This directory contains code that will handle translations of data types across ABI boundaries.
These wrappers are used in modular builds (including Evergreen). They are needed to handle dealing with differences in platform-specific toolchain types and musl-based types. For example, the struct timespec and struct timeval types may contain differently sized member fields and when the caller is Cobalt code compiled with musl it will provide argument data that needs to be adjusted when passed to platform-specific system libraries.
To do this adjustment across compilation toolchains, there are 2 sets of files in this directory that will be compiled in either the Cobalt toolchain or the Starboard plaform-specific toolchain:
cobalt_layer_posix_XYZ_abi_wrappers.cc - these are compiled only in the Cobalt toolchain for modular builds (excluding Evergreen). The logic in these files simply defines the API we want to wrap and re-calls an external function named __abi_wrap_XYZ using the exact same parameter and return value types. This is simply to get the libcobalt.so to not link against the glibc implementations and instead rely on an external implementation that will be defined in libstarboard.so. Similar logic is done specifically for Evergreen in exported_symbols.cc, since we have a custom ELF loader in that case to handle remapping the functions.starboard_layer_posix_XYZ_abi_wrappers.{h,cc} - these are compiled only in the Starboard platform-specific toolchain for modular builds (including Evergreen). The header files define corresponding musl-compatible types (copying definitions provided in //third_party/musl/include/alltypes.h.in) for the cases where data types are not guaranteed to match across ABI boundaries. These files then define the implementation of the __abi_wrap_XYZ functions, which will be called from the Cobalt layer. Since these functions are being implemented/compiled in the Starboard layer, they will rely on the platform-specific toolchain to get the platform-specific type definitions from the system headers (e.g. <time.h>). The implementations will handle doing data type conversions across the ABI boundary (both for parameters and return types).To help illustrate how this works, we can use the POSIX function clock_gettime as an example. When we build modularly, the following table shows how the symbols would look using a tool like nm:
libstarboard.so: U clock_gettime@GLIBC_2.17 (undefined reference to platform library) T __abi_wrap_clock_gettime (concrete definition of ABI wrapper function) libcobalt.so U __abi_wrap_clock_gettime (undefined reference to the Starboard wrapper) t clock_gettime (concrete definition that calls __abi_wrap_*)