The BufferIn argument turns into various things:

 in C      const char *, size_t parameter pair

 in XDR    an opaque<> type (instead of string) which allows \0 chars

 in other bindings
           mostly just a string, since most languages except for C
           permit strings to contain any 8 bit data

truncate, truncate_size: Used to truncate files to a particular
size, or to zero bytes.

mkdir_mode: Like mkdir but allows you to also specify the
initial permissions for the new directory.

utimens: Set timestamp on a file with nanosecond accuracy.

lchown: Corresponding to lchown(2) syscall (we already have chown).

The implementation is complicated by the fact that we had to
add an Int64 parameter type to the generator.