Why does getconf not recognize BUFSIZ?

$ getconf BUFSIZ
getconf: Unrecognized variable 'BUFSIZ'

Is there a standard way to determine the value of BUFSIZ from a shell? It's easy enough to write a simple C program to output the value, but I don't quite understand why getconf doesn't do it.

Because BUFSIZ is a somewhat-arbitary configuration parameter for the stdio library, and not a system configuration parameter

The man/info page for sysconf lists the names of the constants that getconf can return values for.