diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6fafa66..ab8cb38 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -4,6 +4,7 @@ project(snmalloc C CXX)
 option(USE_SNMALLOC_STATS "Track allocation stats" OFF)
 option(USE_MEASURE "Measure performance with histograms" OFF)
 option(USE_SBRK "Use sbrk instead of mmap" OFF)
+option(EXPOSE_EXTERNAL_ALLOCATOR_API "Expose the API that allows using external allocators and sharing pagemaps" OFF)
 
 set(CACHE_FRIENDLY_OFFSET OFF CACHE STRING "Base offset to place linked-list nodes.")
 
@@ -67,6 +68,10 @@ if(USE_SBRK)
   add_definitions(-DUSE_SBRK)
 endif()
 
+if(EXPOSE_EXTERNAL_ALLOCATOR_API)
+  add_definitions(-DSNMALLOC_EXPOSE_RESERVE -DSNMALLOC_EXPOSE_PAGEMAP)
+endif()
+
 macro(add_shim name)
   add_library(${name} SHARED src/override/malloc.cc)
   target_link_libraries(${name} -pthread)