How in vn_rdwr, VOP_ACCESS to implement before writing to a file?

Question

FreeBSD

Is it possible to append to the end of the file somehow using vn_rdwr(9) without calculating the offset itself? As far as I understand, there are also functions for writing from the FreeBSD kernel to a file (vn_write(9), VOP_WRITE()). The VAPPEND flag can be set in VOP_ACCESS. But I don't know how to use it yet. Where are the (source codes) of functions like VOP_ACCESS_APV(), VOP_WRITE_APV() described?