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Funchook - an API hook library


This library depends on one of the following disassemblers.

On x86_64 and x86

  • diStorm3 (default)
  • zydis (when -DFUNCHOOK_DISASM=zydis is passed to the cmake command)
  • capstone (when -DFUNCHOOK_DISASM=capstone is passed to the cmake command)

On arm64


  • write documents.


2.0.0 (20XX-XX-XX)

  • Add funchook_prepare_with_params() to support prehook.
  • Add funchook_get_arg() to get arguments in prehook.

1.1.3 (2023-06-04)

  • Fix build for Windows with raw MSC (#45)
  • Fix debug log format warnings by mingw-w64 compiler.

1.1.2 (2023-03-12)

  • Experimental support for Windows arm64

1.1.1 (2022-10-02)

  • More permissive check for page allocation mmap (#25)
  • Flush instruction cache for arm64. It does nothing for intel CPU.
  • Disassember engine
    • Upgrade capstone to 4.0.2
    • Upgrade distorm to 3.5.2
  • CMake
    • Allow user to specify FUNCHOOK_CPU explicitly (#19)
    • Avoid polluting global include and link dirs (#20)
    • Use target based compile options for gcc's -Wall (#21)
    • Use ExternalProject_add to download captone only (#30)
    • Add option FUNCHOOK_INSTALL (#31)
    • Use "FUNCHOOK_CPU MATCHES " (#32)
  • Documentation
    • added example usage from python (#22)
  • Fix tests on Android (#29)

1.1.0 (2020-03-22)

  • Arm64 Linux support. capstone is used as the disassembler library on arm64.
  • Options to use zydis and capstone as a disassembler library on x86_64 and x86.
  • extern "C" was added in funchook.h for C++. (#15)
  • Libc-compatible functions were removed to simplify code.

1.0.0 (2020-01-19)

  • diStorm3 is used as the disassembler library.
  • Libc-compatible functions were implemented on Linux in order not to hook function calls issued by funchook itself.

Supported Platforms

  • Linux x86_64
  • Linux x86
  • Linux arm64 (since 1.1.0)
  • macOS x86_64 (Functions in executables cannot be hooked when Xcode version >= 11.0. (*1))
  • Windows x64 (except C-runtime functions under Wine)
  • Windows 32-bit

*1 mprotect fails with EACCES.

Tested Platforms

Tested on some versions. Not tested in CI.

  • Windows arm64 (version 1.1.2)

Unsupported Platforms

  • macOS arm64 (*1)
  • x64/x86 emulation for Windows arm64

*1 I received a mail that mprotect failed with EINVAL. This issue may be same with #51 and fixed by this commit.

Compilation and installation


$ git clone --recursive
$ mkdir build
$ cd build
$ cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/path/to/install/directory ../funchook
$ make
$ make install
  • Available CMAKE_BUILD_TYPE values are empty(default), Debug, Release, RelWithDebInfo(release build with debug information) and MinSizeRel.

  • When CMAKE_INSTALL_PREFIX isn't set, funchook is installed at /usr/local.

    installed files:

    • ${CMAKE_INSTALL_PREFIX}/include/funchook.h (header file)
    • ${CMAKE_INSTALL_PREFIX}/lib/ (symbolic link to
    • ${CMAKE_INSTALL_PREFIX}/lib/ (soname; symbolic link to
    • ${CMAKE_INSTALL_PREFIX}/lib/ (shared library)
    • ${CMAKE_INSTALL_PREFIX}/lib/libfunchook.a (static library)


Here is an example to compile funchook with Visual Studio 2017 Win64. Change the argument of -G to use other compilers.

$ git clone --recursive
$ mkdir build
$ cd build
$ cmake -G "Visual Studio 15 2017 Win64" -DCMAKE_INSTALL_PREFIX=c:\path\to\install\directory ..\funchook
$ cmake --build . --config Release --target INSTALL
  • Available -G arguments (generators) are listed in the output of cmake --help.

  • Available --config arguments are Debug(default), Release, RelWithDebInfo and MinSizeRel.

  • When CMAKE_INSTALL_PREFIX isn't set, funchook is installed at c:\Program Files\funchook.

    installed files:

    • ${CMAKE_INSTALL_PREFIX}\include\funchook.h (header file)
    • ${CMAKE_INSTALL_PREFIX}\bin\funchook.dll (shared library)
    • ${CMAKE_INSTALL_PREFIX}\bin\funchook.pdb (debug file for funchook.dll when --config is Debug or RelWithDebInfo)
    • ${CMAKE_INSTALL_PREFIX}\lib\funchook.lib (static library)
    • ${CMAKE_INSTALL_PREFIX}\lib\funchook_dll.lib (import library for funchook.dll)


static ssize_t (*send_func)(int sockfd, const void *buf, size_t len, int flags);
static ssize_t (*recv_func)(int sockfd, void *buf, size_t len, int flags);

static ssize_t send_hook(int sockfd, const void *buf, size_t len, int flags)
    ssize_t rv;

    ... do your task: logging, etc. ...
    rv = send_func(sockfd, buf, len, flags); /* call the original send(). */
    ... do your task: logging, checking the return value, etc. ...
    return rv;

static ssize_t recv_hook(int sockfd, void *buf, size_t len, int flags)
    ssize_t rv;

    ... do your task: logging, etc. ...
    rv = recv_func(sockfd, buf, len, flags); /* call the original recv(). */
    ... do your task: logging, checking received data, etc. ...
    return rv;

int install_hooks()
    funchook_t *funchook = funchook_create();
    int rv;

    /* Prepare hooking.
     * The return value is used to call the original send function
     * in send_hook.
    send_func = send;
    rv = funchook_prepare(funchook, (void**)&send_func, send_hook);
    if (rv != 0) {
       /* error */

    /* ditto */
    recv_func = recv;
    rv = funchook_prepare(funchook, (void**)&recv_func, recv_hook);
    if (rv != 0) {
       /* error */

    /* Install hooks.
     * The first 5-byte code of send() and recv() are changed respectively.
    rv = funchook_install(funchook, 0);
    if (rv != 0) {
       /* error */

Example - Using Python ctypes

# should work on python 2.7/3 windows/linux

# load funchook
import ctypes
fh_lib = ctypes.cdll.LoadLibrary('/path/to/funchook/dll/or/so')

# define signatures
funchook_create = fh_lib.funchook_create
funchook_create.restype = ctypes.c_void_p
funchook_create.argtypes = []

funchook_prepare = fh_lib.funchook_prepare
funchook_prepare.restype = ctypes.c_ssize_t
funchook_prepare.argtypes = [ctypes.c_void_p, ctypes.c_void_p, ctypes.c_void_p]

funchook_install = fh_lib.funchook_install
funchook_install.restype = ctypes.c_ssize_t
funchook_install.argtypes = [ctypes.c_void_p, ctypes.c_int]

PySys_WriteStdout = ctypes.pythonapi.PySys_WriteStdout
PySys_WriteStdout.restype = None

# must keep those references alive, or stuff will be GC'd and weird errors will occur
global orig_write, hook, orig_write_ptr

# create hook (this function will replace the original function)
hook_type = ctypes.PYFUNCTYPE(None, ctypes.c_char_p)
orig_write = None
def hook_impl(msg):
    print('about to write: ' + str(msg)) # do what we want
    orig_write(msg)                      # call the original function

hook = hook_type(hook_impl)

fh = funchook_create()
# create a pointer object with the function address
orig_write_ptr = ctypes.c_void_p(ctypes.c_void_p.from_address(ctypes.addressof(PySys_WriteStdout)).value)
# orig_write_ptr.value will get a ptr to the original PySys_WriteStdout and PySys_WriteStdout will now point to the hook
ret = funchook_prepare(fh, ctypes.addressof(orig_write_ptr), hook)
assert not ret, 'ret is ' + str(ret)
ret = funchook_install(fh, 0)
assert not ret, 'ret is ' + str(ret)
orig_write = hook_type.from_address(ctypes.addressof(orig_write_ptr))
PySys_WriteStdout(b'hi there\n')


GPLv2 or later with a GPL linking exception.

You can use funchook in any software. Though funchook is licensed under the GPL, it doesn't affect outside of funchook due to the linking exception. You have no need to open your souce code under the GPL except funchook itself.

If you modify funchook itself and release it, the modifed part must be open under the GPL with or without the linking exception because funchook itself is under the GPL.

diStorm3 and capstone are released under the 3-clause BSD license. zydis is released under the MIT license. They are compatible with the GPL.