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Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey"
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Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey"


  • Subject: Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey"
  • From: Dave Keck <email@hidden>
  • Date: Fri, 3 Apr 2009 16:32:52 -1000

> As for read-only address space, does the const declaration not take care of
> that? (Serious question.)

I just tested whether it's possible to write at the address of an
NSString declared with either:

NSString *const HelloString = @"ABC123";
#define GoodbyeString @"XYZ345"

(If you're interested, you can download my test project here:
http://www.docdave.com/halla.zip)

In both cases, I was able to write to the address of the string
without crashing. I found this surprising, because I assumed at least
HelloString would be stored in the read-only _TEXT segment...

According to otool (otool -l), the _cstring section of the _TEXT
segment for my test app starts at the decimal offset 3740 in the
binary. With my handy HexEdit, I went to this offset, and sure enough,
there's my 'ABC123' and 'XYZ345'. (Perhaps I should also note that the
_cstring section is the only place where these two strings appear in
the binary.)

So it seems that the strings are stored as C strings in the binary
(not surprising, I guess) but are automagically converted to NSStrings
at runtime. These runtime-generated NSStrings are stored in a
read-write portion of the processes' address space, which is why I was
able to write directly to their addresses.

So I guess this is getting OT, but if someone (perhaps, The Runtime
Wrangler :)) knows how this works, I'd love to know.
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  • Follow-Ups:
    • Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey"
      • From: Quincey Morris <email@hidden>
References: 
 >NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey" (From: Memo Akten <email@hidden>)
 >Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey" (From: Nate Weaver <email@hidden>)
 >Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey" (From: Quincey Morris <email@hidden>)
 >Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey" (From: Memo Akten <email@hidden>)
 >Re: NSString* kMyKey = @"aKey" vs #define MYKEY @"aKey" (From: Nate Weaver <email@hidden>)

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