On Feb 24, 2016, at 5:26 PM, Jens Alfke <
email@hidden> wrote:
Is there a way to control which section of the executable a specific NSString literal goes into? I’ve tried using the ‘section’ attribute, but it has no effect:
I think that syntax is adding the attribute to the type, not to the variable. A pointer-to-something-in-a-section-X is not, itself, necessarily in section X. (But I'm never quite sure about the placement of __attribute__s.)
You can place the kFoo variable itself into a specific section by placing the annotation after its name. But all that does is move the statically-allocated CFString pointer to another section, which I assume isn't what you want.
As far as I know, there's no way to annotate a *value* using the __attribute__ notation, which is what you want if you want the actual textual stuff to go into another section. But a CFString/NSString literal actually creates stuff in two sections: the __cstring section (holding the C string value) and the __cfstring section (holding the statically-allocated CFConstantStringClassReference object, which is a few machine words long).
__attribute__((section(“__TEXT,__foo”))) static NSString* const kFoo = @“Foobar”;
The above doesn’t change where the string appears; there is no __TEXT,__foo section in the binary at all, and the string is still in the default __TEXT,__cstring section.
(FYI, although this doesn't apply to your case I don't think, I've found using repeated @string literals (via a #define, for maintainability) to be somewhat more compact than "static NSString * const foo". CFString literals (including @"foo" strings) are uniqued by the linker, so in general you'll end up sharing a single instance within the framework anyway.)
You can place a C string into a specific section using syntax like this:
static __attribute__((section("__TEXT,foo"))) const char foo_var[] = "Foo, forsooth.";
This works because the quoted string is acting as an initializer, rather than as a string literal, and foo_var[] is annotated as being in a specific section. (This also keeps the linker from merging identical cstrings, since array variables are distinct).
There's also an __attribute__((cold)), which is documented to be useful for situations like this, including basic-block reordering, but I don't know if the Xcode toolchain does anything with it.
If you're desperate, I suppose, you could cobble together your own CFConstantString instances pointing to string data in whatever section you like, using __asm__ blocks, but that might be brittle. :)