An allowlist restricts generated bindings to declarations that match explicit rules. In an NGINX FFI, bindgen initially describes more than 51,000 lines, including the entire C standard library; filtering reduces unnecessary output and compile-time cost. section 4.3.2
Filter declarations by prefix
Add regular-expression rules such as whitelist_type("ngx_.*"), whitelist_function("ngx_.*"), and whitelist_var("ngx_.*"). Rerunning generation produces about 30,000 lines containing declarations whose names begin with ngx_. Explicitly allowing each required declaration is possible but deferred here. section 4.3.2
