# The mechanics of the public_api command¶

The build system, in combination with the header generation mechanism, facilitates the fine grained ability to pick and choose the public API one wants to expose on their platform. The public header files are always generated from the corresponding .h.def files. A header generation command %%public_api is listed in these files. In the generated header file, the header generator replaces this command with the public API relevant for the target platform.

## Under the hood¶

When the header generator sees the %%public_api command, it looks up the API config file for the platform in the path config/<platform>/api.td. The API config file lists two kinds of items:

1. The list of standards from which the public entities available on the platform are derived from.
2. For each header file exposed on the platfrom, the list of public members provided in that header file.

Note that, the header generator only learns the names of the public entities from the header config file (the 2nd item from above.) The exact manner in which the entities are to be declared is got from the standards (the 1st item from above.)

See the ground truth document for more information on how the standards are formally listed in LLVM libc using LLVM table-gen files.