Turn NSArray into C array using ARC

Question

I have an NSArray* containing my own objects that I need to access from a C api. Hitting my objects from C functions is working fine, but when I try to copy them to a C array, I can see the pointers being cleared after some time!

@interface MNCircularBufferPlayer;// just assume this exists

void schedule(MNPlayList* playlist){ /* some C code */}

typedef struct MNPlayList { 
    __unsafe_unretained MNCircularBufferPlayer **players;
    int playersCount;
} MNPlayList;

-(void)sendToCApi(){
//...    
    NSArray* toCopy = self.players; // array of MNCircularBufferPlayer instances
    MNPlayList* playlist=malloc(sizeof(MNPlaylist));
    playlist->playersCount=toCopy.count;
    playlist->players=(__unsafe_unretainedNCircularBufferPlayer**)calloc(toCopy.count+1,sizeof(MNCircularBufferPlayer*));
    for(MNCircularBufferPlayer* player in toCopy){
      playlist->players[i]=player;
    }
    //.. send to C api
}

When inspecting the memory after this point, the buffer contained within my struct looks OK. But when the C function gets the same struct, the struct has the same size, but the contents of the buffer are all 0x30000000! I am entirely positive the MNCircularBufferPlayer instances are still retained! There's even a breakpoint in dealloc just to be 100% sure...

I gave-up and ended using a linked-list instead, and it works:

typedef struct MNPlayList {
    __unsafe_unretained MNCircularBufferPlayer *player;
    struct MNPlayList *next;
} MNPlayList;

But I am losing sleep over this one.

trojanfoe · Accepted Answer · 2014-09-16 09:23:34Z

1

As the objects in the NSArray must be Objective-C objects, it's not clear how you expect to actually use those objects from C, but anyway...

This creates an array of MNCircularBufferPlayer * objects with nil as the last element as a sentinel:

-(void)sendToCApi(){
    MNCircularBufferPlayer *carray = malloc(sizeof(MNCircularBufferPlayer *) * ([self.players count] + 1]));
    for (NSUInteger i = 0; i < [self.players count]; i++)
        carray[i] = self.players[i];
    carray[[self.players count]] = nil;

    //.. send to C api

    free(carray);
}

answered Sep 16, 2014 at 9:23

trojanfoe

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