How do I link botan in cmake?

I am trying to write a program in c++ using botan but I don't know how to properly use it. Take for example the following code:

Botan::AutoSeeded_RNG rng;
Botan::UUID uuid = Botan::UUID(rng);
std::cout << uuid.to_string() << std::endl;

If I try to run this it throws an error which seems to be an issue with botan not being linked. But I'm not sure how I can link this using CMake. In CMakeList.txt I have

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/modules/")
find_package(BOTAN REQUIRED)

based on this, and in modules, I have a file FindBOTAN.cmake.

But this throws the error Could NOT find Botan (missing: BOTAN_LIBRARIES BOTAN_INCLUDE_DIRS), the warning

The package name passed to `find_package_handle_standard_args` (PkgConfig)
does not match the name of the calling package (BOTAN).  This can lead to
problems in calling code that expects `find_package` result variables
(e.g., `_FOUND`) to follow a certain pattern.

and fails to find botan.

I have barely any experience using CMake hence I've based it on someone else's but now I've got the problem that I don't know what is going wrong.

Solution

That find module is totally wrong. You should point whoever wrote it to this answer. Here's something better (but could still be improved, e.g. by validating the version):

find_package(PkgConfig REQUIRED)

if (NOT TARGET Botan::Botan)
  pkg_check_modules(Botan QUIET IMPORTED_TARGET botan-2)
  if (TARGET PkgConfig::Botan)
    add_library(Botan::Botan ALIAS PkgConfig::Botan)
  endif ()
endif ()

if (NOT TARGET Botan::Botan)
  find_path(Botan_INCLUDE_DIRS NAMES botan/botan.h
            PATH_SUFFIXES botan-2
            DOC "The Botan include directory")

  find_library(Botan_LIBRARIES NAMES botan botan-2
               DOC "The Botan library")

  mark_as_advanced(Botan_INCLUDE_DIRS Botan_LIBRARIES)

  add_library(Botan::Botan IMPORTED UNKNOWN)
  set_target_properties(
    Botan::Botan
    PROPERTIES
    IMPORTED_LOCATION "${Botan_LIBRARIES}"
    INTERFACE_INCLUDE_DIRECTORIES "${Botan_INCLUDE_DIRS}"
  )
endif ()

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(
  Botan
  REQUIRED_VARS Botan_LIBRARIES Botan_INCLUDE_DIRS
)

Put this file in cmake/FindBotan.cmake and then you can use it from your top-level CMakeLists.txt like so:

cmake_minimum_required(VERSION 3.23)
project(botan_example)

list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
find_package(Botan REQUIRED)

add_executable(botan_example main.cpp)
target_link_libraries(botan_example PRIVATE Botan::Botan)

As always, you should endeavor to link to imported targets, like the Botan::Botan target that my revised find module defines. For main.cpp, I wrapped your code up like so:

#include <botan/botan.h>
#include <botan/uuid.h>
#include <iostream>

int main() {
  Botan::AutoSeeded_RNG rng;
  Botan::UUID uuid = Botan::UUID(rng);
  std::cout << uuid.to_string() << std::endl;
}

And I confirmed that it does indeed build:

alex@alex-ubuntu:~/test$ cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
-- The C compiler identification is GNU 9.4.0
-- The CXX compiler identification is GNU 9.4.0
-- 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 "0.29.1") 
-- Checking for module 'botan-2'
--   Found botan-2, version 2.12.1
-- Found Botan: botan-2  
-- Configuring done
-- Generating done
-- Build files have been written to: /home/alex/test/build

alex@alex-ubuntu:~/test$ cmake --build build/
[ 50%] Building CXX object CMakeFiles/botan_example.dir/main.cpp.o
[100%] Linking CXX executable botan_example
[100%] Built target botan_example

alex@alex-ubuntu:~/test$ ./build/botan_example 
6ACD38C7-3B70-4B57-A0F8-DEDCDCEF4D34