managing C-strings from blocks, NSAlert, OS X - objective-c

Question

I’ve written a piece of test code "AlertTest" to make sure I’m implementing NSAlert object the proper way. It consists of a button which triggers [doSomething: cstr] printing a C-string, and a method posting an alert, which passes the same string to the same method, but from within the completion handler.
This is the debugger console printout:
-from [buttonPressed]
2015-01-09 18:28:09.832 AlertTest[1260:40881] Application must quit
Application must quit
-from [mPostError:cstr]
2015-01-09 18:28:12.276 AlertTest[1260:40881] #Èø_ˇ
#\351\277_\377
I’m a bit confused on what I have to do to properly pass a string from within the completion handler. No compiler errors reported. I must be missing something very obvious. Thanks in advance. Here’s the code:
//___________________________________________________________
- (IBAction)buttonPressed:(NSButton *)sender {
char cstr[256];
strcpy(cstr, "Application must quit");
[self mDoSomething:cstr];
[self mPostError:cstr];
}
//___________________________________________________________
-(void)mDoSomething: (char*)cstr
{
NSLog(#"%s",cstr);
printf("%s\n", cstr);
}
//___________________________________________________________
-(void) mPostError: (char *)cstr
{
char cMessage[64] = "Critical Error";
NSString *message = [NSString stringWithCString:cMessage encoding:NSASCIIStringEncoding];
NSString *infoText = [NSString stringWithCString:cstr encoding:NSASCIIStringEncoding];
NSAlert* mAlert = [[NSAlert alloc] init];
[mAlert setMessageText:message];
[mAlert setInformativeText:infoText];
[mAlert setAlertStyle: NSCriticalAlertStyle];
[mAlert beginSheetModalForWindow:self.window completionHandler:^(NSModalResponse returnCode) {
[self mDoSomething: cstr]; //this is the suspect line!
}];
}

Answer 1

Your problem is nothing to do with C-strings but with variable lifetime.
Your execution flow goes as follows:
buttonPressed: is called. This creates a local variable cstr and calls mPostError:
mPostError: is called. This calls beginSheetModalForWindow:completionHandler: passing it a block. That block references cstr so a copy is made of the value in cstr, and that value is the address of buttonPressed:'s local variable of the same name.
mPostError: returns which gets us back to...
buttonPressed: which in turn returns destroying its local variable cstr. The block passed to beginSheetModalForWindow:completionHandler: now has the address pointing into released memory.
The user dismisses the alert and the block is called. That block calls mDoSomething: passing it the, now invalid, address.
mDoSomething: calls printf which tries to interpret the memory at the passed address as a C-string, but by now that memory has been re-used for something else and it produces the nonsense you see.
Passing a C-string is not a problem, but whatever value (a char * value in this case) is passed to a block must be valid at the time the block is executed.
You can "fix" the issue in your example by making cstr a global variable:
char cstr[256]; // global
- (IBAction)buttonPressed:(NSButton *)sender {
strcpy(cstr, "Application must quit");
Now cstr always exists and the address the block copies is therefore always valid.
Of course you've now also permanently used up 256 bytes of memory and if this string is only required for a short period that is a (small) waste.
In your real code this may not be an issue - the C strings may be managed already so they live as long as required. If not and using C strings is a requirement then you may need to look at dynamically allocating them (see malloc and friends) and releasing them when no longer needed (see free).
HTH