diff --git a/src/basic/login-util.h b/src/basic/login-util.h
index 4d0c69208b..f9ca34152b 100644
--- a/src/basic/login-util.h
+++ b/src/basic/login-util.h
@@ -4,6 +4,8 @@
 #include <stdbool.h>
 #include <unistd.h>
 
+#include "macro.h"
+
 #define SD_LOGIND_ROOT_CHECK_INHIBITORS           (UINT64_C(1) << 0)
 #define SD_LOGIND_REBOOT_VIA_KEXEC                (UINT64_C(1) << 1)
 #define SD_LOGIND_SOFT_REBOOT                     (UINT64_C(1) << 2)
@@ -16,7 +18,7 @@
 #define SD_LOGIND_SHUTDOWN_AND_SLEEP_FLAGS_PUBLIC (SD_LOGIND_ROOT_CHECK_INHIBITORS|SD_LOGIND_REBOOT_VIA_KEXEC|SD_LOGIND_SOFT_REBOOT|SD_LOGIND_SOFT_REBOOT_IF_NEXTROOT_SET_UP|SD_LOGIND_SKIP_INHIBITORS)
 #define SD_LOGIND_SHUTDOWN_AND_SLEEP_FLAGS_ALL    (SD_LOGIND_SHUTDOWN_AND_SLEEP_FLAGS_PUBLIC|SD_LOGIND_INTERACTIVE)
 
-bool session_id_valid(const char *id);
+bool session_id_valid(const char *id) _pure_;
 
 static inline bool logind_running(void) {
         return access("/run/systemd/seats/", F_OK) >= 0;