Question

If I missed the prototype, XCode (LLVM) prompt me for error

no previous prototype for function for exceptionHandler

But why they are needed in my code below?

void exceptionHandler(NSException * exception); // Why this Line is needed?

void exceptionHandler(NSException * exception)
{
    // ....
}

@implementation AppDelegate

- (void) applicationDidFinishLaunching:(UIApplication *)application
{
    NSSetUncaughtExceptionHandler(&exceptionHandler);
...

Answer 1

If you’re declaring a function only for use within this file, prefix the declaration with the static keyword and the warning will go away. As it is, you’re declaring a global function; theoretically it could be called from anywhere within your app. But as you’ve given it no prototype, nobody else could call it.

So the warning, as I understand it, is trying to make you clarify your intentions between static functions and global functions, and discourage you from declaring a global function when you meant to declare only a static one.