When a porter implements Starboard for a platform, it is more precise to say that they have implemented support for a certain version of Starboard. Changes to the Starboard API are associated with Starboard versions. Any usage of new Starboard APIs must also be protected by a compile-time check for the Starboard version it belongs to. This decoupling of Cobalt and Starboard versions ensures that a porter can update to a newer version of Cobalt, but not be required to implement new Starboard APIs, if the version of Starboard they have implemented is still supported.
The Starboard version describes the set of Starboard APIs available to Starboard applications. It will be incremented with every open-source release that includes changes to the Starboard API. Reasons to increment the Starboard API version include:
Some notable cases that do not justify incrementing the Starboard version include:
starboard::ScopedFileA particular Starboard application may be versioned independently of the Starboard API. A given version of a Starboard application may support a range of Starboard versions. It may be the case that some new functionality in a Starboard application requires Starboard APIs that were added to a particular API version. If a porter wants to use such a feature, they must therefore also implement the required version of the Starboard API. For example, Voice Search was added to Cobalt version 5 and requires the SbMicrophone APIs which were added to Starboard version 2. Platforms that implemented Starboard version 1 continued to build and run Cobalt 5 correctly, but the Voice Search feature would be unavailable.
The minimum supported API version is defined by the SB_MINIMUM_API_VERSION macro, which is defined in starboard/configuration.h. Likewise, the SB_MAXIMUM_API_VERSION macro defines the maximum supported API version. All platforms must declare a SB_API_VERSION macro in the platform’s configuration.h to declare the starboard version the platform has implemented. Declaring implementation for an API version outside this range will result in an error at compilation time. Generally Starboard applications will not support all versions of the Starboard API indefinitely. Starboard application owners may increment the minimum required Starboard version at their discretion. TBD: Timelines and communication around when an upcoming Cobalt release will require porters to implement a newer version of Starboard.
Usage of a Starboard API that is not available in all supported Starboard API versions must be guarded with a check for SB_API_VERSION. Starboard applications must continue to function correctly and must not disable existing functionality if this check evaluates to false, but it’s acceptable to disable new functionality in Starboard applications if this evaluates to false.
At any given time, exactly one version of Starboard will be denoted as the “experimental” version, as defined by the SB_EXPERIMENTAL_API_VERSION macro in starboard/configuration.h. It is generally not recommended to declare support for this version. Any Starboard APIs defined in the experimental version are subject to change and API requirements could be added, removed, or changed at any time.
At any given time, zero or more versions of Starboard will be denoted as the “release candidate” version, as defined by the SB_RELEASE_CANDIDATE_API_VERSION macro in starboard/configuration.h. The “release candidate” version is a set of API changes that have been considered and tested together. It is reasonable to port against this version, it has gone through some stabilization and may become frozen as it currently is. But, be aware that it is possible that minor incompatible changes may be made to this version if an unexpected situation arises. SB_RELEASE_CANDIDATE_API_VERSION is not defined if there is no “release candidate” version. Every API version greater than SB_RELEASE_CANDIDATE_API_VERSION but less than SB_EXPERIMENTAL_API_VERSION is also considered a release candidate.
All Starboard versions that are less than the experimental and release candidate versions are considered frozen. Any Starboard APIs in a frozen version MUST not change.
frozen < release-candidate < experimental < future
As mentioned previously, a release candidate version may or may not exist at any given time. When there is a release candate version, it follows the invariant above.
New Starboard APIs should be defined in the experimental version.
When introducing a new Starboard API (or modifying an existing one), a new feature version define should be created within the “Experimental Feature Defines” section of starboard/configuration.h, and set to SB_EXPERIMENTAL_API_VERSION. A well written comment should be added in front of the feature define that describes exactly what is introduced by the feature. In the comment, all new/modified/removed symbols should be identified, and all modified header files should be named.
For example,
// in starboard/configuration.h #define SB_EXPERIMENTAL_API_VERSION 7 #undef SB_RELEASE_CANDIDATE_API_VERSION // --- Experimental Feature Defines ------------------------------------------ ... // Introduce a new API in starboard/screensaver.h, which declares the following // functions for managing the platform's screensaver settings: // SbScreensaverDisableScreensaver() // SbScreensaverEnableScreensaver() // Additionally, a new event, kSbEventTypeScreensaverStarted, is introduced in // starboard/event.h. #define SB_SCREENSAVER_FEATURE_API_VERSION SB_EXPERIMENTAL_API_VERSION // Introduce a new API in starboard/new_functionality.h which declares the // function SbNewFunctionality(). #define SB_MY_NEW_FEATURE_API_VERSION SB_EXPERIMENTAL_API_VERSION // Introduce another new API in starboard/still_in_development.h which // declares the function SbStillInDevelopment(). #define SB_MY_OTHER_NEW_FEATURE_API_VERSION SB_EXPERIMENTAL_API_VERSION
When declaring the new interface, the following syntax should be used:
// starboard/new_functionality.h #if SB_API_VERSION >= SB_MY_NEW_FEATURE_API_VERSION void SbNewFunctionality(); #endif
Starboard application features that use a new API must have a similar check:
// cobalt/new_feature.cc
#if SB_API_VERSION >= SB_MY_NEW_FEATURE_API_VERSION
void DoSomethingCool() {
SbNewFunctionality();
}
#endif
When promoting the experimental API version to be the release candidate API version, the previously undefined SB_RELEASE_CANDIDATE_API_VERSION is set to the current value of SB_EXPERIMENTAL_API_VERSION, and SB_EXPERIMENTAL_API_VERSION is then incremented by one. As a result, SB_RELEASE_CANDIDATE_API_VERSION on the master branch should always either be undefined, or SB_EXPERIMENTAL_API_VERSION - 1.
One or more features are then moved from SB_EXPERIMENTAL_API_VERSION to SB_RELEASE_CANDIDATE_API_VERSION, and into the “Release Candidate Feature Defines” section of starboard/configuration.h. Some features may be left in experimental if they are not ready for release. The documentation comments of these features should be moved into the (newly created) section for the corresponding version in starboard/CHANGELOG.md.
// in starboard/configuration.h #define SB_EXPERIMENTAL_API_VERSION 8 #define SB_RELEASE_CANDIDATE_API_VERSION 7 // --- Experimental Feature Defines ------------------------------------------ // Introduce another new API in starboard/still_in_development.h which // declares the function SbStillInDevelopment(). #define SB_MY_OTHER_NEW_FEATURE_API_VERSION SB_EXPERIMENTAL_API_VERSION // --- Release Candidate Features Defines ------------------------------------ #define SB_MY_NEW_FEATURE_API_VERSION SB_RELEASE_CANDIDATE_API_VERSION
When a release candidate branch is promoted to a full release, these new Starboard APIs will be irrevocably frozen to the value of SB_RELEASE_CANDIDATE_API_VERSION, and the release candidate version will be undefined. Additionally, the feature defines should be removed.
// starboard/new_functionality.h
#if SB_API_VERSION >= 7
void SbNewFunctionality();
#endif
// starboard/other_new_functionality.h
#if SB_API_VERSION >= SB_MY_OTHER_NEW_FEATURE_API_VERSION
void SbStillInDevelopment();
#endif
// starboard/configuration.h
#define SB_EXPERIMENTAL_API_VERSION 8
#undef SB_RELEASE_CANDIDATE_API_VERSION
// cobalt/new_feature.cc
#if SB_API_VERSION >= 7
void DoSomethingCool() {
SbNewFunctionality();
}
#endif
Whoever increments the experimental version must ensure that stubs and reference platforms declare support for the new experimental version through their respective SB_API_VERSION macros.
When a new version of Starboard is released, starboard/CHANGELOG.md should be updated with the feature define comments for all features enabled in that version.