.. _define_abi_compatibility:

Define package ABI compatibility
================================

Each package recipe can generate *N* binary packages from it, depending on three things:
``settings``, ``options`` and ``requires``.

When any of the :ref:`settings_property` of a package recipe changes, it will reference a
different binary:

.. code-block:: python

    class MyLibConanPackage(ConanFile):	
        name = "MyLib"
        version = "1.0"
        settings = "os", "arch", "compiler", "build_type"

When this package is installed by a *conanfile.txt*, another package *conanfile.py*, or directly:

.. code-block:: bash

    $ conan install MyLib/1.0@user/channel -s arch=x86_64 -s ...

The process will be:

1. Conan will get the user input settings and options. Those settings and options can come from command line, profiles or from the values
   cached in the latest :command:`conan install` execution.
2. Conan will retrieve the ``MyLib/1.0@user/channel`` recipe, read the ``settings`` attribute, and assign the necessary values.
3. With the current package values for ``settings`` (also ``options`` and ``requires``), it will compute a SHA1 hash that will be the binary
   package ID, e.g. ``c6d75a933080ca17eb7f076813e7fb21aaa740f2``.
4. Conan will try to find the ``c6d75...`` binary package. If it's present it will retrieve it, if not, it will fail and indicate that it
   can be built from sources using :command:`conan install --build`.

If the package is installed again with different settings, for example, for 32 bits architecture:

.. code-block:: bash

    $ conan install MyLib/1.0@user/channel -s arch=x86 -s ...

The process will be repeated but generating a different package ID, because the ``arch``
setting will have a different value. The same applies for different compilers, compiler versions, build type, etc., generating multiple
binaries, one for each configuration.

When users of the package define the same settings as one of those binaries that have been uploaded, the computed package ID will be the
same and such binary will be retrieved and reused without building it from sources.

The ``options`` behavior is very similar. The main difference is that options can be more easily defined at the package level and they can
be defaulted. Check the :ref:`conanfile_options` reference.

Note the simple scenario of a **header-only** library. Such package does not need to be built, and it will not have any ABI issues at all.
The recipe of such package will have to generate exactly 1 binary package, no more. This is easily achieved, just by not declaring
``settings`` nor ``options`` in the recipe:

.. code-block:: python

    class MyLibConanPackage(ConanFile): 
        name = "MyLib"
        version = "1.0"
        # no settings defined!

No matter what are the settings defined by the users, which compiler or version: the package settings and options will always be the same
(empty) and they will hash to the same binary package ID. That package will typically contain just the header files.

What happens if we have a library that we know it can be built with GCC 4.8 and we know it will keep the ABI compatibility with GCC 4.9?
(This kind of compatibility is easier to achieve for example for pure C libraries).

Although it could be argued that it is worth rebuilding with 4.9 too -to get fixes and performance improvements for example-, let's suppose
that we don't want to create 2 different binaries, just one built with GCC 4.8 and we want it to be compatible for GCC 4.9 installs.

.. _define_custom_package_id:

Defining a custom package_id()
------------------------------

The default ``package_id()`` uses the ``settings`` and ``options`` directly as defined, and assumes
`semantic versioning <https://semver.org/>`_ for dependencies defined in ``requires``.

This ``package_id()`` method can be overridden to control the package ID generation. Within the ``package_id()`` we have access to the
``self.info`` object, which is the one that is hashed to compute the binary ID and contains:

- **self.info.settings**: Contains all the declared settings, always as string values. We can access/alter the settings. e.g:
  ``self.info.settings.compiler.version``.

- **self.info.options**: Contains all the declared options, always as string values too. e.g: ``self.info.options.shared``.

Initially this ``info`` object will contain the original settings and options, but they can be changed without constraints to any other
string value.

For example, if you are sure your package ABI compatibility is fine for GCC versions > 4.5 and < 5.0 you could do this:

.. code-block:: python

    from conans import ConanFile, CMake, tools
    from conans.model.version import Version

    class PkgConan(ConanFile):
        name = "Pkg"
        version = "1.0"
        settings = "compiler", "build_type"
    
        def package_id(self):
            v = Version(str(self.settings.compiler.version))
            if self.settings.compiler == "gcc" and (v >= "4.5" and v < "5.0"):
                self.info.settings.compiler.version = "GCC version between 4.5 and 5.0"

We have set the ``self.info.settings.compiler.version`` with an arbitrary string, the value it's not important (could be any string). The
only important thing is that it is the same for any GCC version between 4.5 and 5.0. For all those versions, the compiler version will
always be hashed to the same ID.

Let's try and check that it works properly installing the package for GCC 4.5:

.. code-block:: bash

    $ conan export myuser/mychannel
    $ conan install Pkg/1.0@myuser/mychannel -s compiler=gcc -s compiler.version=4.5 ...

    Requirements
        Pkg/1.0@myuser/mychannel from local
    Packages
        Pkg/1.0@myuser/mychannel:mychannel:af044f9619574eceb8e1cca737a64bdad88246ad
    ...

We can see that the computed package ID is ``af04...46ad`` (not real). What would happen if we specify GCC 4.6?

.. code-block:: bash

    $ conan install Pkg/1.0@myuser/mychannel -s compiler=gcc -s compiler.version=4.6 ...

    Requirements
        Pkg/1.0@myuser/mychannel from local
    Packages
        Pkg/1.0@myuser/mychannel:mychannel:af044f9619574eceb8e1cca737a64bdad88246ad

Same result: the required package is again ``af04...46ad``. Now we can try with GCC 4.4 (< 4.5):

.. code-block:: bash

    $ conan install Pkg/1.0@myuser/mychannel -s compiler=gcc -s compiler.version=4.4 ...

    Requirements
        Pkg/1.0@myuser/mychannel from local
    Packages
        Pkg/1.0@myuser/mychannel:mychannel:7d02dc01581029782b59dcc8c9783a73ab3c22dd

Now the computed package ID is different: that means that we need a different binary package for GCC 4.4.

The same way we have adjusted the ``self.info.settings`` we could set the ``self.info.options`` values if needed.

.. seealso::

    Check :ref:`method_package_id` to see the available helper methods and change its behavior for things like:

        - Recipes packaging **header only** libraries.
        - Adjusting **Visual Studio toolsets** compatibility.

.. _problem_of_dependencies:

The problem of dependencies
---------------------------

Let's define a simple scenario where there are two packages: one for ``MyOtherLib/2.0`` and another one ``MyLib/1.0`` which depends
on (requires) ``MyOtherLib/2.0``. Let's assume that their recipes and binaries have already been created and uploaded to a Conan remote.

Now, a new release for ``MyOtherLib/2.1`` comes out with improved recipe and new binaries. The ``MyLib/1.0`` is modified to upgrade the
requires to ``MyOtherLib/2.1``.

.. note::

    This scenario will be the same in the case that a consuming project of ``MyLib/1.0`` defines a dependency to ``MyOtherLib/2.1``, which
    would have precedence over the existing one in ``MyLib/1.0``.

The question is: **Is it necessary to build new MyLib/1.0 binary packages?** Or are the existing packages still valid?

The answer: **It depends**.

Let's suppose that both are being compiled as static libraries and that the API exposed by ``MyOtherLib`` to ``MyLib/1.0`` through the
public headers has not changed at all. Then, it is not necessary to build new binaries for ``MyLib/1.0`` because the final consumer will
link against both ``Mylib/1.0`` and ``MyOtherLib/2.1``.

On the other hand, it could happen that the API exposed by **MyOtherLib** in public headers has changed, but without affecting the
``MyLib/1.0`` binary for any reason (like changes consisting on new functions not used by **MyLib**). The same reasoning would still be
valid if **MyOtherLib** was header only.

But what if one header file of ``MyOtherLib`` -named *myadd.h*- has changed from ``2.0`` to ``2.1``:

.. code-block:: cpp
   :caption: *myadd.h* header file in version 2.0

    int addition (int a, int b) { return a - b; }

.. code-block:: cpp
   :caption: *myadd.h* header file in version 2.1

    int addition (int a, int b) { return a + b; }

And the ``addition()`` function is being called from compiled *.cpp* files of ``MyLib/1.0``?

Then, **a new binary for MyLib/1.0 has to be built for the new dependency version**. Otherwise it will maintain the old, buggy
``addition()`` version. Even in the case that ``MyLib/1.0`` doesn't have any change in its code lines neither in the recipe, the resulting
binary rebuilding ``MyLib`` requiring `MyOtherLib/2.1`` will be different and the package needs to be different.

Using package_id() for package dependencies
-------------------------------------------

The ``self.info`` object has also a ``requires`` object. It is a dictionary with the necessary information for each requirement, all direct
and transitive dependencies. e.g. ``self.info.requires["MyOtherLib"]`` is a ``RequirementInfo`` object.

- Each ``RequirementInfo`` has the following `read only` reference fields:

    - ``full_name``: Full require's name. E.g **MyOtherLib**
    - ``full_version``: Full require's version. E.g **1.2**
    - ``full_user``: Full require's user. E.g **my_user**
    - ``full_channel``: Full require's channel. E.g **stable**
    - ``full_package_id``: Full require's package ID. E.g **c6d75a...**

- The following fields are the ones used in the ``package_id()`` evaluation:

    - ``name``: By default same value as full_name. E.g **MyOtherLib**.
    - ``version``: By default the major version representation of the ``full_version``.
      E.g **1.Y** for a **1.2** ``full_version`` field and **1.Y.Z** for a **1.2.3**
      ``full_version`` field.
    - ``user``: By default ``None`` (doesn't affect the package ID).
    - ``channel``: By default ``None`` (doesn't affect the package ID).
    - ``package_id``: By default ``None`` (doesn't affect the package ID).

When defining a package ID to model dependencies, it is necessary to take into account two factors:

- The versioning schema followed by our requirements (semver?, custom?).
- Type of library being built and type of library being reused (shared (*.so*, *.dll*, *.dylib*), static).

Versioning schema
+++++++++++++++++

By default Conan assumes `semver <https://semver.org/>`_ compatibility. e.g., if a version changes from minor **2.0** to **2.1** Conan will
assume that the API is compatible (headers not changing), and that it is not necessary to build a new binary for it. Exactly the same for
patches, changing from **2.1.10** to **2.1.11** doesn't require a re-build.

If it is necessary to change the default behavior, the applied versioning schema can be customized within the ``package_id()`` method:

.. code-block:: python

    from conans import ConanFile, CMake, tools
    from conans.model.version import Version

    class PkgConan(ConanFile):
        name = "Mylib"
        version = "1.0"
        settings = "os", "compiler", "build_type", "arch"
        requires = "MyOtherLib/2.0@lasote/stable"

        def package_id(self):
            myotherlib = self.info.requires["MyOtherLib"]

            # Any change in the MyOtherLib version will change current Package ID
            myotherlib.version = myotherlib.full_version

            # Changes in major and minor versions will change the Package ID but
            # only a MyOtherLib patch won't. E.j: From 1.2.3 to 1.2.89 won't change.
            myotherlib.version = myotherlib.full_version.minor()

Besides the ``version``, there are some other helpers that can be used to decide whether the **channel** and **user** of one dependency also
affects the binary package, or even the required package ID can change your own package ID.

You can decide if those variables of any requirement will change the ID of your binary package using the following modes:

+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| **Modes / Variables**   | ``name`` | ``version``                             | ``user`` | ``channel`` | ``package_id`` |
+=========================+==========+=========================================+==========+=============+================+
| ``semver_mode()``       | Yes      | Yes, only > 1.0.0 (e.g. **1**.2.Z+b102) | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``major_mode()``        | Yes      | Yes (e.g. **1**.2.Z+b102)               | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``minor_mode()``        | Yes      | Yes (e.g. **1.2**.Z+b102)               | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``patch_mode()``        | Yes      | Yes (e.g. **1.2.3**\+b102)              | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``base_mode()``         | Yes      | Yes (e.g. **1.7**\+b102)                | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``full_version_mode()`` | Yes      | Yes (e.g. **1.2.3+b102**)               | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``full_recipe_mode()``  | Yes      | Yes (e.g. **1.2.3+b102**)               | Yes      | Yes         | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``full_package_mode()`` | Yes      | Yes (e.g. **1.2.3+b102**)               | Yes      | Yes         | Yes            |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+
| ``unrelated_mode()``    | No       | No                                      | No       | No          | No             |
+-------------------------+----------+-----------------------------------------+----------+-------------+----------------+

- ``semver_mode()``: This is the default mode. In this mode only major release version (starting from **1.0.0**) changes the package ID.
  Every version change before 1.0.0 will change the package ID, but only major changes after 1.0.0 will do.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].semver_mode()

- ``major_mode()``: Any change in the major release version (starting from **0.0.0**) changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].major_mode()

- ``minor_mode()``: Any change in major or minor (not patch nor build) version of the required dependency changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].patch_mode()

- ``patch_mode()``: Any change in major, minor or patch (not build) version of the required dependency changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].patch_mode()

- ``base_mode()``: Any change in the base of the version (not build) of the required dependency changes the package ID. Note that in the
  case of semver notation this may produce same result as ``patch_mode()``, but it is actually intended to dismiss the build part of the
  version even without strict semver.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].base_mode()

- ``full_version_mode()``: Any change in the version of the required dependency changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].full_version_mode()

- ``full_recipe_mode()``: Any change in the reference of the requirement (user & channel too) changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].full_recipe_mode()

- ``full_package_mode()``: Any change in the required version, user, channel or package ID changes the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].full_package_mode()

- ``unrelated_mode()``: Requirements do not change the package ID.

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].unrelated_mode()

You can also adjust the individual properties manually:

.. code-block:: python

    def package_id(self):
        myotherlib = self.info.requires["MyOtherLib"]

        # Same as myotherlib.semver_mode()
        myotherlib.name = myotherlib.full_name
        myotherlib.version = myotherlib.full_version.stable()  # major(), minor(), patch(), base, build
        myotherlib.user = myotherlib.channel = myotherlib.package_id = None

        # Only the channel (and the name) matters
        myotherlib.name = myotherlib.full_name
        myotherlib.user = myotherlib.package_id = myotherlib.version = None
        myotherlib.channel = myotherlib.full_channel

The result of the ``package_id()`` is the package ID hash, but the details can be checked in the
generated *conaninfo.txt* file. The ``[requires]``, ``[options]`` and ``[settings]`` are those taken
into account to generate the SHA1 hash for the package ID, while the ``[full_xxxx]`` fields show the
complete reference information.

The default behavior produces a *conaninfo.txt* that looks like:

.. code-block:: text

    [requires]
      MyOtherLib/2.Y.Z

    [full_requires]
      MyOtherLib/2.2@demo/testing:73bce3fd7eb82b2eabc19fe11317d37da81afa56

Library types: Shared, static, header only
++++++++++++++++++++++++++++++++++++++++++

Let's see some examples, corresponding to common scenarios:

- ``MyLib/1.0`` is a shared library that links with a static library ``MyOtherLib/2.0`` package.
  When a new ``MyOtherLib/2.1`` version is released: Do I need to create a new binary for
  ``MyLib/1.0`` to link with it?

  Yes, always, as the implementation is embedded in the ``MyLib/1.0`` shared library. If we
  always want to rebuild our library, even if the channel changes (we assume a channel change could
  mean a source code change):

  .. code-block:: python

      def package_id(self):
          # Any change in the MyOtherLib version, user or
          # channel or Package ID will affect our package ID
          self.info.requires["MyOtherLib"].full_package_mode()

- ``MyLib/1.0`` is a shared library, requiring another shared library ``MyOtherLib/2.0`` package.
  When a new ``MyOtherLib/2.1`` version is released: Do I need to create a new binary for
  ``MyLib/1.0`` to link with it?

  It depends. If the public headers have not changed at all, it is not necessary. Actually it might
  be necessary to consider transitive dependencies that are shared among the public headers, how
  they are linked and if they cross the frontiers of the API, it might also lead to
  incompatibilities. If public headers have changed, it would depend on what changes and how are
  they used in ``MyLib/1.0``. Adding new methods to the public headers will have no impact, but
  changing the implementation of some functions that will be inlined when compiled from
  ``MyLib/1.0`` will definitely require re-building. For this case, it could make sense to have this configuration:

  .. code-block:: python

      def package_id(self):
          # Any change in the MyOtherLib version, user or channel
          # or Package ID will affect our package ID
          self.info.requires["MyOtherLib"].full_package_mode()

          # Or any change in the MyOtherLib version, user or
          # channel will affect our package ID
          self.info.requires["MyOtherLib"].full_recipe_mode()

- ``MyLib/1.0`` is a header-only library, linking with any kind (header, static, shared) of library
  in ``MyOtherLib/2.0`` package. When a new ``MyOtherLib/2.1`` version is released: Do I need to
  create a new binary for ``MyLib/1.0`` to link with it?

  Never. The package should always be the same as there are no settings, no options, and in any way a
  dependency can affect a binary, because there is no such binary. The default behavior should be
  changed to:

  .. code-block:: python

      def package_id(self):
          self.info.requires.clear()

- ``MyLib/1.0`` is a static library, linking with a header only library in ``MyOtherLib/2.0``
  package. When a new ``MyOtherLib/2.1`` version is released: Do I need to create a new binary for
  ``MyLib/1.0`` to link with it? It could happen that the ``MyOtherLib`` headers are strictly used
  in some ``MyLib`` headers, which are not compiled, but transitively included. But in the general
  case it is likely that ``MyOtherLib`` headers are used in ``MyLib`` implementation files, so every
  change in them should imply a new binary to be built. If we know that changes in the channel never
  imply a source code change, because it is the way we have defined our workflow/lifecycle, we could
  write:

  .. code-block:: python

      def package_id(self):
          self.info.requires["MyOtherLib"].full_package()
          self.info.requires["MyOtherLib"].channel = None # Channel doesn't change out package ID