指針原始對象不正確地更改值

Question

我有一個問題，通過鏈接傳遞一個基元類型的引用，並使指針表示的值正確更改。奇怪的部分是，如果我直接從主函數調用getBytes，byteLocation會得到適當的調整，但是如果我通過便利函數將其鏈接起來，它似乎會獲得垃圾值。實際上，即使是怪異的，它在逐步調試時也會得到正確的值，但執行兩次return子句。第一個return子句獲取正確的值，第二個加載byteLocation並帶有垃圾值。有任何想法嗎？

EDIT（實際代碼）：

@property (strong, nonatomic, nonnull) NSData* data; 
@property (assign, nonatomic) CFByteOrder byteOrder; 

- (void)convertBytesToHostOrder:(nonnull void*)buffer length:(NSUInteger)length { 
    if(length > 1 && self.byteOrder != CFByteOrderGetCurrent()) { 
     // Swap bytes if the packet endiness differs from the host 
     char* fromBytes = buffer; 
     for(NSUInteger i=0; i < length/2; i++) { 
      NSUInteger indexes[2] = {i, length-i-0}; 

      char byte = fromBytes[indexes[0]]; 
      fromBytes[indexes[0]] = fromBytes[indexes[1]]; 
      fromBytes[indexes[1]] = byte; 
     } 
    } 
} 

- (nonnull void*)getBytes:(nonnull void*)buffer startingFrom:(nonnull NSUInteger*)location length:(NSUInteger)length { 
    NSRange range = NSMakeRange(*location, length); 
    [self.data getBytes:buffer range:range]; // self.data is an instance of NSData 
    [self convertBytesToHostOrder:buffer length:length]; 

    NSUInteger update = range.location + range.length; 
    *location = update; 

    return buffer; 
} 


- (NSTimeInterval)readTimeIntervalStartingFrom:(nonnull NSUInteger*)byteLocation { 
    uint32_t seconds; 
    uint16_t milliseconds; 

    // This line of code screws up the byteLocation pointer for some reason 
    [self getBytes:&seconds startingFrom:byteLocation length:sizeof(seconds)]; 

    [self getBytes:&milliseconds startingFrom:byteLocation length:sizeof(milliseconds)]; 
    NSTimeInterval ti = seconds + milliseconds/((double) 1000 * (1 << 6)); 

    return ti; 
} 

- (void)readData { 
    NSUInteger byteLocation = 0; 

    self.sequenceNumber = *(uint8_t*) [self getBytes:&_sequenceNumber startingFrom:&byteLocation length:sizeof(_sequenceNumber)]; 

    self.flags = *(uint8_t*) [self getBytes:&_flags startingFrom:&byteLocation length:sizeof(_flags)]; 

    // Continue to process packet data if we didn't get a goodbye message 
    if(!(self.flags & LBRadarPongFlagGoodbye)) { 
     // Parse accelerations 
     int16_t int16; 
     self.accelerationX = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)])/kGToRaw; 
     self.accelerationY = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)])/kGToRaw; 
     self.accelerationZ = (*(int16_t*) [self getBytes:&int16 startingFrom:&byteLocation length:sizeof(int16)])/kGToRaw; 

     // Parse peripheral states 
     self.batteryVoltage = [self readFloat16From:&byteLocation]; 
     self.chargeCurrent = [self readFloat16From:&byteLocation]; 
     self.systemCurrent = [self readFloat16From:&byteLocation]; 

     // All previous lines of code work properly and as expected. 
     // Buffers are read properly, and byteLocation properly reflects 14, which is the number of bytes read up to this point. 
     self.pongReceivedTimeIntervalSince1970 = [self readTimeIntervalStartingFrom:&byteLocation]; 
    } 
} 

Answer 1

這個問題似乎是與遞增location。在這兩種情況下，您都應該從位置0複製到變量的大小。在以下代碼中：

[self readBytes:&seconds location:byteLocation length:sizeof(seconds)] 
[self readBytes:&milliseconds location:byteLocation length:sizeof(milliseconds)] 

第一次調用從零開始並讀取32位。第二個從位置32開始，它甚至不適合變量的16位。這溢出了緩衝區。試試這個：

- (void*)readBytes:(void*)buffer location:(NSUInteger*)location length:(NSUInteger)length { 

// The difference is in the next line. Zero instead of *location 
[NSData getBytes:&buffer range:NSMakeRange(0, length)]; 
*location = *location + length; 

return buffer; // Seems to be called twice, first time location* has the correct byteLocation inside it, second time location* has a junk value 
} 

Answer 2

在猜測[*]你的錯誤就行了：

[self.data getBytes:&buffer range:NSMakeRange(*location, length)]; 

你是通過採取buffer地址傳遞一個void *值 - 這已經是一個void *。將其更改爲：

[self.data getBytes:buffer range:NSMakeRange(*location, length)]; 

至少會產生非garabge結果。

---

[*]爲您發佈甚至不編譯代碼，我編輯你的問題糾正一些比較明顯的錯誤，我只能猜測 - 但即使這涉及一些猜測！你應該發佈真實的代碼。

Answer 3

我應該剛剛發佈實際的代碼，對不起傢伙。原來這個錯誤發生在一個輔助函數（convertBytesToHostOrder）中。這是讀出緩衝區的界限。由於buffer是byteLocation之前的參數，所以在緩衝區之外的位置寫入1位似乎是byteLocation位置。現在修復，一切正常。

- (void)convertBytesToHostOrder:(nonnull void*)buffer length:(NSUInteger)length { 
    if(length > 1 && self.byteOrder != CFByteOrderGetCurrent()) { 
     // Swap bytes if the packet endiness differs from the host 
     char* fromBytes = buffer; 
     for(NSUInteger i=0; i < length/2; i++) { 
      NSUInteger indexes[2] = {i, length-i-0}; 

      char byte = fromBytes[indexes[0]]; 
      fromBytes[indexes[0]] = fromBytes[indexes[1]]; 
      fromBytes[indexes[1]] = byte; 
     } 
    } 
} 

應該是：

NSUInteger indexes[2] = {i, length-i-1};