How to find memory used by NSString? - iphone

Question

I got stuck when trying to look at the memory used by NSString:
My NSString theKey shows fine in lldb:
(lldb) p *theKey
(NSString) $5 = {
NSObject = {
isa = __NSCFConstantString
}
}
The content is visible:
(lldb) po theKey
BBBBBBBBBB
I also get the memory of the NSString object:
(lldb) fr v
(ViewController *const) self = 0x0897eb10
(SEL) _cmd = "viewDidLoad"
(NSString *) theKey = 0x00004664 #"BBBBBBBBBB"
When regarding the memory of the NSString object I only get strange values:
(lldb) memory read 0x00004664
0x00004664: 50 93 87 01 c8 07 00 00 68 37 00 00 0b 00 00 00 P.......h7......
0x00004674: 50 93 87 01 c8 07 00 00 74 37 00 00 0b 00 00 00 P.......t7......
This is, an NSString is not an array of char, and the memory contains … yes … what? How can I find the memory address where the characters stored in my NSString are stored?

Answer 1

You asked:
How can I find the memory address where the characters stored in my
NSString are stored?
In short, you can't. NSString is effectively opaque. The closest you're going to get is probably to call the -UTF8String method on it, which will give you a const char*, but if the string itself is not natively encoded with UTF8 encoding, then that pointer will not point to the "actual" data in the NSString, so...

Answer 2

I recently had the same problem. You can use the following program to find
the C pointer in memory of an NSString.
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
NSString *a=#"###########";
const char *ptr = CFStringGetCStringPtr( (__bridge CFStringRef)a,
kCFStringEncodingMacRoman ) ;
puts( ptr ) ; // find ptr
NSLog(#"address:%p",ptr);
NSLog(#"content:%s",ptr);
return 0;
}
I found that the solution is to use CFStringGetCStringPtr to find the C pointer, which can then be used to find the NSString in memory.
Refer to: http://opensource.apple.com/source/CF/CF-1153.18/CFString.c

Answer 3

I had the same question. I wanted to see the bytes ( in hex ) that were stored in memory for an NSString.
I started with this byte array:
const char bytes1[7] = { 0x05, 0x31, 0x32, 0x33, 0x34, 0x35, 0x00 }; // \0x512345
Placing a breakpoint, after nsStrInput was initialized - like #ipmcc said - the
UTF8String method returned a Character array pointer.
(lldb) po [nsStrInput UTF8String]
0x0000000100769138
(lldb) mem read -c10 0x0000000100769138
0x100769138: 05 31 32 33 34 35 00 00 00 00 .12345....
I could see my instance of NSString did hold the unprintable ASCII charactoer (0x05). That was the answer I was hoping to find.