Why is there this dune build error? 'cannot find -lz'

Question

I am trying to build a project with this dune file:

src/lib/precomputed_values/gen_values/dune:

(executable
 (name gen_values)
 (flags -w -32)
 (libraries
   ;; opam libraries
   stdio
   async_kernel
   compiler-libs
   core_kernel
   ppxlib
   ppxlib.ast
   ppxlib.astlib
   async
   core
   ocaml-migrate-parsetree
   base
   ocaml-compiler-libs.common
   async_unix
   base.caml
   sexplib0
   ;; local libraries
   coda_runtime_config
   consensus
   transaction_snark
   mina_state
   snark_params
   coda_genesis_proof
   blockchain_snark
   mina_base
   global_signer_private_key
   ppx_util
   snarky.backendless
   genesis_constants
   pickles
   test_genesis_ledger
   coda_genesis_ledger
   staged_ledger_diff
 )
 (preprocessor_deps ../../../config.mlh)
 (preprocess
  (pps ppx_version ppx_optcomp ppx_let ppxlib.metaquot ppx_here))
 (instrumentation (backend bisect_ppx))
 (modes native))

But am getting the error:

File "src/lib/precomputed_values/gen_values/dune", line 2, characters 7-17:
2 |  (name gen_values)
           ^^^^^^^^^^
/usr/bin/ld: cannot find -lz
collect2: error: ld returned 1 exit status
File "caml_startup", line 1:
Error: Error during linking (exit code 1)
Makefile:84: recipe for target 'build' failed
make: *** [build] Error 1

Could anyone point me in the right direction? Thanks :)

Answer 1

Do you have the libz available on your system ? it looks like the ld linker does not find it in its search path. libz.a is a C library that implements C function for compressing/decompressing data. Such libraries come with your OS distribution independently of Ocaml.

Ocaml tooling provides way to use C library (via stub) using ld (the standard linker) that will link your ocaml stubs to the functions of libz.a . But if the required library is missing, you get the error you quoted.

-lz is saying to ld to link your application with the libz.a (or libz.so) library that is usually located in /usr/lib - if libz.a (libz.so) is not there, the linker will fail causing the emission of the following message :

/usr/bin/ld: cannot find -lz
collect2: error: ld returned 1 exit status