diff --git a/flang/runtime/extensions.cpp b/flang/runtime/extensions.cpp
--- a/flang/runtime/extensions.cpp
+++ b/flang/runtime/extensions.cpp
@@ -57,7 +57,13 @@ void FORTRAN_PROCEDURE_NAME(getarg)(
 // CALL GETLOG(USRNAME)
 void FORTRAN_PROCEDURE_NAME(getlog)(std::byte *arg, std::int64_t length) {
 #if _REENTRANT || _POSIX_C_SOURCE >= 199506L
-  const int nameMaxLen{LOGIN_NAME_MAX + 1};
+#ifdef LOGIN_NAME_MAX
+  int nameMaxLen{LOGIN_NAME_MAX + 1};
+#else
+  int nameMaxLen = sysconf(_SC_LOGIN_NAME_MAX) + 1;
+  if (nameMaxLen == -1)
+    nameMaxLen = _POSIX_LOGIN_NAME_MAX + 1;
+#endif
   char str[nameMaxLen];
 
   int error{getlogin_r(str, nameMaxLen)};
diff --git a/flang/unittests/Runtime/CommandTest.cpp b/flang/unittests/Runtime/CommandTest.cpp
--- a/flang/unittests/Runtime/CommandTest.cpp
+++ b/flang/unittests/Runtime/CommandTest.cpp
@@ -527,7 +527,13 @@ TEST_F(EnvironmentVariables, GetlogGetNa
 #if _REENTRANT || _POSIX_C_SOURCE >= 199506L
 TEST_F(EnvironmentVariables, GetlogPadSpace) {
   // guarantee 1 char longer than max, last char should be pad space
-  const int charLen{LOGIN_NAME_MAX + 2};
+#ifdef LOGIN_NAME_MAX
+  int charLen{LOGIN_NAME_MAX + 2};
+#else
+  int charLen = sysconf(_SC_LOGIN_NAME_MAX) + 2;
+  if (charLen == -1)
+    charLen = _POSIX_LOGIN_NAME_MAX + 2;
+#endif
   char input[charLen];
 
   FORTRAN_PROCEDURE_NAME(getlog)