Reading large files from NSInputstream is not working in iOS

I am working on iOS app, I am trying to read large Image file more than 300KB from NSInputStream. But i'm getting only upto 300KB. Other data are missing. I am searching solutions of this query on few ios developer forum if you have answer of my query please help me. i'm waiting for your valuable answer. Check below code

- (void)readAllData {

if ( _readData == nil) {
_readData = [[NSMutableData data] retain];
}

while ([[ _session inputStream] hasBytesAvailable])
{
unsigned int bytesRead = 0;
bytesRead = [[ _session inputStream] read:buf maxLength: @"1024"];

if (bytesRead) {

NSMutableString * _string = [NSMutableString stringWithString:@""];
for (int i = 0; i < _readData.length; i++) {
unsigned char _byte;
[_readData getBytes:&_byte range:NSMakeRange(i, 1)];
i f ( _byte >= 32 && _byte < 127) {
[ _string appendFormat:@"%c", _byte];
} else {
[ _string appendFormat:@"[%d]", _byte];
}
}

[ _readData appendBytes:(const void *)buf length:bytesRead];

}

}

NSString *string=[NSString stringWithFormat:@"%d", _readData.length];
UIAlertView *sesView = [[UIAlertView alloc] initWithTitle:@"readDatalength"
message:string
delegate:self
cancelButtonTitle:nil otherButtonTitles:@"OK" , nil];
[sesView show];
}