diff --git a/src/filelock.c b/src/filelock.c
index d4a4747955a..43ede30975e 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -47,6 +47,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #ifdef WINDOWSNT
 #include <share.h>
 #include <sys/socket.h>	/* for fcntl */
+#include "w32common.h"
 #endif
 
 #ifndef MSDOS
@@ -353,7 +354,7 @@ integer_prefixed (char const *s)
 
 /* Whether the integer P could identify an individual process.  On most
    platforms this simply checks for positive pid_t, but on some
-   Microsoft ports our headers #define it to to some other test.  */
+   MS-Windows ports our headers #define it to to some other test.  */
 #ifndef VALID_PROCESS_ID
 # define VALID_PROCESS_ID(p) (0 < (p) && (p) <= TYPE_MAXIMUM (pid_t))
 #endif
diff --git a/src/w32common.h b/src/w32common.h
index 9475481b64e..8a5c7c03bca 100644
--- a/src/w32common.h
+++ b/src/w32common.h
@@ -53,6 +53,8 @@ extern void cache_system_info (void);
 #ifdef WINDOWSNT
 /* Return a static buffer with the MS-Windows version string.  */
 extern char * w32_version_string (void);
+extern bool w32_valid_process_id (intmax_t);
+# define VALID_PROCESS_ID(p) w32_valid_process_id(p)
 #endif
 
 typedef void (* VOIDFNPTR) (void);
diff --git a/src/w32proc.c b/src/w32proc.c
index 55ead13647b..4d237992b14 100644
--- a/src/w32proc.c
+++ b/src/w32proc.c
@@ -2676,6 +2676,19 @@ count_children:
   return nr;
 }
 
+/* This is called from filelock.c:current_lock_owner to validate a PID.
+   Return true if PID could identify a process on the current host,
+   false otherwise.  */
+bool
+w32_valid_process_id (intmax_t id)
+{
+  if (id == -1 || id == 0	/* always invalid */
+      || id > UINT32_MAX 	/* PID is actually a DWORD */
+      || id < INT32_MIN)	/* Windows 9X can report negative PIDs */
+    return false;
+  return true;
+}
+
 /* Substitute for certain kill () operations */
 
 static BOOL CALLBACK