AVAudioRecorder stops working when writing to /dev/null on iPhone 5s with iOS 7 - ios7

Question

I have an app that monitors the background audio level without recording to a file. I use the trick of writing to /dev/null to accomplish this.
This code has worked on the iPhone 3GS with iOS 6, iPhone 4 with iOS 6 and iOS 7, and in the simulator with iOS 7 and iPhone Retina (4-inch 64-bit).
When I try it on a real iPhone 5s, however, the recorder seems to capture audio for a moment, and then silently dies.
This is the code:
// Inititalize the audio
NSURL *url = [NSURL fileURLWithPath:#"/dev/null"];
NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat: 44100.0], AVSampleRateKey,
[NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
[NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
[NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,
nil];
NSError *error;
recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];
if (recorder) {
[recorder prepareToRecord];
recorder.meteringEnabled = YES;
[recorder record];
levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.03 target: self selector: #selector(levelTimerCallback:) userInfo: nil repeats: YES];
// peakLevelTimer = [NSTimer scheduledTimerWithTimeInterval: 2.0 target: self selector: #selector(peakLevelTimerCallback:) userInfo: nil repeats: YES];
} else {
NSLog(#"There was an error setting up the recorder.");
NSLog([error description]);
}
Any ideas what might be going on?
Could anyone suggest a workaround? Writing to a real file works, but I don't want to fill up any space on the device just to monitor audio. Is there a way to write to a small file buffer that just evaporates into thin air? Implement my own /dev/null, effectively?

Answer 1

I had the same issue when testing one of my apps for iOS 7.
I got around the absence of /dev/null by creating an initial file that I then deleted a number of seconds after the recording started. The recording still continued to work for my application and there was no data file being stored.
- (void)startRecording
{
// New recording path.
NSString *recorderFilePath = [NSString stringWithFormat:#"%#/%#.caf", [NSHomeDirectory() stringByAppendingPathComponent:#"Documents"], #"cache"];
NSURL *url = [NSURL fileURLWithPath:recorderFilePath];
AVAudioRecorder recorder = [[ AVAudioRecorder alloc] initWithURL:url settings:recordSetting error:&err];
if([recorder prepareToRecord])
{
[recorder record];
NSTimer deleteFileTimer = [NSTimer scheduledTimerWithTimeInterval:15 target:self selector:#selector(removeRecordingFile) userInfo:nil repeats:NO];
}
}
- (void)removeRecordingFile
{
// Remove the data file from the recording.
NSString *recorderFilePath = [NSString stringWithFormat:#"%#/%#.caf", [NSHomeDirectory() stringByAppendingPathComponent:#"Documents"], #"cache"];
NSFileManager *fileManager = [NSFileManager defaultManager];
NSError *error = nil;
BOOL success = [fileManager removeItemAtPath:recorderFilePath error:&error];
if(success)
{
NSLog(#"Deleted recording file");
}
else
{
NSLog(#"Could not delete file -:%# ",[error localizedDescription]);
}
}

Answer 2

You need to change:
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord error:NULL];
to:
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryRecord error:NULL];