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ios - 与 MIDIPacketNext 一起崩溃

问题描述

十年前，我使用 PGMidi 用 Objective-C 编写了一个应用程序，它是 Core MIDI 的辅助类。PGMidi 的输入函数如下所示：

static void PGMIDIReadProc(const MIDIPacketList *pktlist, void *readProcRefCon, void *srcConnRefCon) {
    PGMidiSource *source = arc_cast<PGMidiSource>(srcConnRefCon);
    [source midiRead:pktlist fromPort:source.name];
}

- (void) midiRead:(const MIDIPacketList *)pktlist fromPort:(NSString *)port {
    NSArray *delegates = self.delegates;
    for (NSValue *delegatePtr in delegates) {
        id<PGMidiSourceDelegate> delegate = (id<PGMidiSourceDelegate>)[delegatePtr pointerValue];
        [delegate midiSource:self midiReceived:pktlist fromPort:port];
    }
}

这会将数据传递给我的应用程序功能：

static NSMutableArray *packetToArray(const MIDIPacket *packet) {
    NSMutableArray *values = [[NSMutableArray alloc] initWithCapacity:3];
    NSNumber *value;
    for (int j=0; j<packet->length; j++) {
        value = [[NSNumber alloc] initWithUnsignedInt:packet->data[j]];
        [values addObject:value];
    }
    return values;
}

- (void)midiSource:(PGMidiSource *)midi midiReceived:(const MIDIPacketList *)packetList fromPort:(NSString *)port {
    const MIDIPacket *packet = &packetList->packet[0];
    for (int i=0; i<packetList->numPackets; ++i) {
        NSDictionary *data = [[NSDictionary alloc] initWithObjectsAndKeys:
            packetToArray(packet), @"values",
            port, @"port",
        nil];
        [self performSelectorOnMainThread:@selector(receiveMidiData:) withObject:data waitUntilDone:FALSE];
    
        packet = MIDIPacketNext(packet);
    }
}

这一直运作良好。但今年早些时候，我将我的应用程序转换为 Swift。我单独离开了 PGMidi 类，但将我的应用程序函数重写为：

func packetToArray(_ packet: MIDIPacket) -> [Int] {
    var values = [Int]()
    let packetMirror = Mirror(reflecting: packet.data)
    let packetValues = packetMirror.children.map({ $0.value as? UInt8 })
    for i in 0..<Int(packet.length) {
        if (packetValues.count > i), let value = packetValues[i] {
            values.append(Int(value))
        }
    }
    return values
}

@objc func midiSource(_ midi: PGMidiSource, midiReceived packetList: UnsafePointer<MIDIPacketList>, fromPort port: String) {
    var packet = packetList.pointee.packet // this gets the first packet
    for _ in 0..<packetList.pointee.numPackets {
        let packetArray = App.packetToArray(packet)
        let data: [AnyHashable : Any] = [
            "values" : packetArray,
            "port" : port
        ]
        self.performSelector(onMainThread: #selector(receiveMidiData(_:)), with: data, waitUntilDone: false)

        packet = MIDIPacketNext(&packet).pointee
    }
}

这主要是可行的，但我遇到了一些崩溃MIDIPacketNext。这并不一致，并且似乎在大量数据进入时发生，例如来自键盘上的表达式轮。一份崩溃日志包括以下内容：

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x000000016b78914c
VM Region Info: 0x16b78914c is not in any region. Bytes after previous region: 37197 Bytes before following region: 360050356
REGION TYPE START - END [ VSIZE] PRT/MAX SHRMOD REGION DETAIL
Stack 16b6f8000-16b780000 [ 544K] rw-/rwx SM=PRV thread 6
---> GAP OF 0x15768000 BYTES
unused shlib __TEXT 180ee8000-180f2c000 [ 272K] r-x/r-x SM=COW ... this process

另一个略有不同，但指向同一行代码：

Exception Type:  EXC_BAD_ACCESS (SIGBUS)
Exception Subtype: KERN_PROTECTION_FAILURE at 0x000000016beb46cc
VM Region Info: 0x16beb46cc is in 0x16beb4000-0x16beb8000;  bytes after start: 1740  bytes before end: 14643
      REGION TYPE                 START - END      [ VSIZE] PRT/MAX SHRMOD  REGION DETAIL
      Stack                    16be2c000-16beb4000 [  544K] rw-/rwx SM=PRV  thread 5
--->  STACK GUARD              16beb4000-16beb8000 [   16K] ---/rwx SM=NUL  ... for thread 9
      Stack                    16beb8000-16bf40000 [  544K] rw-/rwx SM=PRV  thread 9

我是否应该以不同的方式访问数据包以避免这种情况，或者我可以做些什么来检测错误情况并避免崩溃？

标签： iosswiftcoremidi

解决方案

Eugene Dudnyk的评论让我找到了解决方案。我重写了我的 midiSource 委托函数，如下所示，添加了 UnsafePointer 扩展：

@objc func midiSource(_ midi: PGMidiSource, midiReceived packetList: UnsafePointer<MIDIPacketList>, fromPort port: String) {
    let packetList = packetList.loadUnaligned(as: MIDIPacket.self, count: Int(packetList.pointee.numPackets))
    for packet in packetList {
        let packetArray = App.packetToArray(packet)
        let data: [AnyHashable : Any] = [
            "values" : packetArray,
            "port" : port
        ]
        self.performSelector(onMainThread: #selector(receiveMidiData(_:)), with: data, waitUntilDone: false)
    }
}

extension UnsafePointer {
    func loadUnaligned<T>(as: T.Type, count: Int) -> [T] {
        assert(_isPOD(T.self)) // relies on the type being POD (no refcounting or other management)
        let buffer = UnsafeMutablePointer<T>.allocate(capacity: count)
        defer { buffer.deallocate() }
        memcpy(buffer, self, MemoryLayout<T>.size * count)
        return (0..<count).map({ index in buffer.advanced(by: index).pointee })
    }
}

没有更多的崩溃！

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