This document describes the standards for how we develop the Fuchsia SDK within the Platform Source Tree. Some of the information in this document might be of interest to clients of the Fuchsia SDK, but the primary focus of the document is how the Fuchsia project develops the SDK.
Broadly speaking, the binary interface to the system is defined by the FIDL wireformat used by programs to communicate with the rest of the system and the syscalls exposed in libzircon. In particular, the system should not rely upon programs using any specific client libraries, including libc.
The Fuchsia SDK contains a number of client libraries (i.e., libraries that clients of the SDK can link into their programs), but all of these libraries are optional and provided for the convenience of clients, not for the convenience of the system.
FIDL protocols are defined in .fidl files, which are contained in the SDK. All the FIDL definitions that have been published in an SDK should be considered public ABI for the system. The system might also contain additional FIDL definitions that have not been published in an SDK. Those definitions are subject to change without notice and programs that rely upon their ABI might not work properly in future versions of the system.
FIDL definitions in the SDK might evolve in source-incompatible ways. For example, we might rename a method in a protocol while maintaining its ordinal and semantics (the ordinal can be maintained by adding a Selector attribute that is set to the original name). Such a change preserves the ABI but breaks source compatibility.
We do not currently have any standards about when we should break source compatibility.
Public FIDL definitions are located in the source tree at a path with the following pattern: $LAYER/public/lib/$NAME/fidl. The target name should be fidl. Related FIDL libraries can be grouped in a directory. For example, $LAYER/public/lib/$GROUP/$NAME/fidl.
FIDL definitions in the SDK should follow the FIDL API readability rubric.
Client libraries are neither source nor binary stable. Clients that wish to use these libraries should link them into their programs, either statically or dynamically.
Programs load dynamic libraries from their own package, which means different programs on the system might be using different versions of the same dynamic library concurrently. Programs that wish to use dynamic libraries (including libc) should include those libraries in the lib directory of their package.
The Fuchsia SDK does not require clients to use a specific toolchain. For this reason, precompiled libraries that clients link against must have C linkage. For example, a precompiled library cannot export C++ symbols because C++ does not have a standard ABI across toolchains (or even toolchain versions).
The SDK can also contain precompiled shared libraries with C++ linkage that are linked by other precompiled libraries in the SDK. Clients are not expected to link against these libraries directly.
A client that takes a dependency on a client library must also take a dependency on all the dependencies of that library. For this reason, client libraries should have minimal dependencies. For example, client libraries should avoid dependencies on FBL, FXL, FSL, or other “base” libraries.
Client libraries that need to perform asynchronous operations should depend on libasync.a and libasync-default.so. However, these libraries should not assume the client is using any specific implementation of async_dispatcher_t*. For example, these libraries should not assume the async_dispatcher_t* is actually implemented by libasync-loop.a. Libraries that require async_get_default_dispatcher to be populated should state this requirement in their documentation.
Precompiled libraries can have more extensive dependencies if those dependencies are hidden from their client. For example, a precompiled shared library should not export symbols from these dependencies and should not have headers that transitively include headers from these dependencies.
Client libraries should be named according to the language they expect their clients to use. For example, the C++ variant of the $NAME library should be located in the source tree at a path with the following pattern: $LAYER/public/lib/$NAME/cpp.
In some cases, a library only makes sense for one language. In that case, the language suffix may be omitted unless the client library has an associated FIDL protocol.
Client libraries should follow the Fuchsia style guide for the language in which they are written.
Client libraries should avoid logging messages. Instead, client libraries should return errors to their clients, who can decide whether to log the error.
C and C++ client libraries should use ZX_DEBUG_ASSERT and ZX_ASSERT, defined in <zircon/assert.h>, to assert invariants. Client libraries may also use the _MSG variants to provide a message when the assertion fails.
