Question

I print out something to the console using NSLog(). Is there a way to pass all the current method’s arguments to NSLog(), or any other function or method, without looking at each of them explicitly?

For example, I already have a macro that prints useful information to the console when I just put LOGME in my code. The macro will create a call to a singleton class’s logging-method and pass several useful things like _cmd and other stuff.

I would also like to catch all the arguments of that method and pass them on for printing them out automatically. Is that possible?

Answer 1

Use gdb. You can set a break point that logs the arguments to the method and continues.

If you insist on doing it the hard way… It’s certainly not simple, but you could use the stack structure on a given architecture/ABI and make use of the Objective-C runtime to figure out how many arguments and of what size to look for. From here on out, I’m in unchartered territory; I’ve never done this nor would I ever bother. So YMMV…

Within your method, you can get the Method struct, then the number of arguments, then each argument type and the its size. You could then walk the stack from the address of the self parameter (i.e. &self), assuming you knew what you were doing…

Method method = class_getInstanceMethod([self class], _cmd);
unsigned nargs = method_getNumberOfArguments(method);
void *start = &self;
for(unsigned i = 0; i<nargs; i++) {
  char *argtype = method_copyArgumentType(method, i);
  //find arg size from argtype
  // walk stack given arg zie
  free(argtype);
}

Along the way you’d have to convert from the argtype string (using the Objective-C type encodings) to the size of each argument on the stack.

Of course then you’d have to derive the format string for each type, and call NSLogv with an appropriate variable argument array containing the arguments, copied from their location on the stack. Probably a lot more work than its worth. Use the debugger.