How to print and use constants with gdb (through Xcode)?

Question

I am debugging a Cocoa application using xcode-gdb. I am at a break point and I want view the value of some Cocoa constants (ie NSControlKeyMask) and to do some test with the values in the current stackframe. Specifically I am in

- (void) keyDown:(NSEvent *) e 

, and I have done

set $mf = (int)[e modifierFlags]

on the gdb prompt. Now I want to do

p $mf & NSControlKeyMask

and gdb is telling me 'No symbol "NSControlKeyMask" in current context.'

UPDATE:
Xcode has the "Fix and Continue text" feature. So I used Dan M. and n8gray solution with this feature so I don't have to make a proxy of every constant.

Answer 1

If no variables are actually instantiated with a given type, then the debug information for the corresponding symbols doesn't wind up getting generated by gcc. Then, if you ask gdb about such a type, it doesn't know what you are talking about because there is no debug information for that type, and it will give you the "No symbol in current context" error.

A workaround to this problem would normally be to explicitly add a dummy variable, of the type in question, somewhere in the code. Here is a simple example that you can test to see what I'm talking about:

enum an_enum_type {
  foo,
  bar,
  baz
};

int main (int argc, char *argv [])
{
  return baz;
}

Save that program to a file named test.cpp and compile it with this command:

g++ -o test -g -O0 test.cpp

Then run it under gdb and type "p /x baz". You will get the "No symbol baz in current context" error.

Now try it with this modified version that has added a dummy variable, of the enum type:

enum an_enum_type {
  foo,
  bar,
  baz
};

an_enum_type dummy;

int main (int argc, char *argv [])
{
  return baz;
}

Compile with the same command as before and run under gdb. This time when you type "p /x baz" you'll get "0x2" as the answer, which I think is what you are shooting for in your question.

I've looked into it, and the problem is that the NSEvent.h header file doesn't give a name to the enum that contains NSControlKeyMask -- it's an anonymous enum. So there is no way to create a variable of that type (dummy or otherwise). So, I don't see any way of getting the compiler to generate the debug information for that type. I think you're just going to have to rely on the definition of NSControlKeyMask from the header file.