How to debug EXC_BAD_ACCESS in Apple's Accelerator code? - ios

Question

On sporadic occasions I'm getting an EXC_BAD_ACCESS (SIGSEV) in one of Apple's Accelerator framework methods, vDSP_fft2d_zip. The crash report gave the following for Thread 10. Again, most of the time this works like a champ. Any ideas on how to find the cause and fix it?
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x412f2740
Triggered by Thread: 10
. . .
Thread 10 Crashed:
0 libvDSP.dylib 0x2ddfcdb4 ___lldb_unnamed_function431$$libvDSP.dylib + 68
1 libvDSP.dylib 0x2ddfab66 ___lldb_unnamed_function427$$libvDSP.dylib + 562
2 libvDSP.dylib 0x2ddfa8ee vDSP_fft2d_zop + 434
3 libvDSP.dylib 0x2ddfa732 vDSP_fft2d_zip + 18
4 ASSIST for iPad 0x000ac32e -[Processor setupFilterForBubbleSizeMM:rectWidth:rectImageWidth:dataFilled:dataUnfilled:] (Processor.mm:813)
5 ASSIST for iPad 0x000af4d2 -[Processor runConvolutionProcessOnImage:aveImage:] (Processor.mm:1835)
6 ASSIST for iPad 0x000ac5f6 -[Processor processAnswers] (Processor.mm:869)
7 ASSIST for iPad 0x000ae9cc -[Processor process] (Processor.mm:1644)
8 ASSIST for iPad 0x000d7744 -[ReallTimeScanner processImage:] (ReallTimeScanner.mm:1046)
9 Foundation 0x2f296c82 __NSThread__main__ + 1058
10 libsystem_pthread.dylib 0x396b6c1a _pthread_body + 138
11 libsystem_pthread.dylib 0x396b6b8a _pthread_start + 98
12 libsystem_pthread.dylib 0x396b4c8c thread_start + 4
The relevant code looks like this:
#define FFT_SIZE 512
#define FFT_POWER 9
dataFilledIn->imagp=(float *) malloc(sizeof(float)*FFT_SIZE*FFT_SIZE);
dataFilledIn->realp=(float *) malloc(sizeof(float)*FFT_SIZE*FFT_SIZE);
dataUnfilledIn->imagp=(float *) malloc(sizeof(float)*FFT_SIZE*FFT_SIZE);
dataUnfilledIn->realp=(float *) malloc(sizeof(float)*FFT_SIZE*FFT_SIZE);
memset(dataFilledIn->imagp, 0, sizeof(float)*FFT_SIZE*FFT_SIZE);
memset(dataFilledIn->realp, 0, sizeof(float)*FFT_SIZE*FFT_SIZE);
memset(dataUnfilledIn->imagp, 0, sizeof(float)*FFT_SIZE*FFT_SIZE);
memset(dataUnfilledIn->realp, 0, sizeof(float)*FFT_SIZE*FFT_SIZE);
for(float y=0; y<FFT_SIZE; y+=0.25)
{
for(float x=0; x<FFT_SIZE; x+=0.25)
{
dataFilledIn->realp[(int)y*FFT_SIZE+(int)x] += 0.0625;
dataUnfilledIn->realp[(int)y*FFT_SIZE+(int)x] -= 0.0625;
}
}
vDSP_fft2d_zip(setup, dataFilledIn, 1, 0, FFT_POWER, FFT_POWER, kFFTDirection_Forward);

Answer 1

Answering my own question here: Be sure that the very first parameter sent in (type FFTSetup) to vDSP_fft2d_zip() is initialized correctly, and you should use this call
vDSP_destroy_fftsetup(setup);
to clean it up. I was initializing the setup var properly, like this
setup = vDSP_create_fftsetup(FFT_POWER, kFFTRadix2);
and then cleaning it up, but then was not initializing it again before subsequent calls to vDSP_fft2d_zip(). Unfortunately it only rarely caused a crash, which made it seem like the frequent calls to vDSP_fft2d_zip(setup, . . .) were working when they weren't. The crashes were sporadic making the real cause hard to trace.

Answer 2

There are some articles in Raywenderlich may help you:
Objectively Speaking: A Crash Course in Objective C for iOS 6
Objectively Speaking 2: A Crash Course in Objective C for iOS 6