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haskellimportffiobject-files

How do I get the address of a C global variable through the Haskell FFI?

For example, in C, I have something like this declared:

extern char _binary_res_txt_start[];

It came from this command:

ld -r -b binary -o binary.o res.txt

How can I get a Ptr in Haskell that points there? If it were a function, I'd do something like foreign import ccall "&exp" a_exp :: FunPtr (Double -> Double), but I don't think that works for variables.

Solution

  • The foreign import syntax works for variables too, as long as you use &. You can then use peekCString or packCString to read it (or peekCStringLen or packCStringLen if it's not null-terminated but you know its length). Here's a full example:

    printf 'foo\0bar' > res.txt
ld -r -b binary -o binary.o res.txt

    extern char _binary_res_txt_start[];
extern char _binary_res_txt_end[];

    import Foreign.C.Types
import Foreign.C.String
import Foreign.Ptr

foreign import ccall "&_binary_res_txt_start" txt_start :: Ptr CChar
foreign import ccall "&_binary_res_txt_end" txt_end :: Ptr CChar

main = do
  str <- peekCStringLen (txt_start, txt_end `minusPtr` txt_start)
  print str

    "foo\NULbar"

    Note that while ld -b binary also produces a _binary_res_txt_size symbol, it's no longer useful now that ASLR is a thing. See Why doesn't a linked binary file's _size symbol work correctly? for details on that.