Historically Starboard did not allow usage of standard C symbols in order to isolate Cobalt from non-compliant libc implementations and to provide a single, consistent behavior at the Starboard API layer.
Inconsistencies in the libc libraries are rare and all third party libraries need to be ported to the Starboard API. This can be a significant maintenance cost as the dependencies need to be periodically rebased.
Even with all the efforts to use POEM headers from the starboard/client_porting directory many non-Evergreen platforms still have a lot of direct system dependencies. These dependencies do not exist for Evergreen platforms as Cobalt is statically linked with the musl libc library.
Starting with Starboard 13 a limited set of C99 symbols will be allowed. This set will expand gradually while the corresponding Starboard APIs will be deprecated and eventually removed.