This introduces a new option to forcefully disable threads.

lemire · lemire · commit 6c33f518a8f2 · 2020-06-26T13:23:44.000-04:00
diff --git a/cmake/simdjson-flags.cmake b/cmake/simdjson-flags.cmake
@@ -105,15 +105,21 @@ if(NOT SIMDJSON_EXCEPTIONS)
   target_compile_definitions(simdjson-internal-flags INTERFACE SIMDJSON_EXCEPTIONS=0)
 endif()
 
-option(SIMDJSON_ENABLE_THREADS "Enable threaded operation" ON)
+option(SIMDJSON_ENABLE_THREADS "Link with thread support" ON)
 if(SIMDJSON_ENABLE_THREADS)
   set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
   set(THREADS_PREFER_PTHREAD_FLAG TRUE)
   find_package(Threads REQUIRED)
   target_link_libraries(simdjson-flags INTERFACE Threads::Threads)
   target_link_libraries(simdjson-flags INTERFACE ${CMAKE_THREAD_LIBS_INIT})
   target_compile_options(simdjson-flags INTERFACE ${CMAKE_THREAD_LIBS_INIT})
-  target_compile_definitions(simdjson-flags INTERFACE SIMDJSON_THREADS_ENABLED=1)
+  target_compile_definitions(simdjson-flags INTERFACE SIMDJSON_THREADS_ENABLED=1) # This will be set in the code automatically.
+endif()
+
+# Some users compile simdjson with thread support but still do not want simdjson to use threads.
+option(SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT "Whether we enabled thread support or not (SIMDJSON_ENABLE_THREADS), do not use threads. This option does nothing when thread support is not enabled." OFF)
+if(SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT)
+  target_compile_definitions(simdjson-flags INTERFACE SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT=1)
 endif()
 
 if(SIMDJSON_USE_LIBCPP)
diff --git a/doc/parse_many.md b/doc/parse_many.md
@@ -101,6 +101,12 @@ A `document_stream` instance uses at most two threads: there is a main thread an
 You should expect the main thread to be fully occupied while the worker thread is partially busy
 (e.g., 80% of the time).
 
+If you compile simdjson with thread support and you still do not want simdjson to use threads,
+you can forcefully disable them by setting the SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT macro
+to 1 in C++, or by passing SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT=ON to cmake. It is a 
+compile-time decision: if you disable the threads with SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT,
+you will not be able to use threads in simdjson unless you recompile.
+
 Support
 -------
 
diff --git a/include/simdjson/portability.h b/include/simdjson/portability.h
@@ -122,7 +122,6 @@ compiling for a known 64-bit platform."
 #endif
 #endif
 
-
 // workaround for large stack sizes under -O0.
 // https://github.com/simdjson/simdjson/issues/691
 #ifdef __APPLE__
@@ -136,6 +135,13 @@ compiling for a known 64-bit platform."
 #endif
 #endif
 
+
+#if SIMDJSON_DO_NOT_USE_THREADS_NO_MATTER_WHAT
+// No matter what happened, we undefine SIMDJSON_THREADS_ENABLED and so disable threads.
+#undef SIMDJSON_THREADS_ENABLED
+#endif
+
+
 #if defined(__clang__)
 #define NO_SANITIZE_UNDEFINED __attribute__((no_sanitize("undefined")))
 #elif defined(__GNUC__)
