diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..ec13429
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,7 @@
+# Codspeed C++
+
+## Release
+
+For now, releasing is done manually, by tagging a git rev with vX.Y.Z.
+
+When releasing a new version, please make sure the [version](https://github.com/CodSpeedHQ/codspeed-cpp/blob/fix-walltime-pid/core/CMakeLists.txt#L3) is updated in the `core/CMakeLists.txt` file to match the pushed version
diff --git a/core/CMakeLists.txt b/core/CMakeLists.txt
index b6f1657..6ce84c3 100644
--- a/core/CMakeLists.txt
+++ b/core/CMakeLists.txt
@@ -17,20 +17,83 @@ add_library(codspeed src/codspeed.cpp src/walltime.cpp src/uri.cpp)
 # Version
 add_compile_definitions(CODSPEED_VERSION="${CODSPEED_VERSION}")
 
+# Specify the include directories for users of the library
+target_include_directories(
+    codspeed
+    PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+)
+
 # Disable valgrind compilation errors
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
     # Disable the old-style-cast warning for the specific target
     target_compile_options(codspeed PRIVATE -Wno-old-style-cast)
 endif()
 
-# Specify the include directories for users of the library
-target_include_directories(
+execute_process(
+    COMMAND git rev-parse --show-toplevel
+    OUTPUT_VARIABLE GIT_ROOT_DIR
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    RESULT_VARIABLE GIT_COMMAND_RESULT
+)
+
+if(NOT GIT_COMMAND_RESULT EQUAL 0)
+    message(
+        WARNING
+        "Failed to determine the git root directory.\
+        Check that git is in your PATH, and that you are in a git repository.\
+        Continuing, but codspeed features will not be useable"
+    )
+    # Default to user's cmake source directory
+    set(GIT_ROOT_DIR ${CMAKE_SOURCE_DIR})
+endif()
+
+target_compile_definitions(
     codspeed
-    PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+    INTERFACE -DCODSPEED_GIT_ROOT_DIR="${GIT_ROOT_DIR}"
+)
+
+set(CODSPEED_MODE_ALLOWED_VALUES "OFF" "instrumentation" "walltime")
+set(CODSPEED_MODE "OFF" CACHE STRING "Build mode for Codspeed")
+set_property(
+    CACHE CODSPEED_MODE
+    PROPERTY STRINGS ${CODSPEED_MODE_ALLOWED_VALUES}
 )
 
+if(NOT CODSPEED_MODE STREQUAL "OFF")
+    target_compile_definitions(codspeed INTERFACE -DCODSPEED_ENABLED)
+
+    if(NOT CMAKE_BUILD_TYPE)
+        message(
+            WARNING
+            "CMAKE_BUILD_TYPE is not set. Consider setting it to 'RelWithDebInfo'."
+        )
+    elseif(NOT CMAKE_BUILD_TYPE STREQUAL "RelWithDebInfo")
+        message(
+            WARNING
+            "CMAKE_BUILD_TYPE is set to '${CMAKE_BUILD_TYPE}', but 'RelWithDebInfo' is recommended."
+        )
+    endif()
+
+    # Define a preprocessor macro based on the build mode
+    if(CODSPEED_MODE STREQUAL "instrumentation")
+        target_compile_definitions(
+            codspeed
+            INTERFACE -DCODSPEED_INSTRUMENTATION
+        )
+    elseif(CODSPEED_MODE STREQUAL "walltime")
+        target_compile_definitions(codspeed INTERFACE -DCODSPEED_WALLTIME)
+    else()
+        message(
+            FATAL_ERROR
+            "Invalid build mode: ${CODSPEED_MODE}. Use one of ${CODSPEED_MODE_ALLOWED_VALUES}."
+        )
+    endif()
+endif()
+
+message(STATUS "Codspeed mode: ${CODSPEED_MODE}")
+
 option(ENABLE_TESTS "Enable building the unit tests which depend on gtest" OFF)
 if(ENABLE_TESTS)
-    enable_testing() 
+    enable_testing()
     add_subdirectory(test)
 endif()
diff --git a/core/src/walltime.cpp b/core/src/walltime.cpp
index 6e6aa50..92bf7c0 100644
--- a/core/src/walltime.cpp
+++ b/core/src/walltime.cpp
@@ -8,7 +8,11 @@
 #include <numeric>
 #include <sstream>
 #include <string>
-#include <thread>
+#ifdef _WIN32
+#include <process.h>
+#else
+#include <unistd.h>
+#endif
 #include <vector>
 
 const double IQR_OUTLIER_FACTOR = 1.5;
@@ -105,7 +109,11 @@ void write_codspeed_benchmarks_to_json(
 
   std::string creator_name = "codspeed-cpp";
   std::string creator_version = CODSPEED_VERSION;
-  std::thread::id creator_pid = std::this_thread::get_id();
+#ifdef _WIN32
+  pid_t creator_pid = _getpid();
+#else
+  pid_t creator_pid = getpid();
+#endif
   std::string instrument_type = "walltime";
 
   oss << "{\n";
diff --git a/examples/google_benchmark/CMakeLists.txt b/examples/google_benchmark/CMakeLists.txt
index ed72751..f8f8fc4 100644
--- a/examples/google_benchmark/CMakeLists.txt
+++ b/examples/google_benchmark/CMakeLists.txt
@@ -3,6 +3,9 @@ include(FetchContent)
 
 project(codspeed_picobench_compat VERSION 0.0.0 LANGUAGES CXX)
 
+# On the small benchmarks of this repo, most of the benches will be optimized out, but this is expected.
+set(CMAKE_BUILD_TYPE RelWithDebInfo)
+
 option(BENCHMARK_ENABLE_GTEST_TESTS OFF)
 
 FetchContent_Declare(
diff --git a/flake.lock b/flake.lock
index a73bf10..010eb84 100644
--- a/flake.lock
+++ b/flake.lock
@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1739525499,
-        "narHash": "sha256-E9szEEULlfJLUjcacwotBx19aqcElQLR8O22fkR/YmU=",
+        "lastModified": 1741951873,
+        "narHash": "sha256-JbREpcR9+NXW0tSbVaj6GKWpNome5RpZYJ95KEhVMGs=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "bdfb66032a0b71a1826d7f31391a18a39aeb80bc",
+        "rev": "ce52a4d87a33258646dde1a1bfcaa88074e9c821",
         "type": "github"
       },
       "original": {
diff --git a/flake.nix b/flake.nix
index 0c844ef..6cb5538 100644
--- a/flake.nix
+++ b/flake.nix
@@ -34,6 +34,14 @@
             '';
           };
 
+          clang = pkgs.mkShell {
+            buildInputs = with pkgs; [ clang ] ++ commonBuildInputs;
+            shellHook = ''
+              export CXX=clang++;
+              echo "Welcome to the C++ development environment with Clang!"
+            '';
+          };
+
           lsp = pkgs.mkShell {
             buildInputs =
               with pkgs;
diff --git a/google_benchmark/cmake/Codspeed.cmake b/google_benchmark/cmake/Codspeed.cmake
index 958c09a..df978f6 100644
--- a/google_benchmark/cmake/Codspeed.cmake
+++ b/google_benchmark/cmake/Codspeed.cmake
@@ -1,44 +1 @@
 add_subdirectory(${PROJECT_SOURCE_DIR}/../core codspeed)
-
-execute_process(
-    COMMAND git rev-parse --show-toplevel
-    OUTPUT_VARIABLE GIT_ROOT_DIR
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-    RESULT_VARIABLE GIT_COMMAND_RESULT
-)
-
-if(NOT GIT_COMMAND_RESULT EQUAL 0)
-    message(
-        WARNING
-        "Failed to determine the git root directory.\
-        Check that git is in your PATH, and that you are in a git repository.\
-        Continuing, but codspeed features will not be useable"
-    )
-    # Default to user's cmake source directory
-    set(GIT_ROOT_DIR ${CMAKE_SOURCE_DIR})
-endif()
-
-target_compile_definitions(
-    codspeed
-    INTERFACE -DCODSPEED_GIT_ROOT_DIR="${GIT_ROOT_DIR}"
-)
-
-if(DEFINED CODSPEED_MODE)
-    target_compile_definitions(codspeed INTERFACE -DCODSPEED_ENABLED)
-    # Define a preprocessor macro based on the build mode
-    if(CODSPEED_MODE STREQUAL "instrumentation")
-        target_compile_definitions(
-            codspeed
-            INTERFACE -DCODSPEED_INSTRUMENTATION
-        )
-    elseif(CODSPEED_MODE STREQUAL "walltime")
-        target_compile_definitions(codspeed INTERFACE -DCODSPEED_WALLTIME)
-    else()
-        message(
-            FATAL_ERROR
-            "Invalid build mode: ${CODSPEED_MODE}. Use 'instrumentation' or 'walltime'."
-        )
-    endif()
-endif()
-
-message(STATUS "Build mode set to: ${CODSPEED_MODE}")
diff --git a/google_benchmark/src/benchmark_api_internal.h b/google_benchmark/src/benchmark_api_internal.h
index ce64017..0a9446b 100644
--- a/google_benchmark/src/benchmark_api_internal.h
+++ b/google_benchmark/src/benchmark_api_internal.h
@@ -39,6 +39,7 @@ class BenchmarkInstance {
   const std::vector<Statistics>& statistics() const { return statistics_; }
   int repetitions() const {
 #ifdef CODSPEED_INSTRUMENTATION
+    (void)repetitions_;
     return 1;
 #else
     return repetitions_;