How to assign space for #property variables - ios

Question

I am still new to objective-c and I need some insight about variable declaration.
The point of my program is to read a message delivered in several NSData parts. Whenever the program receive a NSData it adds it to the already partially complete message as unsigned chars. This means that the message buffer must be define as a property of the viewController:
#property unsigned char* receiveBuf;
#property int bufSize;
At that point the program has a NSData object named data, and treat it like this:
int len = [data length];
unsigned char* tempBytebuf = (unsigned char*)[data bytes];
for(int i = 0; i < len; i++){ //read all bytes
if (tempBytebuf[i] == 0x7E) { //this char means start of package
_bufSize = 0;
}
else if (tempBytebuf[i] == 0x7D) { //this char means end of package
[self readMsgByte:_receiveBuf :_bufSize];
}
else { //any other char is to be put in the message
_receiveBuf[_bufSize++] = tempBytebuf[i]; //Error happens here
}
}
if I proceed like this it will result in an error EXC_BAD_ACCESS.
If I could tell the program to reserve space for the buffer it would solve the problem:
#property unsigned char receiveBuf[256];
but it seems I can't do this with #properties.
Is there a way of assigning that space in the ViewDidLoad() for example or somewhere else?
Thanks for your support!
EDIT 1:
It seems I just found a solution in some of my previous codes, I should have declare my char table in the implementation instead.
#implementation ViewController
{
unsigned char msgBuf[256];
}
Still if someone could tell me the real difference between the #property and implementation space for variable declaration that would prevent me from doing other mistakes like this.
Thanks a lot.

Answer 1

If you needed to declare that as unsigned char*, then malloc/calloc/realloc/free are your friends.
In reality, making that ivar NSMutableData and using APIs like -appendBytes:length: should be all that's necessary.

Answer 2

Use NSMutableData:
#property (strong) NSMutableData *recievedData;
It provides a nice wrapper around an arbitrary piece of memory who's contents and length can change at any time - and most importantly it fits in perfectly with the objective-c runtime and memory management.
Just create an empty (0 length) data object like this:
self.receivedData = [[NSMutableData alloc] init];
And then at any time you can do:
[self.recivedData appendBytes:bytes length:length];
Or:
[self.recievedBytes appendData:anotherNSDataObj];
From there you can read it with the getBytes: method.
There are other ways to do it, by declaring lower level instance variables, but #property is the modern approach. The older style of instance variable is able to what you tried, but we are moving away from them and it will probably be deprecated soon.