diff --git a/mem/file.go b/mem/file.go
index 62fe4498..829c658b 100644
--- a/mem/file.go
+++ b/mem/file.go
@@ -201,33 +201,30 @@ func (f *File) ReadDir(n int) ([]fs.DirEntry, error) {
 	return di, nil
 }
 
-func (f *File) Read(b []byte) (n int, err error) {
+func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
 	f.fileData.Lock()
 	defer f.fileData.Unlock()
 	if f.closed {
 		return 0, ErrFileClosed
 	}
-	if len(b) > 0 && int(f.at) == len(f.fileData.data) {
+	if len(b) > 0 && int(off) == len(f.fileData.data) {
 		return 0, io.EOF
 	}
-	if int(f.at) > len(f.fileData.data) {
+	if int(off) > len(f.fileData.data) {
 		return 0, io.ErrUnexpectedEOF
 	}
-	if len(f.fileData.data)-int(f.at) >= len(b) {
+	if len(f.fileData.data)-int(off) >= len(b) {
 		n = len(b)
 	} else {
-		n = len(f.fileData.data) - int(f.at)
+		n = len(f.fileData.data) - int(off)
 	}
-	copy(b, f.fileData.data[f.at:f.at+int64(n)])
-	atomic.AddInt64(&f.at, int64(n))
+	copy(b, f.fileData.data[off:off+int64(n)])
 	return
 }
 
-func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
-	prev := atomic.LoadInt64(&f.at)
-	atomic.StoreInt64(&f.at, off)
-	n, err = f.Read(b)
-	atomic.StoreInt64(&f.at, prev)
+func (f *File) Read(b []byte) (n int, err error) {
+	n, err = f.ReadAt(b, atomic.LoadInt64(&f.at))
+	atomic.AddInt64(&f.at, int64(n))
 	return
 }
 
@@ -268,7 +265,7 @@ func (f *File) Seek(offset int64, whence int) (int64, error) {
 	return f.at, nil
 }
 
-func (f *File) Write(b []byte) (n int, err error) {
+func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 	if f.closed {
 		return 0, ErrFileClosed
 	}
@@ -276,30 +273,28 @@ func (f *File) Write(b []byte) (n int, err error) {
 		return 0, &os.PathError{Op: "write", Path: f.fileData.name, Err: errors.New("file handle is read only")}
 	}
 	n = len(b)
-	cur := atomic.LoadInt64(&f.at)
 	f.fileData.Lock()
 	defer f.fileData.Unlock()
-	diff := cur - int64(len(f.fileData.data))
+	diff := off - int64(len(f.fileData.data))
 	var tail []byte
-	if n+int(cur) < len(f.fileData.data) {
-		tail = f.fileData.data[n+int(cur):]
+	if n+int(off) < len(f.fileData.data) {
+		tail = f.fileData.data[n+int(off):]
 	}
 	if diff > 0 {
 		f.fileData.data = append(f.fileData.data, append(bytes.Repeat([]byte{0o0}, int(diff)), b...)...)
 		f.fileData.data = append(f.fileData.data, tail...)
 	} else {
-		f.fileData.data = append(f.fileData.data[:cur], b...)
+		f.fileData.data = append(f.fileData.data[:off], b...)
 		f.fileData.data = append(f.fileData.data, tail...)
 	}
 	setModTime(f.fileData, time.Now())
-
-	atomic.AddInt64(&f.at, int64(n))
 	return
 }
 
-func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
-	atomic.StoreInt64(&f.at, off)
-	return f.Write(b)
+func (f *File) Write(b []byte) (n int, err error) {
+	n, err = f.WriteAt(b, atomic.LoadInt64(&f.at))
+	atomic.AddInt64(&f.at, int64(n))
+	return
 }
 
 func (f *File) WriteString(s string) (ret int, err error) {