray.write(to: NSOutputStream): Int {
+    this.usePinned {
+        return to.write(it.addressOf(0).reinterpret(), size.convert()).convert()
+    }
+}