How to lock an array from audio processing for analysis - ios

Question

In my app I'm doing some audio processing.
In the for loop of the audio buffer, there is a NSMutable array. The loop is called a huge number of time every second (depending on the buffer size).
As an example :
#autoreleasepool
{
for ( int i = 0; i < tempBuffer.mDataByteSize / 2; ++i )
{
if ( samples[i] > trig)
{
[self.k_Array addObject:[NSNumber numberWithInt:k]];
// other stuff
}
}
}
Then, every second, I'm calling a function for other processing.
- (void)realtimeUpdate:(NSTimer*)theTimer
{
// Create a copy of the array
NSMutableArray *k_ArrayCopy = [NSMutableArray arrayWithArray:k_Array]; // CRASH with EXC_BAD_ACCESS code 1 error
//do some stuff with k_ArrayCopy
}
I sometime receive an EXC_BAD_ACCESS error because, I think, a locking problem of the array.
I spent a lot of time trying to get information on queues, locking, working copies, etc... but I'm lost on this specific case.
My questions :
do I have to use atomic or nonatomic for k_array ?
do I have to use a dispatch_sync function ? If so, where exactly ?
should the realtimeUpdate function be called on background ?
Thanks in advance !

Answer 1

Use dispatch queue that will solve problem
//create queue instance variable
dispatch_queue_t q = dispatch_queue_create("com.safearrayaccess.samplequeue", NULL);
//1.
#autoreleasepool
{
for ( int i = 0; i < tempBuffer.mDataByteSize / 2; ++i )
{
if ( samples[i] > trig)
{
dispatch_async(q, ^{
//queue block
[self.k_Array addObject:[NSNumber numberWithInt:k]];
});
// other stuff NOTE: if its operation on array do it in queue block only
}
}
}
//2.
- (void)realtimeUpdate:(NSTimer*)theTimer
{
// Create a copy of the array
__block NSMutableArray *k_ArrayCopy;//when you use any variable inside block add __block before it
dispatch_async(q, ^{
//queue block
k_ArrayCopy = [NSMutableArray arrayWithArray:k_Array];
});
//do some stuff with k_ArrayCopy
}
Now your add and read array operation are on same queue and it will not conflict..
For more details in using dispatch queue go through apples Grand Central Dispatch doc
Other way of doing this is use NSConditonLock