diff --git a/src/libimp/platform/posix/system.h b/src/libimp/platform/posix/system.h
index 404077a..887f28e 100644
--- a/src/libimp/platform/posix/system.h
+++ b/src/libimp/platform/posix/system.h
@@ -38,11 +38,11 @@ result<std::int64_t> conf(info r) noexcept {
   }
   default:
     log.error("invalid info = ", underlyof(r));
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   auto err = sys::error();
   log.error("info = ", underlyof(r), ", error = ", err);
-  return {false, err};
+  return err;
 }
 
 } // namespace sys
diff --git a/src/libimp/platform/win/system.h b/src/libimp/platform/win/system.h
index 19c2043..c1f01fc 100644
--- a/src/libimp/platform/win/system.h
+++ b/src/libimp/platform/win/system.h
@@ -19,7 +19,6 @@
 
 LIBIMP_NAMESPACE_BEG_
 namespace sys {
-namespace {
 
 /**
  * \brief Gets a text description of the system error
@@ -56,8 +55,6 @@ std::string error_string(DWORD code) noexcept {
   return {};
 }
 
-} // namespace
-
 /**
  * \brief Get the system error number.
  * \see https://en.cppreference.com/w/cpp/error/system_category
@@ -82,7 +79,7 @@ result<std::int64_t> conf(info r) noexcept {
   }
   default:
     log.error("invalid info = ", underlyof(r));
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
 }
 
diff --git a/src/libipc/platform/posix/shm_impl.h b/src/libipc/platform/posix/shm_impl.h
index 661027f..91b09bb 100644
--- a/src/libipc/platform/posix/shm_impl.h
+++ b/src/libipc/platform/posix/shm_impl.h
@@ -51,7 +51,7 @@ result<int> shm_open_fd(std::string const &name, mode::type type) noexcept {
   LIBIMP_LOG_();
   if (name.empty()) {
     log.error("name is empty.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
 
   /// \brief Open the object for read-write access.
@@ -70,7 +70,7 @@ result<int> shm_open_fd(std::string const &name, mode::type type) noexcept {
     break;
   default:
     log.error("mode type is invalid. type = ", type);
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
 
   /// \brief Create/Open POSIX shared memory bject
@@ -153,7 +153,9 @@ result<shm_t> shm_open(std::string name, std::size_t size, mode::type type) noex
 result<void> shm_close(shm_t h) noexcept {
   LIBIMP_LOG_();
   auto shm = valid(h);
-  if (shm == nullptr) return {};
+  if (shm == nullptr) {
+    return std::make_error_code(std::errc::invalid_argument);
+  }
   if (::munmap(shm->memp, shm->f_sz) == posix::failed) {
     auto err = sys::error();
     log.error("failed: munmap(", shm->memp, ", ", shm->f_sz, "). error = ", err);
diff --git a/src/libipc/platform/win/event_impl.h b/src/libipc/platform/win/event_impl.h
index 73ffbc8..9ca9c44 100644
--- a/src/libipc/platform/win/event_impl.h
+++ b/src/libipc/platform/win/event_impl.h
@@ -49,7 +49,7 @@ result<evt_t> evt_open(std::string name) noexcept {
   if (h == NULL) {
     auto err = sys::error();
     log.error("failed: CreateEvent(FALSE, FALSE, ", name, "). error = ", err);
-    return {nullptr, err};
+    return err;
   }
   return new evt_handle{std::move(name), h};
 }
@@ -99,7 +99,7 @@ result<bool> evt_wait(evt_t evt, std::int64_t ms) noexcept {
   LIBIMP_LOG_();
   if (!is_valid(evt)) {
     log.error("handle is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   DWORD dwMilliseconds = (ms < 0) ? INFINITE : static_cast<DWORD>(ms);
   auto r = ::WaitForSingleObject(evt->h_event, dwMilliseconds);
@@ -115,7 +115,7 @@ result<bool> evt_wait(evt_t evt, std::int64_t ms) noexcept {
   }
   auto err = sys::error();
   log.error("failed: WaitForSingleObject(", evt->h_event, ", ", dwMilliseconds, "). error = ", err);
-  return {false, err};
+  return err;
 }
 
 /**
@@ -126,14 +126,14 @@ result<bool> evt_wait(::LIBIMP::span<evt_t const> evts, std::int64_t ms) noexcep
   LIBIMP_LOG_();
   if (evts.empty()) {
     log.error("evts handle is empty.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   // Conversion of the event handle array to the windows handle array.
   std::vector<HANDLE> handles(evts.size());
   for (std::size_t i = 0; i < evts.size(); ++i) {
     if (!is_valid(evts[i])) {
       log.error("handle is null.");
-      return {};
+      return std::make_error_code(std::errc::invalid_argument);
     }
     handles[i] = evts[i]->h_event;
   }
@@ -152,7 +152,7 @@ result<bool> evt_wait(::LIBIMP::span<evt_t const> evts, std::int64_t ms) noexcep
   }
   auto err = sys::error();
   log.error("failed: WaitForMultipleObjects(", handles.size(), ", ", dwMilliseconds, "). error = ", err);
-  return {false, err};
+  return err;
 }
 
 LIBIPC_NAMESPACE_END_
diff --git a/src/libipc/platform/win/mmap_impl.h b/src/libipc/platform/win/mmap_impl.h
index e302a31..a28ef1b 100644
--- a/src/libipc/platform/win/mmap_impl.h
+++ b/src/libipc/platform/win/mmap_impl.h
@@ -37,7 +37,7 @@ result<void> mmap_close(HANDLE h) {
   LIBIMP_LOG_();
   if (h == NULL) {
     log.error("handle is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   if (!::CloseHandle(h)) {
     auto err = sys::error();
@@ -64,12 +64,12 @@ result<HANDLE> mmap_open(std::string const &file, std::size_t size, mode::type t
   LIBIMP_LOG_();
   if (file.empty()) {
     log.error("file name is empty.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   auto t_name = detail::to_tstring(file);
   if (t_name.empty()) {
     log.error("file name is empty. (TCHAR conversion failed)");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
 
   /// \brief Opens a named file mapping object.
@@ -78,7 +78,7 @@ result<HANDLE> mmap_open(std::string const &file, std::size_t size, mode::type t
     if (h == NULL) {
       auto err = sys::error();
       log.error("failed: OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, ", file, "). error = ", err);
-      return {nullptr, err};
+      return err;
     }
     return h;
   };
@@ -91,7 +91,7 @@ result<HANDLE> mmap_open(std::string const &file, std::size_t size, mode::type t
     if (h == NULL) {
       auto err = sys::error();
       log.error("failed: CreateFileMapping(PAGE_READWRITE | SEC_COMMIT, ", size, ", ", file, "). error = ", err);
-      return {nullptr, err};
+      return err;
     }
     return h;
   };
@@ -105,7 +105,7 @@ result<HANDLE> mmap_open(std::string const &file, std::size_t size, mode::type t
     return try_open();
   } else if (!(type & mode::create)) {
     log.error("mode type is invalid. type = ", type);
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   auto h = try_create();
   if (!h) return h;
@@ -114,7 +114,7 @@ result<HANDLE> mmap_open(std::string const &file, std::size_t size, mode::type t
   if ((type == mode::create) && (::GetLastError() == ERROR_ALREADY_EXISTS)) {
     log.info("the file being created already exists. file = ", file, ", type = ", type);
     mmap_close(*h);
-    return {};
+    return sys::error();
   }
   return h;
 }
@@ -127,13 +127,13 @@ result<LPVOID> mmap_memof(HANDLE h) {
   LIBIMP_LOG_();
   if (h == NULL) {
     log.error("handle is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   LPVOID mem = ::MapViewOfFile(h, FILE_MAP_ALL_ACCESS, 0, 0, 0);
   if (mem == NULL) {
     auto err = sys::error();
     log.error("failed: MapViewOfFile(", h, ", FILE_MAP_ALL_ACCESS). error = ", err);
-    return {nullptr, err};
+    return err;
   }
   return mem;
 }
@@ -146,13 +146,13 @@ result<SIZE_T> mmap_sizeof(LPCVOID mem) {
   LIBIMP_LOG_();
   if (mem == NULL) {
     log.error("memory pointer is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   MEMORY_BASIC_INFORMATION mem_info {};
   if (::VirtualQuery(mem, &mem_info, sizeof(mem_info)) == 0) {
     auto err = sys::error();
     log.error("failed: VirtualQuery(", mem, "). error = ", err);
-    return {false, err};
+    return err;
   }
   return mem_info.RegionSize;
 }
@@ -165,11 +165,11 @@ result<void> mmap_release(HANDLE h, LPCVOID mem) {
   LIBIMP_LOG_();
   if (h == NULL) {
     log.error("handle is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   if (mem == NULL) {
     log.error("memory pointer is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   if (!::UnmapViewOfFile(mem)) {
     log.warning("failed: UnmapViewOfFile. error = ", sys::error());
diff --git a/src/libipc/platform/win/shm_impl.h b/src/libipc/platform/win/shm_impl.h
index deec8f3..243df96 100644
--- a/src/libipc/platform/win/shm_impl.h
+++ b/src/libipc/platform/win/shm_impl.h
@@ -22,19 +22,19 @@ result<shm_t> shm_open(std::string name, std::size_t size, mode::type type) noex
   auto h = mmap_open(name, size, type);
   if (*h == NULL) {
     log.error("failed: mmap_open(name = ", name, ", size = ", size, ", type = ", type, ").");
-    return {nullptr, h.error()};
+    return h.error();
   }
   auto mem = mmap_memof(*h);
   if (*mem == NULL) {
     log.error("failed: mmap_memof(", *h, ").");
     mmap_close(*h);
-    return {nullptr, mem.error()};
+    return mem.error();
   }
   auto sz = mmap_sizeof(*mem);
   if (!sz) {
     log.error("failed: mmap_sizeof(", *mem, ").");
     mmap_close(*h);
-    return {nullptr, sz.error()};
+    return sz.error();
   }
   return new shm_handle{std::move(name), *sz, *mem, *h};
 }
@@ -43,7 +43,7 @@ result<void> shm_close(shm_t h) noexcept {
   LIBIMP_LOG_();
   if (h == nullptr) {
     log.error("shm handle is null.");
-    return {};
+    return std::make_error_code(std::errc::invalid_argument);
   }
   auto shm = static_cast<shm_handle *>(h);
   auto ret = mmap_release(shm->h_fmap, shm->memp);