How to use Data.Compact.Serialize

[Christopher Done]

I forgot to address the fact that libc never statically links things
like name server libraries.

Therefore would GHC devs recommend using a musl-based build e.g. on
Alpine Linux?

Cheers
On Wed, 19 Dec 2018 at 10:31, Christopher Done <chrisdone at gmail.com> wrote:
>
> Hi,
>
> On the docs for Data.Compact.Serialize it says:
>
> http://hackage.haskell.org/package/compact-0.1.0.1/docs/Data-Compact-Serialize.html
> > Our binary representation contains direct pointers to the info tables
> > of objects in the region. This means that the info tables of the
> > receiving process must be laid out in exactly the same way as from the
> > original process; in practice, this means using static linking, using
> > the exact same binary and turning off ASLR.
>
> It seems to me that in order to use this module in any practical way
> (i.e. write to file from one process, and then a later run of the
> process reads it), you need to know a special way to build your binary
> which isn't fully described here. What flags, for example, should be
> passed to GHC to make this viable?
>
> * Turning off ASLR on Linux is done by writing 0 to
>   /proc/sys/kernel/randomize_va_space, which applies to all
>   programs. That's not the most isolated way to deploy an app, but I
>   discovered that you can set this per process here:
>   https://askubuntu.com/a/507954
> * To compile GHC programs statically, use -optl-static -optl-pthread.
>
> So in total the example would be:
>
> $ stack build compact
> compact-0.1.0.1: download
> compact-0.1.0.1: configure
> compact-0.1.0.1: build
> compact-0.1.0.1: copy/register
>
> Example file from docs:
>
> $ cat main.hs
> {-# LANGUAGE TypeApplications #-}
> import System.Environment
> import Data.Compact
> import Data.Compact.Serialize
> main = do
>   arg:_ <- getArgs
>   case arg of
>     "write" -> do
>       orig_c <- compact ("I want to serialize this", True)
>       writeCompact @(String, Bool) "somefile" orig_c
>     "read" -> do
>       res <- unsafeReadCompact @(String, Bool) "somefile"
>       case res of
>         Left err -> fail err
>         Right c -> print (getCompact c)
>
> Compiling statically:
>
> $ stack ghc -- -optl-static -optl-pthread main.hs
> [1 of 1] Compiling Main             ( main.hs, main.o )
> Linking main ...
> [...hundred more warnings like this ...]
>
> Check that it's static:
>
> $ ldd main
> not a dynamic executable
>
> Write the file:
>
> $ setarch `uname -m` -R ./main write
>
> Read the file:
>
> $ setarch `uname -m` -R ./main read
> ("I want to serialize this",True)
>
> Can a GHC dev confirm that this is the proper way to do this? If so,
> I'll contribute this little example as documentation to the compact package.
>
> Cheers