# Building a C library with opam **URL:** \ **Category:** Learning\ **Tags:** opam\ **Created:** [December 29, 2022, 1:39pm UTC](https://discuss.ocaml.org/t/building-a-c-library-with-opam/11069 "2022-12-29T13:39:53Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![eWert-Online](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/ewert-online/32/4130_2.png) [@eWert-Online](https://discuss.ocaml.org/u/eWert-Online)\ **Post date:** [December 29, 2022, 1:39pm UTC](https://discuss.ocaml.org/t/building-a-c-library-with-opam/11069/1 "2022-12-29T13:39:53Z")
Hello 🙂 I want to build a c library (specifically [libspng](https://github.com/randy408/libspng)) with opam and statically link it into one of my ocaml projects. I have a few questions about how to do this in the best way. My opam file currently looks like this: ```auto opam-version: "2.0" synopsis: "Simple, modern libpng alternative" description: """ libspng (simple png) is a C library for reading and writing Portable Network Graphics (PNG) format files with a focus on security and ease of use. The goal is to provide a fast PNG library with a simpler API than libpng. """ maintainer: ["Torben Ewert "] author: ["randy408@protonmail.com"] license: "BSD-2-Clause" homepage: "https://libspng.org/" bug-reports: "https://github.com/.../issues" dev-repo: "git+https://github.com/randy408/libspng" depends: [ "conf-zlib" {build} "conf-cmake" {build} ] build-env: [ [CFLAGS = "-O3 -fPIC"] ] build: [ [ "%{conf-cmake:cmd}%" "-DCMAKE_MAKE_PROGRAM=make" {os-family = "windows"} "-GUnix Makefiles" {os-family = "windows"} "-DCMAKE_HOST_WIN32=true" {os-family = "windows"} "-DCMAKE_BUILD_TYPE=RELEASE" "-DCMAKE_INSTALL_PREFIX=%{prefix}%" ] [make "-j%{jobs}%"] ] install: [make "install"] url { src: "https://github.com/randy408/libspng/archive/refs/tags/v0.7.3.tar.gz" checksum: ["md5=7d9159a1e08d7dd258ac146537236b67"] } ``` **Questions:** 1. Do I need the source files from libspng or does opam unpack the archive provided in the `url` field? Without the sources present it always fails with `The source directory "..." does not appear to contain CMakeLists.txt.`. So I guess I need the source files. But what is the `url` field for then? 2. Is `%{prefix}%` the correct install prefix? When installing the package into my ocaml project, the files are moved directly into the local switch folder (`_opam/lib/libspng_static.a`, …). That is unlike all other packages, which have a subfolder inside `lib`. So I would think that `_opam/lib/libspng/libspng_static.a` would be better? 3. Should I even do a `make install` or would it be better to provide a `libspng.install` file which moves the built files? 4. When building on windows (cygwin) the `prefix` variable gets evaluated to `D:\a\foo\bar\_opam` which is interpreted as a relative path. So the files get installed into the wrong location. I “fixed” that by moving the build steps into a `build.sh` file and wrapping the prefix with `cygpath` if in a cygwin environment. Is this behaviour a bug in opam? I would expect, that the prefix (and all other opam vars) are already pointing to the correct cygwin directories when building on windows. 5. Should I update the environment variables (ex. `PKG_CONFIG_PATH`) with the `setenv` field? Thank you in advance for your help! I am still pretty new to opam and am maybe a bit confused about some things 😄 - Torben ---
**Author:** ![silene](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/silene/32/2707_2.png) [@silene](https://discuss.ocaml.org/u/silene)\ **Post date:** [December 29, 2022, 7:27pm UTC](https://discuss.ocaml.org/t/building-a-c-library-with-opam/11069/2 "2022-12-29T19:27:02Z")
1. Opam unpacks the archive. No idea why the file `CMakeLists.txt` is not there. 2. `%{prefix}%` should be fine. When you talk about other packages, I assume you mean OCaml libraries. The fact that subdirectories are used is an OCaml/Findlib convention; it does not apply to C libraries. 3. Only bother with an `.install` file if `make install` does not work properly. 4. No idea. 5. You do not need to update `PKG_CONFIG_PATH`. The package `conf-pkg-config` already makes this environment variable point to `%{lib}%/pkgconfig`. So, assuming that `make install` properly populated this directory, that should be sufficient. (That is also assuming that any Opam package built on top of your packaged library depends on `conf-pkg-config {build}`.) ---
**Author:** ![eWert-Online](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/ewert-online/32/4130_2.png) [@eWert-Online](https://discuss.ocaml.org/u/eWert-Online)\ **Post date:** [December 29, 2022, 8:32pm UTC](https://discuss.ocaml.org/t/building-a-c-library-with-opam/11069/3 "2022-12-29T20:32:08Z")
Thank you for your answer! 1: I will investigate the missing sources issue a bit further then. Should the unpacked archive be visible inside some switch folder? 2: Yes I am talking about ocaml libraries. I was just wondering if that is how it is supposed to look 😄 5: Should I depend on `conf-pkg-config` then? ---
**Author:** ![silene](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/silene/32/2707_2.png) [@silene](https://discuss.ocaml.org/u/silene)\ **Post date:** [December 30, 2022, 7:30am UTC](https://discuss.ocaml.org/t/building-a-c-library-with-opam/11069/4 "2022-12-30T07:30:52Z")
Yes, the unpacked archive is somewhere under `.opam/yourswitch/.opam-switch/build`. You might need to pass the option `--keep-build-dir` to `opam install`. As for `conf-pkg-config`, the Opam package for your C library should depend on it only if the C library uses the `pkgconfig` command during its build (e.g., to locate Zlib).