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new file mode 100644
index 0000000..f535598
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+version = "0.1.0"
+edition = "2021"
+authors = ["Keuin <[email protected]>"]
+
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+
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+
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diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..f288702
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The GNU General Public License is a free, copyleft license for
+software and other kinds of works.
+
+  The licenses for most software and other practical works are designed
+to take away your freedom to share and change the works.  By contrast,
+the GNU General Public License is intended to guarantee your freedom to
+share and change all versions of a program--to make sure it remains free
+software for all its users.  We, the Free Software Foundation, use the
+GNU General Public License for most of our software; it applies also to
+any other work released this way by its authors.  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+them if you wish), that you receive source code or can get it if you
+want it, that you can change the software or use pieces of it in new
+free programs, and that you know you can do these things.
+
+  To protect your rights, we need to prevent others from denying you
+these rights or asking you to surrender the rights.  Therefore, you have
+certain responsibilities if you distribute copies of the software, or if
+you modify it: responsibilities to respect the freedom of others.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must pass on to the recipients the same
+freedoms that you received.  You must make sure that they, too, receive
+or can get the source code.  And you must show them these terms so they
+know their rights.
+
+  Developers that use the GNU GPL protect your rights with two steps:
+(1) assert copyright on the software, and (2) offer you this License
+giving you legal permission to copy, distribute and/or modify it.
+
+  For the developers' and authors' protection, the GPL clearly explains
+that there is no warranty for this free software.  For both users' and
+authors' sake, the GPL requires that modified versions be marked as
+changed, so that their problems will not be attributed erroneously to
+authors of previous versions.
+
+  Some devices are designed to deny users access to install or run
+modified versions of the software inside them, although the manufacturer
+can do so.  This is fundamentally incompatible with the aim of
+protecting users' freedom to change the software.  The systematic
+pattern of such abuse occurs in the area of products for individuals to
+use, which is precisely where it is most unacceptable.  Therefore, we
+have designed this version of the GPL to prohibit the practice for those
+products.  If such problems arise substantially in other domains, we
+stand ready to extend this provision to those domains in future versions
+of the GPL, as needed to protect the freedom of users.
+
+  Finally, every program is threatened constantly by software patents.
+States should not allow patents to restrict development and use of
+software on general-purpose computers, but in those that do, we wish to
+avoid the special danger that patents applied to a free program could
+make it effectively proprietary.  To prevent this, the GPL assures that
+patents cannot be used to render the program non-free.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+                       TERMS AND CONDITIONS
+
+  0. Definitions.
+
+  "This License" refers to version 3 of the GNU General Public License.
+
+  "Copyright" also means copyright-like laws that apply to other kinds of
+works, such as semiconductor masks.
+
+  "The Program" refers to any copyrightable work licensed under this
+License.  Each licensee is addressed as "you".  "Licensees" and
+"recipients" may be individuals or organizations.
+
+  To "modify" a work means to copy from or adapt all or part of the work
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+  "Additional permissions" are terms that supplement the terms of this
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+
+  Nothing in this License shall be construed as excluding or limiting
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+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
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+  13. Use with the GNU Affero General Public License.
+
+  Notwithstanding any other provision of this License, you have
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+  14. Revised Versions of this License.
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+Foundation.  If the Program does not specify a version number of the
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+  If the Program specifies that a proxy can decide which future
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+  Later license versions may give you additional or different
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+  15. Disclaimer of Warranty.
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+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
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+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
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+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
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+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
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+reviewing courts shall apply local law that most closely approximates
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+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
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+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
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+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
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+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
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+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
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+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
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+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..744e9b1
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,17 @@
+JAVAC = javac
+
+HEADER_DIR = src/header
+JAVA_SRC_DIR = src/java
+
+JAVA_SRC_FILE = $(JAVA_SRC_DIR)/FileCowCopier.java
+JNI_HEADER = $(HEADER_DIR)/com_keuin_kbackupfabric_util_cow_FileCowCopier.h
+
+all: $(JNI_HEADER)
+
+$(JNI_HEADER): $(JAVA_SRC_FILE)
+	$(JAVAC) -h $(HEADER_DIR) $(JAVA_SRC_FILE)
+
+clean:
+	rm -rf $(HEADER_DIR)
+
+.PHONY: all clean
diff --git a/src/java/FileCowCopier.java b/src/java/FileCowCopier.java
new file mode 100644
index 0000000..7295307
--- /dev/null
+++ b/src/java/FileCowCopier.java
@@ -0,0 +1,19 @@
+package com.keuin.kbackupfabric.util.cow;
+
+
+public final class FileCowCopier {
+
+    public static native void init();
+
+    public static native void copy(String dst, String src);
+
+    public static native String getVersion();
+
+    public static void main(String[] args) {
+        System.loadLibrary("kbackup_cow");
+        FileCowCopier.init();
+        System.out.println("kbackup-cow version: " + FileCowCopier.getVersion());
+        FileCowCopier.copy("b", "a");
+    }
+
+}
diff --git a/src/lib/lib.rs b/src/lib/lib.rs
new file mode 100644
index 0000000..3557335
--- /dev/null
+++ b/src/lib/lib.rs
@@ -0,0 +1,133 @@
+use crate::CopyError::{FileOpenError, IoctlError};
+use jni::descriptors::Desc;
+use jni::objects::{JClass, JString};
+use jni::strings::JNIString;
+use jni::sys::jstring;
+use jni::JNIEnv;
+use log::debug;
+use nix::libc::{ioctl, FICLONE};
+use std::fmt::{Formatter, Write};
+use std::fs::OpenOptions;
+use std::os::fd::AsRawFd;
+use std::{fmt, io};
+
+fn throw<S: Into<JNIString>>(env: &mut JNIEnv, msg: S) {
+    throw_new(env, "java/lang/RuntimeException", msg);
+}
+
+fn throw_new<'a, 'b, S: Into<JNIString>, T: Desc<'a, JClass<'b>>>(
+    env: &mut JNIEnv<'a>,
+    ex: T,
+    msg: S,
+) {
+    if let Err(why) = env.throw_new(ex, msg) {
+        env.fatal_error(format!("failed to throw new exception: {why}"));
+    }
+}
+
+#[no_mangle]
+pub extern "system" fn Java_com_keuin_kbackupfabric_util_cow_FileCowCopier_init<'local>(
+    _env: JNIEnv<'local>, _class: JClass<'local>) {
+    env_logger::init();
+    debug!("kbackup-cow init");
+}
+
+#[no_mangle]
+pub extern "system" fn Java_com_keuin_kbackupfabric_util_cow_FileCowCopier_getVersion<'local>(
+    env: JNIEnv<'local>,
+    _class: JClass<'local>,
+) -> jstring {
+    match env.new_string(option_env!("CARGO_PKG_VERSION").unwrap_or("unknown")) {
+        Ok(s) => s.into_raw(),
+        Err(why) => env.fatal_error(format!("failed to create String: {why}")),
+    }
+}
+
+#[no_mangle]
+pub extern "system" fn Java_com_keuin_kbackupfabric_util_cow_FileCowCopier_copy<'local>(
+    mut env: JNIEnv<'local>,
+    _class: JClass<'local>,
+    dst: JString<'local>,
+    src: JString<'local>,
+) {
+    unsafe {
+        let src = match env.get_string_unchecked(&src) {
+            Ok(s) => s,
+            Err(why) => {
+                throw_new(
+                    &mut env,
+                    "java/lang/IllegalArgumentException",
+                    format!("invalid src: {why}"),
+                );
+                return;
+            }
+        };
+        let dst = match env.get_string_unchecked(&dst) {
+            Ok(s) => s,
+            Err(why) => {
+                throw_new(
+                    &mut env,
+                    "java/lang/IllegalArgumentException",
+                    format!("invalid dst: {why}"),
+                );
+                return;
+            }
+        };
+        match copy(dst.into(), src.into()) {
+            Ok(_) => {}
+            Err(why) => {
+                throw_new(&mut env, "java/io/IOException", format!("{}", why));
+            }
+        };
+    }
+}
+
+enum CopyError {
+    IoctlError(io::Error),
+    FileOpenError(String, io::Error),
+}
+
+impl fmt::Display for CopyError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match &self {
+            IoctlError(why) => {
+                write!(f, "ioctl error: {why}")
+            }
+            FileOpenError(path, why) => {
+                write!(f, "error opening file `{path}`: {why}")
+            }
+        }
+    }
+}
+
+fn copy(dst: String, src: String) -> Result<(), CopyError> {
+    debug!("copy src: {src}, dst: {dst}");
+    let f_src = match OpenOptions::new().read(true).open(&src) {
+        Ok(f) => f,
+        Err(why) => {
+            return Err(FileOpenError(src.clone(), why));
+        }
+    };
+    let f_dst = match OpenOptions::new()
+        .read(true)
+        .write(true)
+        .create(true)
+        .open(&dst)
+    {
+        Ok(f) => f,
+        Err(why) => {
+            return Err(FileOpenError(dst.clone(), why));
+        }
+    };
+    unsafe {
+        let fd_dst = f_dst.as_raw_fd();
+        let fd_src = f_src.as_raw_fd();
+        debug!("fd_src: {fd_src}, fd_dst: {fd_dst}");
+        if ioctl(fd_dst, FICLONE, fd_src) != 0 {
+            let err = io::Error::last_os_error();
+            debug!("ioctl failed: {:?}", err);
+            return Err(IoctlError(err));
+        }
+    }
+    Ok(())
+}