diff --git a/vfs/shm_bsd.go b/vfs/shm_bsd.go
index 94e5274c..ed6b8970 100644
--- a/vfs/shm_bsd.go
+++ b/vfs/shm_bsd.go
@@ -20,11 +20,7 @@ import (
 // [EXCLUSIVE locking mode]: https://sqlite.org/pragma.html#pragma_locking_mode
 const SupportsSharedMemory = true
 
-const (
-	_SHM_NLOCK = 8
-	_SHM_BASE  = 120
-	_SHM_DMS   = _SHM_BASE + _SHM_NLOCK
-)
+const _SHM_NLOCK = 8
 
 func (f *vfsFile) SharedMemory() SharedMemory { return f.shm }
 
@@ -100,22 +96,19 @@ func (s *vfsShm) Close() error {
 	return err
 }
 
-func (s *vfsShm) shmOpen() error {
+func (s *vfsShm) shmOpen() (err error) {
 	if s.vfsShmFile != nil {
 		return nil
 	}
 
-	var flag int
-	if s.readOnly {
-		flag = unix.O_RDONLY
-	} else {
-		flag = unix.O_RDWR
-	}
+	// Open file read-write, as it will be shared.
 	f, err := os.OpenFile(s.path,
-		flag|unix.O_CREAT|unix.O_NOFOLLOW, 0666)
+		unix.O_RDWR|unix.O_CREAT|unix.O_NOFOLLOW, 0666)
 	if err != nil {
 		return _CANTOPEN
 	}
+	// Close if file if it's not nil.
+	defer func() { f.Close() }()
 
 	fi, err := f.Stat()
 	if err != nil {
@@ -125,19 +118,34 @@ func (s *vfsShm) shmOpen() error {
 	vfsShmFilesMtx.Lock()
 	defer vfsShmFilesMtx.Unlock()
 
+	// Find a shared file, increase the reference count.
 	for _, g := range vfsShmFiles {
 		if g != nil && os.SameFile(fi, g.info) {
-			f.Close()
 			g.refs++
 			s.vfsShmFile = g
 			return nil
 		}
 	}
+
+	// Lock and truncate the file, if not readonly.
+	if s.readOnly {
+		err = _READONLY_CANTINIT
+	} else {
+		if rc := osWriteLock(f, 0, 0, 0); rc != _OK {
+			return rc
+		}
+		if err := f.Truncate(0); err != nil {
+			return _IOERR_SHMOPEN
+		}
+	}
+
+	// Add the new shared file.
 	s.vfsShmFile = &vfsShmFile{
 		File: f,
 		info: fi,
 		refs: 1,
 	}
+	f = nil
 	add := true
 	for i, g := range vfsShmFiles {
 		if g == nil {
@@ -148,17 +156,7 @@ func (s *vfsShm) shmOpen() error {
 	if add {
 		vfsShmFiles = append(vfsShmFiles, s.vfsShmFile)
 	}
-
-	if s.readOnly {
-		return _READONLY_CANTINIT
-	}
-	if rc := osWriteLock(f, _SHM_DMS, 1, 0); rc != _OK {
-		return rc
-	}
-	if err := f.Truncate(0); err != nil {
-		return _IOERR_SHMOPEN
-	}
-	return nil
+	return err
 }
 
 func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (uint32, error) {