automerge/rust/automerge-c

Overview

automerge-c exposes a C API that can either be used directly or as the basis for other language bindings that have good support for calling C functions.

Installing

See the main README for instructions on getting your environment set up and then you can build the automerge-c library and install its constituent files within a root directory of your choosing (e.g. "/usr/local") like so:

cmake -E make_directory automerge-c/build
cmake -S automerge-c -B automerge-c/build 
cmake --build automerge-c/build
cmake --install automerge-c/build --prefix "/usr/local"

Installation is important because the name, location and structure of CMake's out-of-source build subdirectory is subject to change based on the platform and the release version; generated headers like automerge-c/config.h and automerge-c/utils/enum_string.h are only sure to be found within their installed locations.

It's not obvious because they are versioned but the Cargo.toml and cbindgen.toml configuration files are also generated in order to ensure that the project name, project version and library name that they contain match those specified within the top-level CMakeLists.txt file.

If you'd like to cross compile the library for different platforms you can do so using cross. For example:

• cross build --manifest-path rust/automerge-c/Cargo.toml -r --target aarch64-unknown-linux-gnu

This will output a shared library in the directory rust/target/aarch64-unknown-linux-gnu/release/.

You can replace aarch64-unknown-linux-gnu with any cross supported targets. The targets below are known to work, though other targets are expected to work too:

• x86_64-apple-darwin
• aarch64-apple-darwin
• x86_64-unknown-linux-gnu
• aarch64-unknown-linux-gnu

As a caveat, CMake generates the automerge.h header file in terms of the processor architecture of the computer on which it was built so, for example, don't use a header generated for a 64-bit processor if your target is a 32-bit processor.

Usage

You can build and view the C API's HTML reference documentation like so:

cmake -E make_directory automerge-c/build
cmake -S automerge-c -B automerge-c/build 
cmake --build automerge-c/build --target automerge_docs
firefox automerge-c/build/src/html/index.html

To get started quickly, look at the examples.

Almost all operations in automerge-c act on an Automerge document (AMdoc struct) which is structurally similar to a JSON document.

You can get a document by calling either AMcreate() or AMload(). Operations on a given document are not thread-safe so you must use a mutex or similar to avoid calling more than one function on the same one concurrently.

A C API function that could succeed or fail returns a result (AMresult struct) containing a status code (AMstatus enum) and either a sequence of at least one item (AMitem struct) or a read-only view onto a UTF-8 error message string (AMbyteSpan struct). An item contains up to three components: an index within its parent object (AMbyteSpan struct or size_t), a unique identifier (AMobjId struct) and a value. The result of a successful function call that doesn't produce any values will contain a single item that is void (AM_VAL_TYPE_VOID). A returned result must be passed to AMresultFree() once the item(s) or error message it contains is no longer needed in order to avoid a memory leak.

#include <stdio.h>
#include <stdlib.h>
#include <automerge-c/automerge.h>
#include <automerge-c/utils/string.h>

int main(int argc, char** argv) {
  AMresult *docResult = AMcreate(NULL);

  if (AMresultStatus(docResult) != AM_STATUS_OK) {
    char* const err_msg = AMstrdup(AMresultError(docResult), NULL);
    printf("failed to create doc: %s", err_msg);
    free(err_msg);
    goto cleanup;
  }

  AMdoc *doc;
  AMitemToDoc(AMresultItem(docResult), &doc);

  // useful code goes here!

cleanup:
  AMresultFree(docResult);
}

If you are writing an application in C, the AMstackItem(), AMstackItems() and AMstackResult() functions enable the lifetimes of anonymous results to be centrally managed and allow the same validation logic to be reused without relying upon the goto statement (see examples/quickstart.c).

If you are wrapping automerge-c in another language, particularly one that has a garbage collector, you can call the AMresultFree() function within a finalizer to ensure that memory is reclaimed when it is no longer needed.

Automerge documents consist of a mutable root which is always a map from string keys to values. A value can be one of the following types:

• A number of type double / int64_t / uint64_t
• An explicit true / false / null
• An immutable UTF-8 string (AMbyteSpan).
• An immutable array of arbitrary bytes (AMbyteSpan).
• A mutable map from string keys to values.
• A mutable list of values.
• A mutable UTF-8 string.

If you read from a location in the document with no value, an item with type AM_VAL_TYPE_VOID will be returned, but you cannot write such a value explicitly.

Under the hood, automerge references a mutable object by its object identifier where AM_ROOT signifies a document's root map object.

There are functions to put each type of value into either a map or a list, and functions to read the current or a historical value from a map or a list. As (in general) collaborators may edit the document at any time, you cannot guarantee that the type of the value at a given part of the document will stay the same. As a result, reading from the document will return an AMitem struct that you can inspect to determine the type of value that it contains.

Strings in automerge-c are represented using an AMbyteSpan which contains a pointer and a length. Strings must be valid UTF-8 and may contain NUL (0) characters. For your convenience, you can call AMstr() to get the AMbyteSpan struct equivalent of a null-terminated byte string or AMstrdup() to get the representation of an AMbyteSpan struct as a null-terminated byte string wherein its NUL characters have been removed/replaced as you choose.

Putting all of that together, to read and write from the root of the document you can do this:

#include <stdio.h>
#include <stdlib.h>
#include <automerge-c/automerge.h>
#include <automerge-c/utils/string.h>

int main(int argc, char** argv) {
  // ...previous example...
  AMdoc *doc; 
  AMitemToDoc(AMresultItem(docResult), &doc);

  AMresult *putResult = AMmapPutStr(doc, AM_ROOT, AMstr("key"), AMstr("value"));
  if (AMresultStatus(putResult) != AM_STATUS_OK) {
    char* const err_msg = AMstrdup(AMresultError(putResult), NULL);
    printf("failed to put: %s", err_msg);
    free(err_msg);
    goto cleanup;
  }

  AMresult *getResult = AMmapGet(doc, AM_ROOT, AMstr("key"), NULL);
  if (AMresultStatus(getResult) != AM_STATUS_OK) {
    char* const err_msg = AMstrdup(AMresultError(putResult), NULL);
    printf("failed to get: %s", err_msg);
    free(err_msg);
    goto cleanup;
  }

  AMbyteSpan got;
  if (AMitemToStr(AMresultItem(getResult), &got)) {
    char* const c_str = AMstrdup(got, NULL);
    printf("Got %zu-character string \"%s\"", got.count, c_str);
    free(c_str);
  } else {
    printf("expected to read a string!");
    goto cleanup;
  }


cleanup:
  AMresultFree(getResult);
  AMresultFree(putResult);
  AMresultFree(docResult);
}

Functions that do not return an AMresult (for example AMitemKey()) do not allocate memory but rather reference memory that was previously allocated. It's therefore important to keep the original AMresult alive (in this case the one returned by AMmapRange()) until after you are finished with the items that it contains. However, the memory for an individual AMitem can be shared with a new AMresult by calling AMitemResult() on it. In other words, a select group of items can be filtered out of a collection and only each one's corresponding AMresult must be kept alive from that point forward; the originating collection's AMresult can be safely freed.

Beyond that, good luck!