Warning: "format not a string literal and no format arguments"

Question

Since upgrading to the latest Xcode 3.2.1 and Snow Leopard, I've been getting the warning

"format not a string literal and no format arguments"

from the following code:

NSError *error = nil;

if (![self.managedObjectContext save:&error]) 
{
    NSLog([NSString stringWithFormat:@"%@ %@, %@", 
       errorMsgFormat, 
       error, 
       [error userInfo]]);      

}

If errorMsgFormat is an NSString with format specifiers (eg: "print me like this: %@"), what is wrong with the above NSLog call? And what is the recommended way to fix it so that the warning isn't generated?

Answer 1

Xcode is complaining because this is a security problem.

Here's code similar to yours:

NSString *nameFormat = @"%@ %@";
NSString *firstName = @"Jon";
NSString *lastName = @"Hess %@";
NSString *name = [NSString stringWithFormat:nameFormat, firstName, lastName];
NSLog(name);

That last NSLog statement is going to be executing the equivalent of this:

NSLog(@"Jon Hess %@");

That's going to cause NSLog to look for one more string argument, but there isn't one. Because of the way the C language works, it's going to pick up some random garbage pointer from the stack and try to treat it like an NSString. This will most likely crash your program. Now your strings probably don't have %@'s in them, but some day they might. You should always use a format string with data you explicitly control as the first argument to functions that take format strings (printf, scanf, NSLog, -[NSString stringWithFormat:], ...).

As Otto points out, you should probably just do something like:

NSLog(errorMsgFormat, error, [error userInfo]);