NSMutableData *data = [[@"foo" dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES] mutableCopy];
char nul = '\0';
[data appendBytes:&nul length:sizeof(nul)];
const char *cString = [data bytes];
Or, simply:
const char *cString = [@"foo" UTF8String];
... with exactly the same result.