rclone/librclone

librclone

This directory contains code to build rclone as a C library and the shims for accessing rclone from C and other languages.

Note for the moment, the interfaces defined here are experimental and may change in the future. Eventually they will stabilise and this notice will be removed.

C

The shims are a thin wrapper over the rclone RPC.

The implementation is based on cgo; to build it you need Go and a GCC compatible C compiler (GCC or Clang). On Windows you can use the MinGW port of GCC, installing it via the MSYS2 distribution is recommended (make sure you install GCC in the classic mingw64 subsystem, the ucrt64 version is not compatible with cgo).

Build a shared library like this:

go build --buildmode=c-shared -o librclone.so github.com/rclone/rclone/librclone

Build a static library like this:

go build --buildmode=c-archive -o librclone.a github.com/rclone/rclone/librclone

Both the above commands will also generate librclone.h which should be #included in C programs wishing to use the library (with some exceptions).

The library will depend on libdl and libpthread on Linux/macOS, unless linking with a C standard library where their functionality is integrated, which is the case for glibc version 2.34 and newer.

You may add arguments -ldflags -s to make the library file smaller. This will omit symbol table and debug information, reducing size by about 25% on Linux and 50% on Windows.

Documentation

For documentation see the Go documentation for:

• RcloneInitialize
• RcloneFinalize
• RcloneRPC
• RcloneFreeString

Linux C example

There is an example program ctest.c, with Makefile, in the ctest subdirectory. It can be built on Linux/macOS, but not Windows without changes - as described next.

Windows C/C++ guidelines

The official C example is targeting Linux/macOS, and will not work on Windows. It is very possible to use librclone from a C/C++ application on Windows, but there are some pitfalls that you can avoid by following these guidelines:

• Build librclone as shared library, and use run-time dynamic linking (see linking).
• Do not try to unload the library with FreeLibrary (see unloading).
• Deallocate returned strings with API function RcloneFreeString (see memory management).
• Define struct RcloneRPCResult, instead of including librclone.h (see include file).
• Use UTF-8 encoded strings (see encoding).
• Properly escape JSON strings, beware of the native path separator (see escaping).

Linking

Use of different compilers, compiler versions, build configuration, and dependency on different C runtime libraries for a library and the application that references it, may easily break compatibility. When building the librclone library with MinGW GCC compiler (via go build command), if you link it into an application built with Visual C++ for example, there will be more than enough differences to cause problems.

Linking with static library requires most compatibility, and is less likely to work. Linking with shared library is therefore recommended. The library exposes a plain C interface, and by using run-time dynamic linking (by using Windows API functions LoadLibrary and GetProcAddress), you can make a boundary that ensures compatibility (and in any case, you will not have an import library). The only remaining concern is then memory allocations; you should make sure memory is deallocated in the same library where it was allocated, as explained below.

Unloading

Do not try to unload the library with FreeLibrary, when using run-time dynamic linking. The library includes Go-specific runtime components, with garbage collection and other background threads, which do not handle unloading. Trying to call FreeLibrary will crash the application. I.e. after you have loaded librclone.dll into your application it must stay loaded until your application exits.

Memory management

The output string returned from RcloneRPC is allocated within the librclone library, and caller is responsible for freeing the memory. Due to C runtime library differences, as mentioned above, it is not recommended to do this by calling free from the consuming application. You should instead use the API function RcloneFreeString, which will call free from within the librclone library, using the same runtime that allocated it in the first place.

Include file

Do not include librclone.h. It contains some plain C, golang/cgo and GCC specific type definitions that will not compile with all other compilers without adjustments, where Visual C++ is one notable example. When using run-time dynamic linking, you have no use of the extern declared functions either.

The interface of librclone is so simple, that all you need is to define the small struct RcloneRPCResult, from librclone.go:

struct RcloneRPCResult {
    char* Output;
    int	Status;
};

Encoding

The API uses plain C strings (type char*, called "narrow" strings), and rclone assumes content is UTF-8 encoded. On Linux systems this normally matches the standard string representation, and no special considerations must be made. On Windows it is more complex.

On Windows, narrow strings are traditionally used with native non-Unicode encoding, the so-called ANSI code page, while Unicode strings are instead represented with the alternative wchar_t* type, called "wide" strings, and encoded as UTF-16. This means, to correctly handle characters that are encoded differently in UTF-8, you will need to perform conversion at some level: Conversion between UTF-8 encoded narrow strings used by rclone, and either ANSI encoded narrow strings or wide UTF-16 encoded strings used in runtime function, Windows API, third party APIs, etc.

Escaping

The RPC method takes a string containing JSON. In addition to the normal escaping of strings constants in your C/C++ source code, the JSON needs its own escaping. This is not a Windows-specific issue, but there is the additional challenge that native filesystem path separator is the same as the escape character, and you may end up with strings like this:

const char* input = "{"
"\"fs\": \"C:\\\\Temp\","
"\"remote\": \"sub/folder\","
"\"opt\": \"{\\\"showHash\\\": true}\""
"}";

With C++11 you can use raw string literals to avoid the C++ escaping of string constants, leaving escaping only necessary for the contained JSON.

gomobile

The gomobile subdirectory contains the equivalent of the C binding but suitable for using with gomobile using something like this.

gomobile bind -v -target=android -javapkg=org.rclone github.com/rclone/rclone/librclone/gomobile

The command generates an Android library (aar) that can be imported into an Android application project. Librclone will be contained within libgojni.so and loaded automatically.

// imports
import org.rclone.gomobile.Gomobile;
import org.rclone.gomobile.RcloneRPCResult;

// initialize rclone
Gomobile.rcloneInitialize();

// call RC method and log response.
RcloneRPCResult response = Gomobile.rcloneRPC("core/version", "{}");
Log.i("rclone", "response status: " + response.getStatus());
Log.i("rclone", "output: " + response.getOutput());

// Clean up when finished.
Gomobile.rcloneFinalize();

This is a low level interface - serialization, job management etc must be built on top of it.

iOS has not been tested (but should probably work).

Further docs:

• gomobile main website
• gomobile wiki
• go issue #16876 where the feature was added
• gomobile design doc for extra details not in the docs.

python

The python subdirectory contains a simple Python wrapper for the C API using rclone linked as a shared library with ctypes.

You are welcome to use this directly.

This needs expanding and submitting to pypi...

Rust

Rust bindings are available in the librclone crate: https://crates.io/crates/librclone

TODO

• Async jobs must currently be cancelled manually at the moment - RcloneFinalize doesn't do it.
• This will use the rclone config system and rclone logging system.
• Need examples showing how to configure things,