the fcntl function is heavy, so make the syscall directly instead. also, avoid the code size and runtime overhead of querying the old flags, since it's reasonable to assume nothing will be set on a newly-created socket. this code is only used on old kernels which lack proper atomic close-on-exec support, so future changes that might invalidate such an assumption do not need to be considered.
as usual, this is non-atomic, but better than producing an error or failing to set the close-on-exec flag at all.
the input name is validated, the other parameters are assumed to be valid (the list of already compressed names are not checked for infinite reference loops or out-of-bound offsets). names are handled case-sensitively for now.
trailing . should be accepted in domain name strings by convention (RFC 1034), host name lookup accepts "." but rejects empty "", res_* interfaces also accept empty name following existing practice.
A domain name is at most 255 bytes long (RFC 1035), but the string representation is two bytes smaller so the strlen maximum is 253.
Due to an error introduced in commit fcc522c92335783293ac19df318415cd97fbf66b, checking of the remaining output buffer space was not performed correctly, allowing malformed input to write past the end of the buffer. In addition, the loop detection logic failed to account for the possibility of infinite loops with no output, which would hang the function. The output size is now limited more strictly so only names with valid length are accepted.
this also affects the legacy getservbyport family, which uses getnameinfo as its backend.
this also affects the legacy gethostbyaddr family, which uses getnameinfo as its backend. some other minor changes associated with the refactoring of source files are also made; in particular, the resolv.conf parser now uses the same code that's used elsewhere to handle ip literals, so as a side effect it can now accept a scope id for nameserver addressed with link-local scope.
the service and protocol functions are defined also in other files, and the protocol ones are actually non-nops elsewhere, so the weak definitions in ent.c could have prevented the strong definitions from getting pulled in and used in some static programs.