Objective-c Substring Range Exception - iphone

Question

I am creating an imap client. I want to parse body and header of incoming data but it crashes. I couldn't understand why it crashes and gives substring out of range error. How can I fix it?
I only want to check if the incoming string contains "FETCH" so I parse data, since string comes like
* FETCH or * 1 FETCH I thought checking isEqualToString range of (4,6) would be enough but that didn't work.
- (NSString*) readLine
{
NSMutableData* data = [[NSMutableData alloc] init];
unsigned char c;
for (;;) {
recv(socket_, &c, sizeof(c), 0);
if (c == '\n') {
NSString* s = [[NSString alloc] initWithData: data
encoding: NSUTF8StringEncoding];
NSString *str = [s substringWithRange:NSMakeRange(4, 6)];
if( [str isEqualToString:#"FETCH "]){
NSMutableArray *substrings = [NSMutableArray new];
NSScanner *scanner = [NSScanner scannerWithString:s];
[scanner scanUpToString:#"}" intoString:nil];
while(![scanner isAtEnd]) {
NSString *substring = nil;
[scanner scanString:#"}" intoString:nil];
if([scanner scanUpToString:#"*" intoString:&substring]) {
// If the space immediately followed the }, this will be skipped
[substrings addObject:substring];
}
[scanner scanUpToString:#"}" intoString:nil]; // Scan all characters before next }
}
NSString *email;
[emailList addObject:#"Select an Email"];
for(int i=0; i<substrings.count;i++){
email = [substrings objectAtIndex:i];
[emailList addObject:email]; // add emails in emailList
}
[substrings release];
}
if (nil != s) {
NSLog(#"%#",s);
}
[data release];
return [s autorelease];
}
else {
[data appendBytes: &c length: 1];
}
}
return nil;
}
output is:
* 1 FETCH (BODY[HEADER.FIELDS (FROM SUBJECT DATE)] {149}
2011-11-07 23:32:24.363 SwitchDeneme[327:bc03] Date: Mon, 07 Nov 2011 17:00:25 -0500 (EST)
2011-11-07 23:32:24.364 SwitchDeneme[327:bc03] From: "AOLWelcomeInfo" <AOLWelcomeInfo#message.aol.com>
2011-11-07 23:32:24.365 SwitchDeneme[327:bc03] Subject: Welcome to Your New Email Account!
2011-11-07 23:32:24.367 SwitchDeneme[327:bc03] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[NSCFString substringWithRange:]: Range or index out of bounds'
terminate called throwing an exceptionsharedlibrary apply-load-rules all

Answer 1

The problem is that your string is likely shorter than 7 characters, meaning it does not have an index of 6.
Try something more like this:
NSRange range = [someString rangeOfString:#"FETCH "];
if( range.location != NSNotFound ) {
//found it... so now do you processing...
}

Answer 2

You allocate and initialize an NSData object, then use that empty data object to initialize a string, so that string is empty.