Defining Package ABI Compatibility

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

When any of the settings of a package recipe changes, it will reference a different binary:

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:

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

The process is:

1. Conan gets the user input settings and options. Those settings and options can come from the command line, profiles or from the values cached in the latest conan install execution.

2. Conan retrieves the MyLib/1.0@user/channel recipe, reads the settings attribute, and assigns the necessary values.

3. With the current package values for settings (also options and requires), it will compute a SHA1 hash that will serve as the binary package ID, e.g., c6d75a933080ca17eb7f076813e7fb21aaa740f2.

4. Conan will try to find the c6d75... binary package. If it exists, it will be retrieved. If it cannot be found, it will fail and indicate that it can be built from sources using conan install --build.

If the package is installed again using different settings, for example, on a 32-bit architecture:

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

The process will be repeated with a different generated package ID, because the arch setting will have a different value. The same applies to different compilers, compiler versions, build types. When generating multiple binaries - a separate ID is generated for each configuration.

When developers using the package use the same settings as one of those uploaded binaries, the computed package ID will be identical causing the binary to be retrieved and reused without the need of rebuilding it from the 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 options reference.

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

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

No matter the settings are defined by the users, including the compiler or version, the package settings and options will always be the same (left 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 can be built with GCC 4.8 and will preserve 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-. Let’s suppose that we don’t want to create 2 different binaries, but just a single built with GCC 4.8 which also needs to be compatible for GCC 4.9 installations.

Defining a Custom package_id()

The default package_id() uses the settings and options directly as defined, and assumes the semantic versioning for dependencies is 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 hashed to compute the binary ID and contains:

• self.info.settings: Contains all the declared settings, always as string values. We can access/modify 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 contains 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 the following:

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 of which is 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 when installing the package for GCC 4.5:

$ 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 happens if we specify GCC 4.6?

$ 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

The required package has the same result again af04...46ad. Now we can try using GCC 4.4 (< 4.5):

$ 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

The computed package ID is different which 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.

See also

Check 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.

Dependency Issues

Let’s define a simple scenario whereby there are two packages: MyOtherLib/2.0 and MyLib/1.0 which depends on 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 is released with an improved recipe and new binaries. The MyLib/1.0 is modified and is required to be upgraded 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 takes precedence over the existing project 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 assume that both packages are compiled as static libraries and that the API exposed by MyOtherLib to MyLib/1.0 through the public headers, has not changed at all. In this case, it is not required 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 the 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 apply if MyOtherLib was only the header.

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

myadd.h header file in version 2.0

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

myadd.h header file in version 2.1

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

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

Then, a new binary for MyLib/1.0 is required 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 requires MyOtherLib/2.1` and the package to be different.

Using package_id() for Package Dependencies

The self.info object has also a requires object. It is a dictionary containing the necessary information for each requirement, all direct and transitive dependencies. For example, 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 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 for model dependencies, it is necessary to take into account two factors:

• The versioning schema followed by our requirements (semver?, custom?).

• The type of library being built or reused (shared (.so, .dll, .dylib), static).

Versioning Schema

By default Conan assumes semver compatibility. For example, 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. This also applies to patches, whereby 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:

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.g., from 1.2.3 to 1.2.89 won't change.
        myotherlib.version = myotherlib.full_version.minor()

Besides version, there are additional helpers that can be used to determine 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 determine if the following variables within any requirement 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 a major release version (starting from 1.0.0) changes the package ID. Every version change prior to 1.0.0 changes the package ID, but only major changes after 1.0.0 will be applied.

  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.

  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.

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

• patch_mode(): Any changes to major, minor or patch (not build) versions of the required dependency change the package ID.

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

• base_mode(): Any changes to 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 the same result as patch_mode(), but it is actually intended to dismiss the build part of the version even without strict semver.

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

• full_version_mode(): Any changes to the version of the required dependency changes the package ID.

  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.

  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.

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

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

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

You can also adjust the individual properties manually:

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 taken into account when generating 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:

[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):

  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 the 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:

  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:

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

• MyLib/1.0 is a static library linking to 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 general, it is more 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, as set in our workflow/lifecycle, we could write:

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