How can I determine if a compiled Objective-C app is using garbage collection?

Question

For any application that I have on my Mac, is there a way to tell if it was compiled with GC enabled, or if it's doing manual memory management?

Answer 1

Within the mach-o is a flag used to determine if a binary compiled with GC support, for non-GC, or mixed mode.

I don't know of anything that queries these bits via a more friendly API.

The markgc.c source within the Objective-C runtime can read said flags. You could refactor it to your needs, as desired.

Kind of curious why you need to know?

Answer 2

I found the answer here. Mind you that the original post is wrong, but contains a comment by Mark Rowe, an Apple engineer, that points the way.

I have re-run the otool commands he mentions on my machine with the current OS (10.6.4). Here's the output:

$ uname -a
Darwin meaningless.local 10.4.0 Darwin Kernel Version 10.4.0: Fri Apr 23 18:28:53 PDT 2010; root:xnu-1504.7.4~1/RELEASE_I386 i386

### Mail doesn't use GC
$ otool -oV /Applications/Mail.app/Contents/MacOS/Mail | tail -3
Contents of (__DATA,__objc_imageinfo) section
  version 0
    flags 0x0

### Xcode supports GC and retain/release
$ otool -oV /Developer/Applications/Xcode.app/Contents/MacOS/Xcode | tail -3
Contents of (__DATA,__objc_imageinfo) section
  version 0
    flags 0x2 OBJC_IMAGE_SUPPORTS_GC

Mark Rowe's explanation:

The field of interest here is the “flags” field of the __image_info section of the __OBJC segment. If garbage collection is supported it will have the value 0×2 and will be shown as “GC RR” to represent that both garbage collection and retain/release are supported. If garbage collection is required then the field will have the value 0×4 and will be shown as “GC-only” indicating that only garbage collection is supported and that retain/release is not available. The field can also contain other values, but those two are the only values that are relevant to garbage collection.