Generating a tone using DTMF and playing it with AVAudioPlayer.

Question

I want to generate a custom DTMF tone and play it on the iPhone. In order to do so, I have created and allocated a memory buffer with a custom tone (ptr). Now I want to create a NSData object, initialized with the memory buffer, and pass it to AVAudioPlayer using initWithData:error: instance method.

I wrote the following code, but when I press the button "Play", it crashes.

#import "AudioPlayerViewController.h"
#include <stdlib.h>
#include <math.h>
#define SIZE 10
#define LENGTH 65535
const int PLAYBACKFREQ = 44100;
const float PI2 = 3.14159265359f * 2;
const int freq1 = 697;
const int freq2 = 1209;



@implementation AudioPlayerViewController

@synthesize playButton, stopButton;

- (void)viewDidLoad {
    [super viewDidLoad];
 // Allocate space for an array with ten elements of type int.
int *ptr = malloc(SIZE * sizeof(int));
if (ptr == NULL) NSLog(@"Error: Memory buffer could not be allocated.");
else NSLog(@"Allocation succeeded.");

 // The formula for the tone, the content of the buffer.
for(int i=0; i<SIZE; i++) ptr[i] = (sin(i*(PI2*(PLAYBACKFREQ/freq1))) + sin(i*    (PI2*(PLAYBACKFREQ/freq2)))) * 16383;
NSData *myData = [[NSData alloc] initWithBytesNoCopy:ptr length:SIZE];
free(ptr);
ptr = NULL;
audioPlayer = [[AVAudioPlayer alloc] initWithData:myData error:&error];
audioPlayer.numberOfLoops = -1;
}
-(IBAction) playAudio: (id) sender {
    if (audioPlayer == nil) NSLog([error description]);             
    else [audioPlayer play];
}
-(IBAction) stopAudio: (id) sender { [audioPlayer stop]; }

- (void)dealloc {
    [audioPlayer release];
    [myData release];
    [super dealloc];
}

@end

In the documentation, the description of method initWithBytesNoCopy reads:

A buffer containing data for the new object. bytes must point to a memory block allocated with malloc.

So I have already done this, but it doesn't work.

Any kind of help will be appreciated!

Answer 1

You create an NSData without copying the data and then you free the data, so NSData now has a dangling pointer. Remove the free(ptr); line and try it again. NSData will free the data by itself when it is done with it.