Index: libomptarget/src/exports
===================================================================
--- libomptarget/src/exports
+++ libomptarget/src/exports
@@ -13,6 +13,8 @@
     __tgt_target_data_update_nowait;
     __tgt_target_nowait;
     __tgt_target_teams_nowait;
+    __tgt_mapper_num_components;
+    __tgt_push_mapper_component;
     omp_get_num_devices;
     omp_get_initial_device;
     omp_target_alloc;
Index: libomptarget/src/interface.cpp
===================================================================
--- libomptarget/src/interface.cpp
+++ libomptarget/src/interface.cpp
@@ -304,6 +304,28 @@
                             arg_sizes, arg_types, team_num, thread_limit);
 }
 
+// Get the current number of components for a user-defined mapper.
+EXTERN int64_t __tgt_mapper_num_components(void *rt_mapper_handle) {
+  auto *MapperComponentsPtr = (struct MapperComponentsTy *)rt_mapper_handle;
+  int64_t size = MapperComponentsPtr->Components.size();
+  DP("__tgt_mapper_num_components(Handle=" DPxMOD ") returns %" PRId64 "\n",
+     DPxPTR(rt_mapper_handle), size);
+  return size;
+}
+
+// Push back one component for a user-defined mapper.
+EXTERN void __tgt_push_mapper_component(void *rt_mapper_handle, void *base,
+                                        void *begin, int64_t size,
+                                        int64_t type) {
+  DP("__tgt_push_mapper_component(Handle=" DPxMOD
+     ") adds an entry (Base=" DPxMOD ", Begin=" DPxMOD ", Size=%" PRId64
+     ", Type=0x%" PRIx64 ").\n",
+     DPxPTR(rt_mapper_handle), DPxPTR(base), DPxPTR(begin), size, type);
+  auto *MapperComponentsPtr = (struct MapperComponentsTy *)rt_mapper_handle;
+  MapperComponentsPtr->Components.push_back(
+      MapComponentInfoTy(base, begin, size, type));
+}
+
 EXTERN void __kmpc_push_target_tripcount(int64_t device_id,
     uint64_t loop_tripcount) {
   if (IsOffloadDisabled())
Index: libomptarget/src/private.h
===================================================================
--- libomptarget/src/private.h
+++ libomptarget/src/private.h
@@ -41,6 +41,24 @@
 typedef enum kmp_target_offload_kind kmp_target_offload_kind_t;
 extern kmp_target_offload_kind_t TargetOffloadPolicy;
 
+// This structure stores information of a mapped memory region.
+struct MapComponentInfoTy {
+  void *Base;
+  void *Begin;
+  int64_t Size;
+  int64_t Type;
+  MapComponentInfoTy() = default;
+  MapComponentInfoTy(void *Base, void *Begin, int64_t Size, int64_t Type)
+      : Base(Base), Begin(Begin), Size(Size), Type(Type) {}
+};
+
+// This structure stores all components of a user-defined mapper. The number of
+// components are dynamically decided, so we utilize C++ STL vector
+// implementation here.
+struct MapperComponentsTy {
+  std::vector<MapComponentInfoTy> Components;
+};
+
 ////////////////////////////////////////////////////////////////////////////////
 // implemtation for fatal messages
 ////////////////////////////////////////////////////////////////////////////////
Index: libomptarget/test/mapping/declare_mapper_api.cpp
===================================================================
--- /dev/null
+++ libomptarget/test/mapping/declare_mapper_api.cpp
@@ -0,0 +1,46 @@
+// RUN: %libomptarget-compile-run-and-check-aarch64-unknown-linux-gnu
+// RUN: %libomptarget-compile-run-and-check-powerpc64-ibm-linux-gnu
+// RUN: %libomptarget-compile-run-and-check-powerpc64le-ibm-linux-gnu
+// RUN: %libomptarget-compile-run-and-check-x86_64-pc-linux-gnu
+
+#include <cstdio>
+#include <cstdlib>
+
+// Data structure definitions copied from OpenMP RTL.
+struct MapComponentInfoTy {
+  void *Base;
+  void *Begin;
+  int64_t Size;
+  int64_t Type;
+  MapComponentInfoTy() = default;
+  MapComponentInfoTy(void *Base, void *Begin, int64_t Size, int64_t Type)
+      : Base(Base), Begin(Begin), Size(Size), Type(Type) {}
+};
+
+struct MapperComponentsTy {
+  std::vector<MapComponentInfoTy> Components;
+};
+
+// OpenMP RTL interfaces
+#ifdef __cplusplus
+extern "C" {
+#endif
+int64_t __tgt_mapper_num_components(void *rt_mapper_handle);
+void __tgt_push_mapper_component(void *rt_mapper_handle, void *base,
+                                 void *begin, int64_t size, int64_t type);
+#ifdef __cplusplus
+}
+#endif
+
+int main(int argc, char *argv[]) {
+  MapperComponentsTy MC;
+  void *base, *begin;
+  int64_t size, type;
+  // Push 2 elements into MC.
+  __tgt_push_mapper_component((void *)&MC, base, begin, size, type);
+  __tgt_push_mapper_component((void *)&MC, base, begin, size, type);
+  int64_t num = __tgt_mapper_num_components((void *)&MC);
+  // CHECK: num=2
+  printf("num=%lld\n", num);
+  return 0;
+}