add item to NSMutableArray inside a thread-CodePudding

I want to scan the Photo library with threads, when my app logic is ok I want to add the PHAsset to finalarray:

NSMutableArray <PHAsset *> *finalarray = [[NSMutableArray alloc] init];

for (NSMutableArray *photoArray in arrayOfArrays) {
    dispatch_group_enter(aGroup);
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        for (PHAsset *asset in photoArray) {
            [[PHImageManager defaultManager] requestImageDataForAsset:asset
                                                              options:options
                                                        resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {

                //Make some logic

                if (logic) {
                    [finalarray addObject:asset];
                }
            }];
        }
        dispatch_group_leave(aGroup);
    });
}

arrayOfArrays is array of array of PHAsset.

My code works fine, but sometimes I get a SIGABRT crash and I get this log in code console:

malloc: Incorrect checksum for freed object 0x1206d5968: probably modified after being freed.
Corrupt value: 0x100017580
malloc: *** set a breakpoint in malloc_error_break to debug
malloc: Incorrect checksum for freed object 0x1206d5968: probably modified after being freed.
Corrupt value: 0x100017580

CodePudding user response：

If you have multiple threads updating a property, you must synchronize their access. You can use a NSLock:

NSMutableArray <PHAsset *> *finalarray = [[NSMutableArray alloc] init];
NSLock *lock = [[NSLock alloc] init];

for (NSMutableArray *photoArray in arrayOfArrays) {
    dispatch_group_enter(aGroup);
    for (PHAsset *asset in photoArray) {
        [[PHImageManager defaultManager] requestImageDataForAsset:asset
                                                          options:options
                                                    resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {

            //Make some logic

            if (logic) {
                [lock lock];
                [finalarray addObject:asset];
                [lock unlock];
                dispatch_group_leave(aGroup);
            }
        }];
    }
}

Or you can use a GCD queue:

NSMutableArray <PHAsset *> *finalarray = [[NSMutableArray alloc] init];
dispatch_queue_t queue = dispatch_queue_create("...", NULL);

for (NSMutableArray *photoArray in arrayOfArrays) {
    dispatch_group_enter(aGroup);
    for (PHAsset *asset in photoArray) {
        [[PHImageManager defaultManager] requestImageDataForAsset:asset
                                                          options:options
                                                    resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {

            //Make some logic

            if (logic) {
                dispatch_async(queue, ^{
                    [finalarray addObject:asset];
                    dispatch_group_leave(aGroup);
                });
            }
        }];
    }
}

Note, above I assume you are fetching your images asynchronously, in which case your use of the global queue is unnecessary, but you need to make sure to call dispatch_group_leave inside the requestImageDataForAsset.

But, regardless, make sure you synchronize your access (either with GCD queue or some lock) when accessing properties from multiple threads.

CodePudding user response：

Try synchronizing your code with a mutex:

#import <pthread.h>

...
NSMutableArray <PHAsset *> *finalarray = [[NSMutableArray alloc] init];
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);

for (NSMutableArray *photoArray in arrayOfArrays) {
    dispatch_group_enter(aGroup);
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        for (PHAsset *asset in photoArray) {
            [[PHImageManager defaultManager] requestImageDataForAsset:asset
                                                              options:options
                                                        resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {

                //Make some logic

                if (logic) {
                    pthread_mutex_lock(&mutex);
                    [finalarray addObject:asset];
                    pthread_mutex_unlock(&mutex);
                }
            }];
        }
        dispatch_group_leave(aGroup);
    });
}

In case this didn't help, have you tried setting a breakpoint in malloc_error_break to debug, as your error suggests?