* Being consumed by CMake

To support libraries built with JustBuild to be consumed by CMake, the rule
~["CC", "install-with-deps"]~ can be used. This rule will install the library's
artifacts, its public headers, and a pkg-config file, which can be used by CMake
to properly consume the library.

In this example, we show how to build and install the ~ssl~ library with
JustBuild, so it can be picked up and used by CMake afterwards.

** Example: Build and install the ~ssl~ Library with JustBuild

First make sure that ~just~, ~just-mr~, and ~just-import-git~ are available in
your ~PATH~. Then, define a new workspace by creating the ~ROOT~ marker.

#+BEGIN_SRC sh
$ touch ROOT
#+END_SRC

The ~ssl~ library that we want to use is a defined dependency of the JustBuild
repository ~just/ssl~. To define a repository for that library, create the
following ~repos.template.json~ file.

#+SRCNAME: repos.template.json
#+BEGIN_SRC js
{ "main": "ssl"
, "repositories":
  { "ssl":
    { "repository": {"type": "file", "path": "."}
    , "bindings": {"rules": "rules-cc", "just-ssl": "just/ssl"}
    }
  }
}
#+END_SRC

and import the ~rules-cc~ and JustBuild repository, including its dependency
~just/ssl~, via:

#+BEGIN_SRC sh
$ cat repos.template.json \
  | just-import-git -C - --as just -b master https://github.com/just-buildsystem/justbuild \
  | just-import-git -C - --as rules-cc -b master https://github.com/just-buildsystem/rules-cc \
  > repos.json
$
#+END_SRC

Next, the following ~TARGETS~ file needs to be provided.

#+SRCNAME: TARGETS
#+BEGIN_SRC js
{ "ssl-lib":
  { "type": ["@", "rules", "CC", "library"]
  , "name": ["ssl"]
  , "deps": [["@", "just-ssl", "", "ssl"]]
  }
, "ssl-pkg":
  {"type": ["@", "rules", "CC", "install-with-deps"], "targets": ["ssl-lib"]}
}
#+END_SRC

The library target ~ssl-lib~ specifies the name ~"ssl"~ (also used as package
name) and a dependency on the ~ssl~ library from ~just-ssl~. Note that although
this library target is merely a "wrapper" (header-only library without headers),
it could equally well be a full-blown library target. Furthermore, the package
target ~ssl-pkg~ installs the ~ssl-lib~ target including its dependencies and
generates a pkg-config file that can be later used by CMake.

Finally, the ~ssl-pkg~ target can be built and installed to a specific ~PREFIX~
(note that ~OS~ and ~ARCH~ must be set as well).

#+BEGIN_SRC sh
$ export PREFIX=/tmp/just_ssl
$ just-mr install -D'{"OS":"linux", "ARCH":"x64_64", "PREFIX":"'$PREFIX'"}' -o $PREFIX ssl-pkg
INFO: Requested target is [["@","ssl","","ssl-pkg"],{"ARCH":"x64_64","OS":"linux","PREFIX":"/tmp/just_ssl"}]
INFO: Analysed target [["@","ssl","","ssl-pkg"],{"ARCH":"x64_64","OS":"linux","PREFIX":"/tmp/just_ssl"}]
INFO: Export targets found: 0 cached, 2 uncached, 0 not eligible for caching
INFO: Discovered 269 actions, 3 trees, 1 blobs
INFO: Building [["@","ssl","","ssl-pkg"],{"ARCH":"x64_64","OS":"linux","PREFIX":"/tmp/just_ssl"}].
PROG: [ 79%] 0 cached, 215 run, 8 processing (["@","just/ssl","","crypto-lib"]#105, ...).
INFO: Processed 269 actions, 0 cache hits.
INFO: Artifacts can be found in:
        /tmp/just_ssl/include/openssl/aead.h [6d78db27d24746b39eacc20374697476e080c9f6:21054:f]
        ...
        /tmp/just_ssl/lib/libcrypto.a [df64da43f0168a717d09d609bf0c631d29c86b61:2336092:f]
        /tmp/just_ssl/lib/libssl.a [77d2c2bfbe3ef3608895c854f1d1f6e1c200efd0:852620:f]
        /tmp/just_ssl/share/pkgconfig/ssl.pc [9b69c758430f5b5fb6ff7a9b1f1ffc89471509af:406:f]
$
#+END_SRC

** Example: Consume the installed ~ssl~ Library with CMake

As an example, a minimal ~main.c~ file is created that depends on the ~ssl~
library.

#+SRCNAME: main.c
#+BEGIN_SRC C
#include <openssl/evp.h>

int main(int argc, char** argv) {
  OpenSSL_add_all_algorithms();
  return 0;
}
#+END_SRC

In the ~CMakeLists.txt~ file, the ~pkg_check_modules()~ macro can be used to
determine the compile and linker flags needed to consume our ~ssl~ library with
CMake.

#+SRCNAME: CMakeLists.txt
#+BEGIN_SRC cmake
cmake_minimum_required(VERSION 3.1)

project(test_ssl)

find_package(PkgConfig REQUIRED)
pkg_check_modules(SSL REQUIRED ssl)

add_executable(main main.c)
target_include_directories(main PRIVATE ${SSL_INCLUDE_DIRS})
target_compile_options(main PRIVATE ${SSL_CFLAGS_OTHER})
target_link_directories(main PRIVATE ${SSL_LIBRARY_DIRS})
target_link_libraries(main PRIVATE ${SSL_LDFLAGS_OTHER} ${SSL_LIBRARIES})
#+END_SRC

Note that ~${SSL_LDFLAGS_OTHER}~ can be omitted if the ~ssl-pkg~ target was
defined with a non-empty value for the field ~"flat-libs"~ (see rule
documentation of ~["CC", "install-with-deps"]~).

Finally, when running CMake, make sure to set the ~CMAKE_PREFIX_PATH~ to the
previously used install prefix in order to successfully find the installed ~ssl~
library and its pkg-config file.

#+BEGIN_SRC sh
$ cmake -DCMAKE_PREFIX_PATH=/tmp/just_ssl -S . -B /tmp/test_ssl
-- The C compiler identification is GNU 10.2.1
-- The CXX compiler identification is GNU 10.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.2") 
-- Checking for module 'ssl'
--   Found ssl, version unknown
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/test_ssl
$ cmake --build /tmp/test_ssl
[ 50%] Building CXX object CMakeFiles/main.dir/main.c.o
[100%] Linking CXX executable main
[100%] Built target main
$
#+END_SRC

Note that if the package is moved to a different location, the ~prefix~ variable
within the pkg-config file ~share/pkgconfig/ssl.pc~ must be updated as well.
Alternatively, CMake can be instructed to automatically guess the correct prefix
by setting the variable ~PKG_CONFIG_ARGN~ to ~"--define-prefix"~ (either on the
command line or in the ~CMakeLists.txt~). However, this is a fairly recent CMake
feature and requires at least CMake version 3.22.