AudioFileWriteBytes fails with error code -40 - iphone

Question

I'm trying to write raw audio bytes to a file using AudioFileWriteBytes(). Here's what I'm doing:
void writeSingleChannelRingBufferDataToFileAsSInt16(AudioFileID audioFileID, AudioConverterRef audioConverter, ringBuffer *rb, SInt16 *holdingBuffer) {
// First, figure out which bits of audio we'll be
// writing to file from the ring buffer
UInt32 lastFreshSample = rb->lastWrittenIndex;
OSStatus status;
int numSamplesToWrite;
UInt32 numBytesToWrite;
if (lastFreshSample < rb->lastReadIndex) {
numSamplesToWrite = kNumPointsInWave + lastFreshSample - rb->lastReadIndex - 1;
}
else {
numSamplesToWrite = lastFreshSample - rb->lastReadIndex;
}
numBytesToWrite = numSamplesToWrite*sizeof(SInt16);
Then we copy the audio data (stored as floats) to a holding buffer (SInt16) that will be written directly to the file. The copying looks funky because it's from a ring buffer.
UInt32 buffLen = rb->sizeOfBuffer - 1;
for (int i=0; i < numSamplesToWrite; ++i) {
holdingBuffer[i] = rb->data[(i + rb->lastReadIndex) & buffLen];
}
Okay, now we actually try to write the audio from the SInt16 buffer "holdingBuffer" to the audio file. The NSLog will spit out an error -40, but also claims that it's writing bytes. No data is written to file.
status = AudioFileWriteBytes(audioFileID, NO, 0, &numBytesToWrite, &holdingBuffer);
rb->lastReadIndex = lastFreshSample;
NSLog(#"Error = %d, wrote %d bytes", status, numBytesToWrite);
return;
What is this error -40? By the way, everything works fine if I write straight from the ringBuffer to the file. Of course it sounds like junk, because I'm writing floats, not SInt16s, but AudioFileWriteBytes doesn't complain.

Answer 1

The key is to explicitly change the endianness of the incoming data to big endian. All I had to do was wrap CFSwapInt16HostToBig around my data to get:
float audioVal = rb->data[(i + rb->lastReadIndex) & buffLen];
holdingBuffer[i] = CFSwapInt16HostToBig((SInt16) audioVal );