The use of TCP_ in udp.h is not known, fortunately udp.h is not specified by posix so there are no strict namespace rules, added in linux commit e27cca96cd68fa2c6814c90f9a1cfd36bb68c593 xfrm: add espintcp (RFC 8229)
sockopt to enable gro for udp. added in linux commit e20cf8d3f1f763ad28a9cb3b41305b8a8a42653e
used for optimizing the rxrpc protocol added in linux commit 5271953cad31b97dea80f848c16e96ad66401199
add UDP_NO_CHECK6_* to restrict zero UDP6 checksums, new in linux commit 1c19448c9ba6545b80ded18488a64a7f3d8e6998 (pre-v4.18 change, was missed) add UDP_SEGMENT to support generic segmentation offload for udp datagrams, bec1f6f697362c5bc635dacd7ac8499d0a10a4e7 (new in v4.18)
for GPRS tunneling protocol, new in linux commit 459aa660eb1d8ce67080da1983bb81d716aa5a69
the affected part of the header is responsible for providing both GNU and BSD versions of the udphdr structure. previously, the namespace-polluting GNU names were always used for the actual struct members, and the BSD names, which are named in a manner resembling a sane namespace, were always macros defined to expand to the GNU names. now, unless _GNU_SOURCE is defined, the BSD names are used as the actual structure members, and the macros and GNU names only come into play when the application requests them.