How to call methods or execute code in LLDB debugger?

Question

I know I can type print someFloatVariable when I set a breakpoint or po [self someIvarHoldingAnObject], but I can't do useful things like:

[self setAlpha:1];

Then it spits out:

error: '[self' is not a valid command.

Weird thing is that I can call po [self someIvarHoldingAnObject] and it will print it's description.

I believe I've seen a video a year ago where someone demonstrated how to execute code through the console at runtime, and if I am not mistaken this guy also provided arguments and assigned objects to pointers. How to do that?

Answer 1

The canonical reference for gdb v. lldb commands is http://lldb.llvm.org/lldb-gdb.html

You want to use the expr command which evaluates an expression. It's one of the lldb commands that takes "raw input" in addition to arguments so you often need a "--" to indicate where the arguments (to expr) end and the command(s) begin. e.g.

(lldb) expr -- [self setAlpha:1]

There is a shortcut, "p", which does the -- for you (but doesn't allow any arguments), e.g.

(lldb) p [self setAlpha:1]

If the function(s) you're calling are not part of your program, you'll often need to explicitly declare their return type so lldb knows how to call them. e.g.

(lldb) p printf("hi\n")
error: 'printf' has unknown return type; cast the call to its declared return type
error: 1 errors parsing expression
(lldb) p (int)printf("hi\n")
(int) $0 = 3
hi
(lldb)

There is a neat way to work around the floating point argument problem, BTW. You create an "expression prefix" file which is added to every expression you enter in lldb, with a prototype of your class methods. For instance, I have a class MyClass which inherits from NSObject, it has two methods of interest, "setArg:" and "getArg" which set and get a float ivar. This is a silly little example, but it shows how to use it. Here's a prefix file I wrote for lldb:

@interface NSObject
@end
@interface MyClass : NSObject
- init;
- setArg: (float)arg;
- (float) getArg;
@end

extern "C" {
  int strcmp (const char *, const char *);
  int printf(const char * __restrict, ...);
  void puts (const char *);
}

in my ~/.lldbinit file I add

settings set target.expr-prefix /Users/jason/lldb-prefix.h

and now I can do

(lldb)    p [var getArg]
(float) $0 = 0.5
(lldb)    p [var setArg:0.7]
(id) $1 = 0x0000000100104740
(lldb)    p [var getArg]
(float) $2 = 0.7

You'll notice I included a couple of standard C library functions in here too. After doing this, I don't need to cast the return types of these any more, e.g.

(lldb) p printf("HI\n")
<no result>
HI
(lldb) p strcmp ("HI", "THERE")
(int) $3 = -12

(a fix for that "<no result>" thing has been committed to the lldb TOT sources already.)