-[NSInputStream read:maxLength:] throws an exception saying length is too big, but it isn't

Question

I use an `NSInputStream` to read data from a file. It will crash if `maxLength` is greater than 49152. When it crashes -- sometimes, but not every time, it gives this message: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSConcreteData initWithBytes:length:copy:freeWhenDone:bytesAreVM:]: absurd length: 4294967295, maximum size: 2147483648 bytes' From my calculation, 524288 is still less than that maximum, and can fit in the return value. What did I miss? ``` - (void)stream:(NSStream *)aStream handleEvent:(NSStreamEvent)eventCode { switch (eventCode) { case NSStreamEventHasBytesAvailable: { NSInteger bufferSizeNumber = 524288; //this one will crash. // NSInteger bufferSizeNumber = 491520; // this one will work. uint8_t buf[bufferSizeNumber]; unsigned int len = 0; len = [_stream read:buf maxLength:bufferSizeNumber]; //crashing at this line // more code ... } // more code... } } ``` Edit: (I think this is critical part of that behavior) If I…

Accepted Answer

Your problem is a so called "stack overflow" (you might have heard this before).

Your method allocates a buffer on the stack using a variable length array:

```
uint8_t buf[bufferSizeNumber];
```

When the size of the buffer is so large that it overflows the size of the current stack the behavior is undefined. Undefined behavior might result in a crash or just work as expected: just what you are observing.

512kB is a huge buffer, especially on iOS where background threads get a stack of exactly this size.

You should allocate it on the heap:

```
NSInteger bufferSizeNumber = 524288;
NSMutableData *myBuffer = [NSMutableData dataWithLength:bufferSizeNumber];

uint8_t *buf = [myBuffer mutableBytes];
unsigned int len = 0;

len = [_stream read:buf maxLength:bufferSizeNumber];
// more code ...
```

-[NSInputStream read:maxLength:] throws an exception saying length is too big, but it isn't

Solution

Problem