diff --git a/config.go b/config.go
index 153fbd6..b390ffa 100644
--- a/config.go
+++ b/config.go
@@ -265,7 +265,7 @@ func traceCallback(ctx context.Context, mod api.Module, evt TraceEvent, pDB, pAr
 			}
 		}
 		if arg1 != nil {
-			_, rc = errorCode(c.trace(evt, arg1, arg2), ERROR)
+			_ = c.trace(evt, arg1, arg2)
 		}
 	}
 	return rc
diff --git a/context.go b/context.go
index 269bf52..f7a8cc5 100644
--- a/context.go
+++ b/context.go
@@ -198,14 +198,14 @@ func (ctx Context) ResultError(err error) {
 		return
 	}
 
-	msg, code := errorCode(err, _OK)
+	msg, code := errorCode(err, ERROR)
 	if msg != "" {
 		defer ctx.c.arena.mark()()
 		ptr := ctx.c.arena.string(msg)
 		ctx.c.call("sqlite3_result_error",
 			stk_t(ctx.handle), stk_t(ptr), stk_t(len(msg)))
 	}
-	if code != _OK {
+	if code != res_t(ERROR) {
 		ctx.c.call("sqlite3_result_error_code",
 			stk_t(ctx.handle), stk_t(code))
 	}
diff --git a/driver/driver_test.go b/driver/driver_test.go
index 1748fa7..863fcfc 100644
--- a/driver/driver_test.go
+++ b/driver/driver_test.go
@@ -44,8 +44,8 @@ func Test_Open_dir(t *testing.T) {
 	if err == nil {
 		t.Fatal("want error")
 	}
-	if !errors.Is(err, sqlite3.CANTOPEN) {
-		t.Errorf("got %v, want sqlite3.CANTOPEN", err)
+	if !errors.Is(err, sqlite3.CANTOPEN_ISDIR) {
+		t.Errorf("got %v, want sqlite3.CANTOPEN_ISDIR", err)
 	}
 }
 
diff --git a/error.go b/error.go
index 83b057d..3dfea7b 100644
--- a/error.go
+++ b/error.go
@@ -11,6 +11,7 @@ import (
 //
 // https://sqlite.org/c3ref/errcode.html
 type Error struct {
+	sys  error
 	msg  string
 	sql  string
 	code res_t
@@ -33,7 +34,7 @@ func (e *Error) ExtendedCode() ExtendedErrorCode {
 // Error implements the error interface.
 func (e *Error) Error() string {
 	var b strings.Builder
-	b.WriteString(util.ErrorCodeString(uint32(e.code)))
+	b.WriteString(util.ErrorCodeString(e.code))
 
 	if e.msg != "" {
 		b.WriteString(": ")
@@ -43,6 +44,14 @@ func (e *Error) Error() string {
 	return b.String()
 }
 
+// Unwrap returns the underlying operating system error
+// that caused the I/O error or failure to open a file.
+//
+// https://sqlite.org/c3ref/system_errno.html
+func (e *Error) Unwrap() error {
+	return e.sys
+}
+
 // Is tests whether this error matches a given [ErrorCode] or [ExtendedErrorCode].
 //
 // It makes it possible to do:
@@ -90,7 +99,16 @@ func (e *Error) SQL() string {
 
 // Error implements the error interface.
 func (e ErrorCode) Error() string {
-	return util.ErrorCodeString(uint32(e))
+	return util.ErrorCodeString(e)
+}
+
+// As converts this error to an [ExtendedErrorCode].
+func (e ErrorCode) As(err any) bool {
+	c, ok := err.(*xErrorCode)
+	if ok {
+		*c = xErrorCode(e)
+	}
+	return ok
 }
 
 // Temporary returns true for [BUSY] errors.
@@ -105,7 +123,7 @@ func (e ErrorCode) ExtendedCode() ExtendedErrorCode {
 
 // Error implements the error interface.
 func (e ExtendedErrorCode) Error() string {
-	return util.ErrorCodeString(uint32(e))
+	return util.ErrorCodeString(e)
 }
 
 // Is tests whether this error matches a given [ErrorCode].
@@ -150,14 +168,10 @@ func errorCode(err error, def ErrorCode) (msg string, code res_t) {
 		return code.msg, res_t(code.code)
 	}
 
-	var ecode ErrorCode
 	var xcode xErrorCode
-	switch {
-	case errors.As(err, &xcode):
+	if errors.As(err, &xcode) {
 		code = res_t(xcode)
-	case errors.As(err, &ecode):
-		code = res_t(ecode)
-	default:
+	} else {
 		code = res_t(def)
 	}
 	return err.Error(), code
diff --git a/internal/util/error.go b/internal/util/error.go
index 76769ed..d9bce5b 100644
--- a/internal/util/error.go
+++ b/internal/util/error.go
@@ -33,8 +33,12 @@ func AssertErr() ErrorString {
 	return ErrorString(msg)
 }
 
-func ErrorCodeString(rc uint32) string {
-	switch rc {
+type errorCode interface {
+	~uint8 | ~uint16 | ~uint32 | ~int32
+}
+
+func ErrorCodeString[T errorCode](rc T) string {
+	switch uint32(rc) {
 	case ABORT_ROLLBACK:
 		return "sqlite3: abort due to ROLLBACK"
 	case ROW:
@@ -42,7 +46,7 @@ func ErrorCodeString(rc uint32) string {
 	case DONE:
 		return "sqlite3: no more rows available"
 	}
-	switch rc & 0xff {
+	switch uint8(rc) {
 	case OK:
 		return "sqlite3: not an error"
 	case ERROR:
diff --git a/internal/util/module.go b/internal/util/module.go
index ba5754a..06b90bb 100644
--- a/internal/util/module.go
+++ b/internal/util/module.go
@@ -12,6 +12,7 @@ type ConnKey struct{}
 
 type moduleKey struct{}
 type moduleState struct {
+	sysError error
 	mmapState
 	handleState
 }
@@ -23,3 +24,15 @@ func NewContext(ctx context.Context) context.Context {
 	ctx = context.WithValue(ctx, moduleKey{}, state)
 	return ctx
 }
+
+func GetSystemError(ctx context.Context) error {
+	s := ctx.Value(moduleKey{}).(*moduleState)
+	return s.sysError
+}
+
+func SetSystemError(ctx context.Context, err error) {
+	s, ok := ctx.Value(moduleKey{}).(*moduleState)
+	if ok {
+		s.sysError = err
+	}
+}
diff --git a/sqlite.go b/sqlite.go
index fb64ac5..67f961c 100644
--- a/sqlite.go
+++ b/sqlite.go
@@ -125,14 +125,15 @@ func (sqlt *sqlite) error(rc res_t, handle ptr_t, sql ...string) error {
 		panic(util.OOMErr)
 	}
 
+	var msg, query string
 	if handle != 0 {
-		var msg, query string
 		if ptr := ptr_t(sqlt.call("sqlite3_errmsg", stk_t(handle))); ptr != 0 {
 			msg = util.ReadString(sqlt.mod, ptr, _MAX_LENGTH)
+			msg = strings.TrimPrefix(msg, "sqlite3: ")
+			msg = strings.TrimPrefix(msg, util.ErrorCodeString(rc)[len("sqlite3: "):])
+			msg = strings.TrimPrefix(msg, ": ")
 			if msg == "not an error" {
 				msg = ""
-			} else {
-				msg = strings.TrimPrefix(msg, util.ErrorCodeString(uint32(rc))[len("sqlite3: "):])
 			}
 		}
 
@@ -141,10 +142,16 @@ func (sqlt *sqlite) error(rc res_t, handle ptr_t, sql ...string) error {
 				query = sql[0][i:]
 			}
 		}
+	}
 
-		if msg != "" || query != "" {
-			return &Error{code: rc, msg: msg, sql: query}
-		}
+	var sys error
+	switch ErrorCode(rc) {
+	case CANTOPEN, IOERR:
+		sys = util.GetSystemError(sqlt.ctx)
+	}
+
+	if sys != nil || msg != "" || query != "" {
+		return &Error{code: rc, sys: sys, msg: msg, sql: query}
 	}
 	return xErrorCode(rc)
 }
diff --git a/tests/conn_test.go b/tests/conn_test.go
index 1271f2f..bacd80c 100644
--- a/tests/conn_test.go
+++ b/tests/conn_test.go
@@ -22,8 +22,8 @@ func TestConn_Open_dir(t *testing.T) {
 	if err == nil {
 		t.Fatal("want error")
 	}
-	if !errors.Is(err, sqlite3.CANTOPEN) {
-		t.Errorf("got %v, want sqlite3.CANTOPEN", err)
+	if !errors.Is(err, sqlite3.CANTOPEN_ISDIR) {
+		t.Errorf("got %v, want sqlite3.CANTOPEN_ISDIR", err)
 	}
 }
 
diff --git a/vfs/README.md b/vfs/README.md
index 68cb92d..44d5944 100644
--- a/vfs/README.md
+++ b/vfs/README.md
@@ -73,7 +73,7 @@ to check if your build supports shared memory.
 
 ### Blocking Locks
 
-On Windows and macOS, this package implements
+On macOS, this package implements
 [Wal-mode blocking locks](https://sqlite.org/src/doc/tip/doc/wal-lock.md).
 
 ### Batch-Atomic Write
diff --git a/vfs/api.go b/vfs/api.go
index a0d36b2..46a716e 100644
--- a/vfs/api.go
+++ b/vfs/api.go
@@ -193,8 +193,8 @@ type FileSharedMemory interface {
 // SharedMemory is a shared-memory WAL-index implementation.
 // Use [NewSharedMemory] to create a shared-memory.
 type SharedMemory interface {
-	shmMap(context.Context, api.Module, int32, int32, bool) (ptr_t, _ErrorCode)
-	shmLock(int32, int32, _ShmFlag) _ErrorCode
+	shmMap(context.Context, api.Module, int32, int32, bool) (ptr_t, error)
+	shmLock(int32, int32, _ShmFlag) error
 	shmUnmap(bool)
 	shmBarrier()
 	io.Closer
diff --git a/vfs/const.go b/vfs/const.go
index 9ed67e3..87e19d9 100644
--- a/vfs/const.go
+++ b/vfs/const.go
@@ -20,7 +20,7 @@ type (
 type _ErrorCode uint32
 
 func (e _ErrorCode) Error() string {
-	return util.ErrorCodeString(uint32(e))
+	return util.ErrorCodeString(e)
 }
 
 const (
diff --git a/vfs/file.go b/vfs/file.go
index bdebdf6..518f7dc 100644
--- a/vfs/file.go
+++ b/vfs/file.go
@@ -40,7 +40,7 @@ func evalSymlinks(path string) (string, error) {
 func (vfsOS) Delete(path string, syncDir bool) error {
 	err := os.Remove(path)
 	if errors.Is(err, fs.ErrNotExist) {
-		return _IOERR_DELETE_NOENT
+		return sysError{err, _IOERR_DELETE_NOENT}
 	}
 	if err != nil {
 		return err
@@ -53,7 +53,7 @@ func (vfsOS) Delete(path string, syncDir bool) error {
 		defer f.Close()
 		err = osSync(f, 0, SYNC_FULL)
 		if err != nil {
-			return _IOERR_DIR_FSYNC
+			return sysError{err, _IOERR_DIR_FSYNC}
 		}
 	}
 	return nil
@@ -108,14 +108,14 @@ func (vfsOS) OpenFilename(name *Filename, flags OpenFlag) (File, OpenFlag, error
 	}
 	if err != nil {
 		if name == nil {
-			return nil, flags, _IOERR_GETTEMPPATH
+			return nil, flags, sysError{err, _IOERR_GETTEMPPATH}
 		}
 		if errors.Is(err, syscall.EISDIR) {
-			return nil, flags, _CANTOPEN_ISDIR
+			return nil, flags, sysError{err, _CANTOPEN_ISDIR}
 		}
 		if isCreate && isJournl && errors.Is(err, fs.ErrPermission) &&
 			osAccess(name.String(), ACCESS_EXISTS) != nil {
-			return nil, flags, _READONLY_DIRECTORY
+			return nil, flags, sysError{err, _READONLY_DIRECTORY}
 		}
 		return nil, flags, err
 	}
@@ -123,7 +123,7 @@ func (vfsOS) OpenFilename(name *Filename, flags OpenFlag) (File, OpenFlag, error
 	if modeof := name.URIParameter("modeof"); modeof != "" {
 		if err = osSetMode(f, modeof); err != nil {
 			f.Close()
-			return nil, flags, _IOERR_FSTAT
+			return nil, flags, sysError{err, _IOERR_FSTAT}
 		}
 	}
 	if isUnix && flags&OPEN_DELETEONCLOSE != 0 {
@@ -193,7 +193,7 @@ func (f *vfsFile) Sync(flags SyncFlag) error {
 		defer d.Close()
 		err = osSync(f.File, f.flags, flags)
 		if err != nil {
-			return _IOERR_DIR_FSYNC
+			return sysError{err, _IOERR_DIR_FSYNC}
 		}
 	}
 	return nil
diff --git a/vfs/os_unix.go b/vfs/os_unix.go
index ec312cc..8aa07e4 100644
--- a/vfs/os_unix.go
+++ b/vfs/os_unix.go
@@ -33,7 +33,7 @@ func osReadAt(file *os.File, p []byte, off int64) (int, error) {
 			unix.ERANGE,
 			unix.EIO,
 			unix.ENXIO:
-			return n, _IOERR_CORRUPTFS
+			return n, sysError{err, _IOERR_CORRUPTFS}
 		}
 	}
 	return n, err
@@ -42,7 +42,7 @@ func osReadAt(file *os.File, p []byte, off int64) (int, error) {
 func osWriteAt(file *os.File, p []byte, off int64) (int, error) {
 	n, err := file.WriteAt(p, off)
 	if errno, ok := err.(unix.Errno); ok && errno == unix.ENOSPC {
-		return n, _FULL
+		return n, sysError{err, _FULL}
 	}
 	return n, err
 }
diff --git a/vfs/os_windows.go b/vfs/os_windows.go
index 0a6693d..19ad7a2 100644
--- a/vfs/os_windows.go
+++ b/vfs/os_windows.go
@@ -20,7 +20,7 @@ func osWriteAt(file *os.File, p []byte, off int64) (int, error) {
 		case
 			windows.ERROR_HANDLE_DISK_FULL,
 			windows.ERROR_DISK_FULL:
-			return n, _FULL
+			return n, sysError{err, _FULL}
 		}
 	}
 	return n, err
diff --git a/vfs/shm_bsd.go b/vfs/shm_bsd.go
index be1495d..4545517 100644
--- a/vfs/shm_bsd.go
+++ b/vfs/shm_bsd.go
@@ -141,7 +141,7 @@ func (s *vfsShm) shmOpen() (rc _ErrorCode) {
 	return _OK
 }
 
-func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, _ErrorCode) {
+func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, error) {
 	// Ensure size is a multiple of the OS page size.
 	if int(size)&(unix.Getpagesize()-1) != 0 {
 		return 0, _IOERR_SHMMAP
@@ -154,26 +154,30 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	// Check if file is big enough.
 	o, err := s.Seek(0, io.SeekEnd)
 	if err != nil {
-		return 0, _IOERR_SHMSIZE
+		return 0, sysError{err, _IOERR_SHMSIZE}
 	}
 	if n := (int64(id) + 1) * int64(size); n > o {
 		if !extend {
-			return 0, _OK
+			return 0, nil
 		}
-		if osAllocate(s.File, n) != nil {
-			return 0, _IOERR_SHMSIZE
+		if err := osAllocate(s.File, n); err != nil {
+			return 0, sysError{err, _IOERR_SHMSIZE}
 		}
 	}
 
 	r, err := util.MapRegion(ctx, mod, s.File, int64(id)*int64(size), size, false)
 	if err != nil {
-		return 0, _IOERR_SHMMAP
+		return 0, err
 	}
 	s.regions = append(s.regions, r)
-	return r.Ptr, _OK
+	return r.Ptr, nil
 }
 
-func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) _ErrorCode {
+func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) error {
+	if s.vfsShmParent == nil {
+		return _IOERR_SHMLOCK
+	}
+
 	s.Lock()
 	defer s.Unlock()
 
diff --git a/vfs/shm_copy.go b/vfs/shm_copy.go
index db8ddb4..4e20bcb 100644
--- a/vfs/shm_copy.go
+++ b/vfs/shm_copy.go
@@ -31,8 +31,8 @@ const (
 //
 // https://sqlite.org/walformat.html#the_wal_index_file_format
 
-func (s *vfsShm) shmAcquire(ptr *_ErrorCode) {
-	if ptr != nil && *ptr != _OK {
+func (s *vfsShm) shmAcquire(errp *error) {
+	if errp != nil && *errp != _OK {
 		return
 	}
 	if len(s.ptrs) == 0 || shmEqual(s.shadow[0][:], s.shared[0][:]) {
diff --git a/vfs/shm_dotlk.go b/vfs/shm_dotlk.go
index cb697a9..4dbfb3a 100644
--- a/vfs/shm_dotlk.go
+++ b/vfs/shm_dotlk.go
@@ -59,7 +59,7 @@ func (s *vfsShm) Close() error {
 	}
 
 	if err := dotlk.Unlock(s.path); err != nil {
-		return _IOERR_UNLOCK
+		return sysError{err, _IOERR_UNLOCK}
 	}
 	delete(vfsShmList, s.path)
 	s.vfsShmParent = nil
@@ -96,7 +96,7 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 	return _OK
 }
 
-func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, _ErrorCode) {
+func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, error) {
 	if size != _WALINDEX_PGSZ {
 		return 0, _IOERR_SHMMAP
 	}
@@ -116,7 +116,7 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	// Extend shared memory.
 	if int(id) >= len(s.shared) {
 		if !extend {
-			return 0, _OK
+			return 0, nil
 		}
 		s.shared = append(s.shared, make([][_WALINDEX_PGSZ]byte, int(id)-len(s.shared)+1)...)
 	}
@@ -140,16 +140,20 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	}
 
 	s.shadow[0][4] = 1
-	return s.ptrs[id], _OK
+	return s.ptrs[id], nil
 }
 
-func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) (rc _ErrorCode) {
+func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) (err error) {
+	if s.vfsShmParent == nil {
+		return _IOERR_SHMLOCK
+	}
+
 	s.Lock()
 	defer s.Unlock()
 
 	switch {
 	case flags&_SHM_LOCK != 0:
-		defer s.shmAcquire(&rc)
+		defer s.shmAcquire(&err)
 	case flags&_SHM_EXCLUSIVE != 0:
 		s.shmRelease()
 	}
diff --git a/vfs/shm_ofd.go b/vfs/shm_ofd.go
index b0f50fc..d7a3781 100644
--- a/vfs/shm_ofd.go
+++ b/vfs/shm_ofd.go
@@ -73,7 +73,7 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 	return rc
 }
 
-func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, _ErrorCode) {
+func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (ptr_t, error) {
 	// Ensure size is a multiple of the OS page size.
 	if int(size)&(unix.Getpagesize()-1) != 0 {
 		return 0, _IOERR_SHMMAP
@@ -86,29 +86,32 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	// Check if file is big enough.
 	o, err := s.Seek(0, io.SeekEnd)
 	if err != nil {
-		return 0, _IOERR_SHMSIZE
+		return 0, sysError{err, _IOERR_SHMSIZE}
 	}
 	if n := (int64(id) + 1) * int64(size); n > o {
 		if !extend {
-			return 0, _OK
+			return 0, nil
 		}
-		if s.readOnly || osAllocate(s.File, n) != nil {
+		if s.readOnly {
 			return 0, _IOERR_SHMSIZE
 		}
+		if err := osAllocate(s.File, n); err != nil {
+			return 0, sysError{err, _IOERR_SHMSIZE}
+		}
 	}
 
 	r, err := util.MapRegion(ctx, mod, s.File, int64(id)*int64(size), size, s.readOnly)
 	if err != nil {
-		return 0, _IOERR_SHMMAP
+		return 0, err
 	}
 	s.regions = append(s.regions, r)
 	if s.readOnly {
 		return r.Ptr, _READONLY
 	}
-	return r.Ptr, _OK
+	return r.Ptr, nil
 }
 
-func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) _ErrorCode {
+func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) error {
 	// Argument check.
 	switch {
 	case n <= 0:
@@ -129,6 +132,10 @@ func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) _ErrorCode {
 		panic(util.AssertErr())
 	}
 
+	if s.File == nil {
+		return _IOERR_SHMLOCK
+	}
+
 	var timeout time.Duration
 	if s.blocking {
 		timeout = time.Millisecond
diff --git a/vfs/shm_windows.go b/vfs/shm_windows.go
index 0be523a..bf54206 100644
--- a/vfs/shm_windows.go
+++ b/vfs/shm_windows.go
@@ -65,7 +65,7 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 	return rc
 }
 
-func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (_ ptr_t, rc _ErrorCode) {
+func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (_ ptr_t, err error) {
 	// Ensure size is a multiple of the OS page size.
 	if size != _WALINDEX_PGSZ || (windows.Getpagesize()-1)&_WALINDEX_PGSZ != 0 {
 		return 0, _IOERR_SHMMAP
@@ -79,19 +79,19 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 		return 0, rc
 	}
 
-	defer s.shmAcquire(&rc)
+	defer s.shmAcquire(&err)
 
 	// Check if file is big enough.
 	o, err := s.Seek(0, io.SeekEnd)
 	if err != nil {
-		return 0, _IOERR_SHMSIZE
+		return 0, sysError{err, _IOERR_SHMSIZE}
 	}
 	if n := (int64(id) + 1) * int64(size); n > o {
 		if !extend {
-			return 0, _OK
+			return 0, nil
 		}
-		if osAllocate(s.File, n) != nil {
-			return 0, _IOERR_SHMSIZE
+		if err := osAllocate(s.File, n); err != nil {
+			return 0, sysError{err, _IOERR_SHMSIZE}
 		}
 	}
 
@@ -99,7 +99,7 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	for int(id) >= len(s.shared) {
 		r, err := util.MapRegion(ctx, mod, s.File, int64(id)*int64(size), size)
 		if err != nil {
-			return 0, _IOERR_SHMMAP
+			return 0, err
 		}
 		s.regions = append(s.regions, r)
 		s.shared = append(s.shared, r.Data)
@@ -124,13 +124,17 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 	}
 
 	s.shadow[0][4] = 1
-	return s.ptrs[id], _OK
+	return s.ptrs[id], nil
 }
 
-func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) (rc _ErrorCode) {
+func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) (err error) {
+	if s.File == nil {
+		return _IOERR_SHMLOCK
+	}
+
 	switch {
 	case flags&_SHM_LOCK != 0:
-		defer s.shmAcquire(&rc)
+		defer s.shmAcquire(&err)
 	case flags&_SHM_EXCLUSIVE != 0:
 		s.shmRelease()
 	}
diff --git a/vfs/vfs.go b/vfs/vfs.go
index aef81c3..49ca25b 100644
--- a/vfs/vfs.go
+++ b/vfs/vfs.go
@@ -102,7 +102,7 @@ func vfsFullPathname(ctx context.Context, mod api.Module, pVfs, zRelative ptr_t,
 	}
 	util.WriteString(mod, zFull, path)
 
-	return vfsErrorCode(err, _CANTOPEN_FULLPATH)
+	return vfsErrorCode(ctx, err, _CANTOPEN_FULLPATH)
 }
 
 func vfsDelete(ctx context.Context, mod api.Module, pVfs, zPath ptr_t, syncDir int32) _ErrorCode {
@@ -110,7 +110,7 @@ func vfsDelete(ctx context.Context, mod api.Module, pVfs, zPath ptr_t, syncDir i
 	path := util.ReadString(mod, zPath, _MAX_PATHNAME)
 
 	err := vfs.Delete(path, syncDir != 0)
-	return vfsErrorCode(err, _IOERR_DELETE)
+	return vfsErrorCode(ctx, err, _IOERR_DELETE)
 }
 
 func vfsAccess(ctx context.Context, mod api.Module, pVfs, zPath ptr_t, flags AccessFlag, pResOut ptr_t) _ErrorCode {
@@ -119,7 +119,7 @@ func vfsAccess(ctx context.Context, mod api.Module, pVfs, zPath ptr_t, flags Acc
 
 	ok, err := vfs.Access(path, flags)
 	util.WriteBool(mod, pResOut, ok)
-	return vfsErrorCode(err, _IOERR_ACCESS)
+	return vfsErrorCode(ctx, err, _IOERR_ACCESS)
 }
 
 func vfsOpen(ctx context.Context, mod api.Module, pVfs, zPath, pFile ptr_t, flags OpenFlag, pOutFlags, pOutVFS ptr_t) _ErrorCode {
@@ -134,7 +134,7 @@ func vfsOpen(ctx context.Context, mod api.Module, pVfs, zPath, pFile ptr_t, flag
 		file, flags, err = vfs.Open(name.String(), flags)
 	}
 	if err != nil {
-		return vfsErrorCode(err, _CANTOPEN)
+		return vfsErrorCode(ctx, err, _CANTOPEN)
 	}
 
 	if file, ok := file.(FilePowersafeOverwrite); ok {
@@ -155,7 +155,7 @@ func vfsOpen(ctx context.Context, mod api.Module, pVfs, zPath, pFile ptr_t, flag
 
 func vfsClose(ctx context.Context, mod api.Module, pFile ptr_t) _ErrorCode {
 	err := vfsFileClose(ctx, mod, pFile)
-	return vfsErrorCode(err, _IOERR_CLOSE)
+	return vfsErrorCode(ctx, err, _IOERR_CLOSE)
 }
 
 func vfsRead(ctx context.Context, mod api.Module, pFile, zBuf ptr_t, iAmt int32, iOfst int64) _ErrorCode {
@@ -167,7 +167,7 @@ func vfsRead(ctx context.Context, mod api.Module, pFile, zBuf ptr_t, iAmt int32,
 		return _OK
 	}
 	if err != io.EOF {
-		return vfsErrorCode(err, _IOERR_READ)
+		return vfsErrorCode(ctx, err, _IOERR_READ)
 	}
 	clear(buf[n:])
 	return _IOERR_SHORT_READ
@@ -178,45 +178,45 @@ func vfsWrite(ctx context.Context, mod api.Module, pFile, zBuf ptr_t, iAmt int32
 	buf := util.View(mod, zBuf, int64(iAmt))
 
 	_, err := file.WriteAt(buf, iOfst)
-	return vfsErrorCode(err, _IOERR_WRITE)
+	return vfsErrorCode(ctx, err, _IOERR_WRITE)
 }
 
 func vfsTruncate(ctx context.Context, mod api.Module, pFile ptr_t, nByte int64) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	err := file.Truncate(nByte)
-	return vfsErrorCode(err, _IOERR_TRUNCATE)
+	return vfsErrorCode(ctx, err, _IOERR_TRUNCATE)
 }
 
 func vfsSync(ctx context.Context, mod api.Module, pFile ptr_t, flags SyncFlag) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	err := file.Sync(flags)
-	return vfsErrorCode(err, _IOERR_FSYNC)
+	return vfsErrorCode(ctx, err, _IOERR_FSYNC)
 }
 
 func vfsFileSize(ctx context.Context, mod api.Module, pFile, pSize ptr_t) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	size, err := file.Size()
 	util.Write64(mod, pSize, size)
-	return vfsErrorCode(err, _IOERR_SEEK)
+	return vfsErrorCode(ctx, err, _IOERR_SEEK)
 }
 
 func vfsLock(ctx context.Context, mod api.Module, pFile ptr_t, eLock LockLevel) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	err := file.Lock(eLock)
-	return vfsErrorCode(err, _IOERR_LOCK)
+	return vfsErrorCode(ctx, err, _IOERR_LOCK)
 }
 
 func vfsUnlock(ctx context.Context, mod api.Module, pFile ptr_t, eLock LockLevel) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	err := file.Unlock(eLock)
-	return vfsErrorCode(err, _IOERR_UNLOCK)
+	return vfsErrorCode(ctx, err, _IOERR_UNLOCK)
 }
 
 func vfsCheckReservedLock(ctx context.Context, mod api.Module, pFile, pResOut ptr_t) _ErrorCode {
 	file := vfsFileGet(ctx, mod, pFile).(File)
 	locked, err := file.CheckReservedLock()
 	util.WriteBool(mod, pResOut, locked)
-	return vfsErrorCode(err, _IOERR_CHECKRESERVEDLOCK)
+	return vfsErrorCode(ctx, err, _IOERR_CHECKRESERVEDLOCK)
 }
 
 func vfsFileControl(ctx context.Context, mod api.Module, pFile ptr_t, op _FcntlOpcode, pArg ptr_t) _ErrorCode {
@@ -268,20 +268,20 @@ func vfsFileControlImpl(ctx context.Context, mod api.Module, file File, op _Fcnt
 		if file, ok := file.(FileSizeHint); ok {
 			size := util.Read64[int64](mod, pArg)
 			err := file.SizeHint(size)
-			return vfsErrorCode(err, _IOERR_TRUNCATE)
+			return vfsErrorCode(ctx, err, _IOERR_TRUNCATE)
 		}
 
 	case _FCNTL_HAS_MOVED:
 		if file, ok := file.(FileHasMoved); ok {
 			moved, err := file.HasMoved()
 			util.WriteBool(mod, pArg, moved)
-			return vfsErrorCode(err, _IOERR_FSTAT)
+			return vfsErrorCode(ctx, err, _IOERR_FSTAT)
 		}
 
 	case _FCNTL_OVERWRITE:
 		if file, ok := file.(FileOverwrite); ok {
 			err := file.Overwrite()
-			return vfsErrorCode(err, _IOERR)
+			return vfsErrorCode(ctx, err, _IOERR)
 		}
 
 	case _FCNTL_SYNC:
@@ -291,29 +291,29 @@ func vfsFileControlImpl(ctx context.Context, mod api.Module, file File, op _Fcnt
 				name = util.ReadString(mod, pArg, _MAX_PATHNAME)
 			}
 			err := file.SyncSuper(name)
-			return vfsErrorCode(err, _IOERR)
+			return vfsErrorCode(ctx, err, _IOERR)
 		}
 
 	case _FCNTL_COMMIT_PHASETWO:
 		if file, ok := file.(FileCommitPhaseTwo); ok {
 			err := file.CommitPhaseTwo()
-			return vfsErrorCode(err, _IOERR)
+			return vfsErrorCode(ctx, err, _IOERR)
 		}
 
 	case _FCNTL_BEGIN_ATOMIC_WRITE:
 		if file, ok := file.(FileBatchAtomicWrite); ok {
 			err := file.BeginAtomicWrite()
-			return vfsErrorCode(err, _IOERR_BEGIN_ATOMIC)
+			return vfsErrorCode(ctx, err, _IOERR_BEGIN_ATOMIC)
 		}
 	case _FCNTL_COMMIT_ATOMIC_WRITE:
 		if file, ok := file.(FileBatchAtomicWrite); ok {
 			err := file.CommitAtomicWrite()
-			return vfsErrorCode(err, _IOERR_COMMIT_ATOMIC)
+			return vfsErrorCode(ctx, err, _IOERR_COMMIT_ATOMIC)
 		}
 	case _FCNTL_ROLLBACK_ATOMIC_WRITE:
 		if file, ok := file.(FileBatchAtomicWrite); ok {
 			err := file.RollbackAtomicWrite()
-			return vfsErrorCode(err, _IOERR_ROLLBACK_ATOMIC)
+			return vfsErrorCode(ctx, err, _IOERR_ROLLBACK_ATOMIC)
 		}
 
 	case _FCNTL_CKPT_START:
@@ -338,7 +338,7 @@ func vfsFileControlImpl(ctx context.Context, mod api.Module, file File, op _Fcnt
 
 			out, err := file.Pragma(strings.ToLower(name), value)
 
-			ret := vfsErrorCode(err, _ERROR)
+			ret := vfsErrorCode(ctx, err, _ERROR)
 			if ret == _ERROR {
 				out = err.Error()
 			}
@@ -407,14 +407,15 @@ func vfsShmBarrier(ctx context.Context, mod api.Module, pFile ptr_t) {
 
 func vfsShmMap(ctx context.Context, mod api.Module, pFile ptr_t, iRegion, szRegion, bExtend int32, pp ptr_t) _ErrorCode {
 	shm := vfsFileGet(ctx, mod, pFile).(FileSharedMemory).SharedMemory()
-	p, rc := shm.shmMap(ctx, mod, iRegion, szRegion, bExtend != 0)
+	p, err := shm.shmMap(ctx, mod, iRegion, szRegion, bExtend != 0)
 	util.Write32(mod, pp, p)
-	return rc
+	return vfsErrorCode(ctx, err, _IOERR_SHMMAP)
 }
 
 func vfsShmLock(ctx context.Context, mod api.Module, pFile ptr_t, offset, n int32, flags _ShmFlag) _ErrorCode {
 	shm := vfsFileGet(ctx, mod, pFile).(FileSharedMemory).SharedMemory()
-	return shm.shmLock(offset, n, flags)
+	err := shm.shmLock(offset, n, flags)
+	return vfsErrorCode(ctx, err, _IOERR_SHMLOCK)
 }
 
 func vfsShmUnmap(ctx context.Context, mod api.Module, pFile ptr_t, bDelete int32) _ErrorCode {
@@ -454,13 +455,36 @@ func vfsFileClose(ctx context.Context, mod api.Module, pFile ptr_t) error {
 	return util.DelHandle(ctx, id)
 }
 
-func vfsErrorCode(err error, def _ErrorCode) _ErrorCode {
-	if err == nil {
-		return _OK
+func vfsErrorCode(ctx context.Context, err error, code _ErrorCode) _ErrorCode {
+	var sys error
+
+	switch err := err.(type) {
+	case nil:
+		code = _OK
+	case _ErrorCode:
+		code = err
+	case sysError:
+		code = err.code
+		sys = err.error
+	default:
+		switch v := reflect.ValueOf(err); v.Kind() {
+		case reflect.Uint8, reflect.Uint16:
+			code = _ErrorCode(v.Uint())
+		}
 	}
-	switch v := reflect.ValueOf(err); v.Kind() {
-	case reflect.Uint8, reflect.Uint16, reflect.Uint32:
-		return _ErrorCode(v.Uint())
-	}
-	return def
+
+	util.SetSystemError(ctx, sys)
+	return code
+}
+
+func SystemError[T interface{ ~uint8 | ~uint16 }](err error, code T) error {
+	if err == nil {
+		return nil
+	}
+	return sysError{error: err, code: _ErrorCode(code)}
+}
+
+type sysError struct {
+	error
+	code _ErrorCode
 }
diff --git a/vtab.go b/vtab.go
index 16ff280..0725eae 100644
--- a/vtab.go
+++ b/vtab.go
@@ -465,19 +465,19 @@ func vtabModuleCallback(i vtabConstructor) func(_ context.Context, _ api.Module,
 			vtabPutHandle(ctx, mod, ppVTab, val[0].Interface())
 		}
 
-		return vtabError(ctx, mod, pzErr, _PTR_ERROR, err)
+		return vtabError(ctx, mod, pzErr, _PTR_ERROR, err, ERROR)
 	}
 }
 
 func vtabDisconnectCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	err := vtabDelHandle(ctx, mod, pVTab)
-	return vtabError(ctx, mod, 0, _PTR_ERROR, err)
+	return vtabError(ctx, mod, 0, _PTR_ERROR, err, ERROR)
 }
 
 func vtabDestroyCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabDestroyer)
 	err := errors.Join(vtab.Destroy(), vtabDelHandle(ctx, mod, pVTab))
-	return vtabError(ctx, mod, 0, _PTR_ERROR, err)
+	return vtabError(ctx, mod, 0, _PTR_ERROR, err, ERROR)
 }
 
 func vtabBestIndexCallback(ctx context.Context, mod api.Module, pVTab, pIdxInfo ptr_t) res_t {
@@ -490,7 +490,7 @@ func vtabBestIndexCallback(ctx context.Context, mod api.Module, pVTab, pIdxInfo
 	err := vtab.BestIndex(&info)
 
 	info.save()
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabUpdateCallback(ctx context.Context, mod api.Module, pVTab ptr_t, nArg int32, pArg, pRowID ptr_t) res_t {
@@ -504,13 +504,13 @@ func vtabUpdateCallback(ctx context.Context, mod api.Module, pVTab ptr_t, nArg i
 		util.Write64(mod, pRowID, rowID)
 	}
 
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabRenameCallback(ctx context.Context, mod api.Module, pVTab, zNew ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabRenamer)
 	err := vtab.Rename(util.ReadString(mod, zNew, _MAX_NAME))
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabFindFuncCallback(ctx context.Context, mod api.Module, pVTab ptr_t, nArg int32, zName, pxFunc ptr_t) int32 {
@@ -534,51 +534,49 @@ func vtabIntegrityCallback(ctx context.Context, mod api.Module, pVTab, zSchema,
 	err := vtab.Integrity(schema, table, int(mFlags))
 	// xIntegrity should return OK - even if it finds problems in the content of the virtual table.
 	// https://sqlite.org/vtab.html#xintegrity
-	vtabError(ctx, mod, pzErr, _PTR_ERROR, err)
-	_, code := errorCode(err, _OK)
-	return code
+	return vtabError(ctx, mod, pzErr, _PTR_ERROR, err, _OK)
 }
 
 func vtabBeginCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabTxn)
 	err := vtab.Begin()
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabSyncCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabTxn)
 	err := vtab.Sync()
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabCommitCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabTxn)
 	err := vtab.Commit()
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabRollbackCallback(ctx context.Context, mod api.Module, pVTab ptr_t) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabTxn)
 	err := vtab.Rollback()
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabSavepointCallback(ctx context.Context, mod api.Module, pVTab ptr_t, id int32) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabSavepointer)
 	err := vtab.Savepoint(int(id))
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabReleaseCallback(ctx context.Context, mod api.Module, pVTab ptr_t, id int32) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabSavepointer)
 	err := vtab.Release(int(id))
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func vtabRollbackToCallback(ctx context.Context, mod api.Module, pVTab ptr_t, id int32) res_t {
 	vtab := vtabGetHandle(ctx, mod, pVTab).(VTabSavepointer)
 	err := vtab.RollbackTo(int(id))
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func cursorOpenCallback(ctx context.Context, mod api.Module, pVTab, ppCur ptr_t) res_t {
@@ -589,12 +587,12 @@ func cursorOpenCallback(ctx context.Context, mod api.Module, pVTab, ppCur ptr_t)
 		vtabPutHandle(ctx, mod, ppCur, cursor)
 	}
 
-	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, pVTab, _VTAB_ERROR, err, ERROR)
 }
 
 func cursorCloseCallback(ctx context.Context, mod api.Module, pCur ptr_t) res_t {
 	err := vtabDelHandle(ctx, mod, pCur)
-	return vtabError(ctx, mod, 0, _VTAB_ERROR, err)
+	return vtabError(ctx, mod, 0, _PTR_ERROR, err, ERROR)
 }
 
 func cursorFilterCallback(ctx context.Context, mod api.Module, pCur ptr_t, idxNum int32, idxStr ptr_t, nArg int32, pArg ptr_t) res_t {
@@ -609,7 +607,7 @@ func cursorFilterCallback(ctx context.Context, mod api.Module, pCur ptr_t, idxNu
 
 	cursor := vtabGetHandle(ctx, mod, pCur).(VTabCursor)
 	err := cursor.Filter(int(idxNum), idxName, *args...)
-	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err)
+	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err, ERROR)
 }
 
 func cursorEOFCallback(ctx context.Context, mod api.Module, pCur ptr_t) int32 {
@@ -623,14 +621,14 @@ func cursorEOFCallback(ctx context.Context, mod api.Module, pCur ptr_t) int32 {
 func cursorNextCallback(ctx context.Context, mod api.Module, pCur ptr_t) res_t {
 	cursor := vtabGetHandle(ctx, mod, pCur).(VTabCursor)
 	err := cursor.Next()
-	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err)
+	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err, ERROR)
 }
 
 func cursorColumnCallback(ctx context.Context, mod api.Module, pCur, pCtx ptr_t, n int32) res_t {
 	cursor := vtabGetHandle(ctx, mod, pCur).(VTabCursor)
 	db := ctx.Value(connKey{}).(*Conn)
 	err := cursor.Column(Context{db, pCtx}, int(n))
-	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err)
+	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err, ERROR)
 }
 
 func cursorRowIDCallback(ctx context.Context, mod api.Module, pCur, pRowID ptr_t) res_t {
@@ -641,7 +639,7 @@ func cursorRowIDCallback(ctx context.Context, mod api.Module, pCur, pRowID ptr_t
 		util.Write64(mod, pRowID, rowID)
 	}
 
-	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err)
+	return vtabError(ctx, mod, pCur, _CURSOR_ERROR, err, ERROR)
 }
 
 const (
@@ -650,10 +648,10 @@ const (
 	_CURSOR_ERROR
 )
 
-func vtabError(ctx context.Context, mod api.Module, ptr ptr_t, kind uint32, err error) res_t {
+func vtabError(ctx context.Context, mod api.Module, ptr ptr_t, kind uint32, err error, def ErrorCode) res_t {
 	const zErrMsgOffset = 8
-	msg, code := errorCode(err, ERROR)
-	if msg != "" && ptr != 0 {
+	msg, code := errorCode(err, def)
+	if ptr != 0 && msg != "" {
 		switch kind {
 		case _VTAB_ERROR:
 			ptr = ptr + zErrMsgOffset // zErrMsg