src/net/cgo_unix.go - go.git - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // This file is called cgo_unix.go, but to allow syscalls-to-libc-based
 // implementations to share the code, it does not use cgo directly.
 // Instead of C.foo it uses _C_foo, which is defined in either
 // cgo_unix_cgo.go or cgo_unix_syscall.go

 //go:build !netgo && ((cgo && unix) || darwin)

 package net

 import (
 	"context"
 	"errors"
 	"internal/bytealg"
 	"net/netip"
 	"runtime"
 	"syscall"
 	"unsafe"

 	"golang.org/x/net/dns/dnsmessage"
 )

 // cgoAvailable set to true to indicate that the cgo resolver
 // is available on this system.
 const cgoAvailable = true

 // An addrinfoErrno represents a getaddrinfo, getnameinfo-specific
 // error number. It's a signed number and a zero value is a non-error
 // by convention.
 type addrinfoErrno int

 func (eai addrinfoErrno) Error() string   { return _C_gai_strerror(_C_int(eai)) }
 func (eai addrinfoErrno) Temporary() bool { return eai == _C_EAI_AGAIN }
 func (eai addrinfoErrno) Timeout() bool   { return false }

 // isAddrinfoErrno is just for testing purposes.
 func (eai addrinfoErrno) isAddrinfoErrno() {}

 // doBlockingWithCtx executes a blocking function in a separate goroutine when the provided
 // context is cancellable. It is intended for use with calls that don't support context
 // cancellation (cgo, syscalls). blocking func may still be running after this function finishes.
 // For the duration of the execution of the blocking function, the thread is 'acquired' using [acquireThread],
 // blocking might not be executed when the context gets canceled early.
 func doBlockingWithCtx[T any](ctx context.Context, lookupName string, blocking func() (T, error)) (T, error) {
 	if err := acquireThread(ctx); err != nil {
 		var zero T
 		return zero, &DNSError{
 			Name:      lookupName,
 			Err:       mapErr(err).Error(),
 			IsTimeout: err == context.DeadlineExceeded,
 		}
 	}

 	if ctx.Done() == nil {
 		defer releaseThread()
 		return blocking()
 	}

 	type result struct {
 		res T
 		err error
 	}

 	res := make(chan result, 1)
 	go func() {
 		defer releaseThread()
 		var r result
 		r.res, r.err = blocking()
 		res <- r
 	}()

 	select {
 	case r := <-res:
 		return r.res, r.err
 	case <-ctx.Done():
 		var zero T
 		return zero, &DNSError{
 			Name:      lookupName,
 			Err:       mapErr(ctx.Err()).Error(),
 			IsTimeout: ctx.Err() == context.DeadlineExceeded,
 		}
 	}
 }

 func cgoLookupHost(ctx context.Context, name string) (hosts []string, err error) {
 	addrs, err := cgoLookupIP(ctx, "ip", name)
 	if err != nil {
 		return nil, err
 	}
 	for _, addr := range addrs {
 		hosts = append(hosts, addr.String())
 	}
 	return hosts, nil
 }

 func cgoLookupPort(ctx context.Context, network, service string) (port int, err error) {
 	var hints _C_struct_addrinfo
 	switch network {
 	case "ip": // no hints
 	case "tcp", "tcp4", "tcp6":
 		*_C_ai_socktype(&hints) = _C_SOCK_STREAM
 		*_C_ai_protocol(&hints) = _C_IPPROTO_TCP
 	case "udp", "udp4", "udp6":
 		*_C_ai_socktype(&hints) = _C_SOCK_DGRAM
 		*_C_ai_protocol(&hints) = _C_IPPROTO_UDP
 	default:
 		return 0, &DNSError{Err: "unknown network", Name: network + "/" + service}
 	}
 	switch ipVersion(network) {
 	case '4':
 		*_C_ai_family(&hints) = _C_AF_INET
 	case '6':
 		*_C_ai_family(&hints) = _C_AF_INET6
 	}

 	return doBlockingWithCtx(ctx, network+"/"+service, func() (int, error) {
 		return cgoLookupServicePort(&hints, network, service)
 	})
 }

 func cgoLookupServicePort(hints *_C_struct_addrinfo, network, service string) (port int, err error) {
 	cservice, err := syscall.ByteSliceFromString(service)
 	if err != nil {
 		return 0, &DNSError{Err: err.Error(), Name: network + "/" + service}
 	}
 	// Lowercase the C service name.
 	for i, b := range cservice[:len(service)] {
 		cservice[i] = lowerASCII(b)
 	}
 	var res *_C_struct_addrinfo
 	gerrno, err := _C_getaddrinfo(nil, (*_C_char)(unsafe.Pointer(&cservice[0])), hints, &res)
 	if gerrno != 0 {
 		switch gerrno {
 		case _C_EAI_SYSTEM:
 			if err == nil { // see golang.org/issue/6232
 				err = syscall.EMFILE
 			}
 			return 0, newDNSError(err, network+"/"+service, "")
 		case _C_EAI_SERVICE, _C_EAI_NONAME: // Darwin returns EAI_NONAME.
 			return 0, newDNSError(errUnknownPort, network+"/"+service, "")
 		default:
 			return 0, newDNSError(addrinfoErrno(gerrno), network+"/"+service, "")
 		}
 	}
 	defer _C_freeaddrinfo(res)

 	for r := res; r != nil; r = *_C_ai_next(r) {
 		switch *_C_ai_family(r) {
 		case _C_AF_INET:
 			sa := (*syscall.RawSockaddrInet4)(unsafe.Pointer(*_C_ai_addr(r)))
 			p := (*[2]byte)(unsafe.Pointer(&sa.Port))
 			return int(p[0])<<8 | int(p[1]), nil
 		case _C_AF_INET6:
 			sa := (*syscall.RawSockaddrInet6)(unsafe.Pointer(*_C_ai_addr(r)))
 			p := (*[2]byte)(unsafe.Pointer(&sa.Port))
 			return int(p[0])<<8 | int(p[1]), nil
 		}
 	}
 	return 0, newDNSError(errUnknownPort, network+"/"+service, "")
 }

 func cgoLookupHostIP(network, name string) (addrs []IPAddr, err error) {
 	var hints _C_struct_addrinfo
 	*_C_ai_flags(&hints) = cgoAddrInfoFlags
 	*_C_ai_socktype(&hints) = _C_SOCK_STREAM
 	*_C_ai_family(&hints) = _C_AF_UNSPEC
 	switch ipVersion(network) {
 	case '4':
 		*_C_ai_family(&hints) = _C_AF_INET
 	case '6':
 		*_C_ai_family(&hints) = _C_AF_INET6
 	}

 	h, err := syscall.BytePtrFromString(name)
 	if err != nil {
 		return nil, &DNSError{Err: err.Error(), Name: name}
 	}
 	var res *_C_struct_addrinfo
 	gerrno, err := _C_getaddrinfo((*_C_char)(unsafe.Pointer(h)), nil, &hints, &res)
 	if gerrno != 0 {
 		switch gerrno {
 		case _C_EAI_SYSTEM:
 			if err == nil {
 				// err should not be nil, but sometimes getaddrinfo returns
 				// gerrno == _C_EAI_SYSTEM with err == nil on Linux.
 				// The report claims that it happens when we have too many
 				// open files, so use syscall.EMFILE (too many open files in system).
 				// Most system calls would return ENFILE (too many open files),
 				// so at the least EMFILE should be easy to recognize if this
 				// comes up again. golang.org/issue/6232.
 				err = syscall.EMFILE
 			}
 			return nil, newDNSError(err, name, "")
 		case _C_EAI_NONAME, _C_EAI_NODATA:
 			return nil, newDNSError(errNoSuchHost, name, "")
 		case _C_EAI_ADDRFAMILY:
 			if runtime.GOOS == "freebsd" {
 				// FreeBSD began returning EAI_ADDRFAMILY for valid hosts without
 				// an A record in 13.2. We previously returned "no such host" for
 				// this case.
 				//
 				// https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273912
 				return nil, newDNSError(errNoSuchHost, name, "")
 			}
 			fallthrough
 		default:
 			return nil, newDNSError(addrinfoErrno(gerrno), name, "")
 		}

 	}
 	defer _C_freeaddrinfo(res)

 	for r := res; r != nil; r = *_C_ai_next(r) {
 		// We only asked for SOCK_STREAM, but check anyhow.
 		if *_C_ai_socktype(r) != _C_SOCK_STREAM {
 			continue
 		}
 		switch *_C_ai_family(r) {
 		case _C_AF_INET:
 			sa := (*syscall.RawSockaddrInet4)(unsafe.Pointer(*_C_ai_addr(r)))
 			addr := IPAddr{IP: copyIP(sa.Addr[:])}
 			addrs = append(addrs, addr)
 		case _C_AF_INET6:
 			sa := (*syscall.RawSockaddrInet6)(unsafe.Pointer(*_C_ai_addr(r)))
 			addr := IPAddr{IP: copyIP(sa.Addr[:]), Zone: zoneCache.name(int(sa.Scope_id))}
 			addrs = append(addrs, addr)
 		}
 	}
 	return addrs, nil
 }

 func cgoLookupIP(ctx context.Context, network, name string) (addrs []IPAddr, err error) {
 	return doBlockingWithCtx(ctx, name, func() ([]IPAddr, error) {
 		return cgoLookupHostIP(network, name)
 	})
 }

 // These are roughly enough for the following:
 //
 //	 Source		Encoding			Maximum length of single name entry
 //	 Unicast DNS		ASCII or			<=253 + a NUL terminator
 //				Unicode in RFC 5892		252 * total number of labels + delimiters + a NUL terminator
 //	 Multicast DNS	UTF-8 in RFC 5198 or		<=253 + a NUL terminator
 //				the same as unicast DNS ASCII	<=253 + a NUL terminator
 //	 Local database	various				depends on implementation
 const (
 	nameinfoLen    = 64
 	maxNameinfoLen = 4096
 )

 func cgoLookupPTR(ctx context.Context, addr string) (names []string, err error) {
 	ip, err := netip.ParseAddr(addr)
 	if err != nil {
 		return nil, &DNSError{Err: "invalid address", Name: addr}
 	}
 	sa, salen := cgoSockaddr(IP(ip.AsSlice()), ip.Zone())
 	if sa == nil {
 		return nil, &DNSError{Err: "invalid address " + ip.String(), Name: addr}
 	}

 	return doBlockingWithCtx(ctx, addr, func() ([]string, error) {
 		return cgoLookupAddrPTR(addr, sa, salen)
 	})
 }

 func cgoLookupAddrPTR(addr string, sa *_C_struct_sockaddr, salen _C_socklen_t) (names []string, err error) {
 	var gerrno int
 	var b []byte
 	for l := nameinfoLen; l <= maxNameinfoLen; l *= 2 {
 		b = make([]byte, l)
 		gerrno, err = cgoNameinfoPTR(b, sa, salen)
 		if gerrno == 0 || gerrno != _C_EAI_OVERFLOW {
 			break
 		}
 	}
 	if gerrno != 0 {
 		switch gerrno {
 		case _C_EAI_SYSTEM:
 			if err == nil { // see golang.org/issue/6232
 				err = syscall.EMFILE
 			}
 			return nil, newDNSError(err, addr, "")
 		case _C_EAI_NONAME:
 			return nil, newDNSError(errNoSuchHost, addr, "")
 		default:
 			return nil, newDNSError(addrinfoErrno(gerrno), addr, "")
 		}
 	}
 	if i := bytealg.IndexByte(b, 0); i != -1 {
 		b = b[:i]
 	}
 	return []string{absDomainName(string(b))}, nil
 }

 func cgoSockaddr(ip IP, zone string) (*_C_struct_sockaddr, _C_socklen_t) {
 	if ip4 := ip.To4(); ip4 != nil {
 		return cgoSockaddrInet4(ip4), _C_socklen_t(syscall.SizeofSockaddrInet4)
 	}
 	if ip6 := ip.To16(); ip6 != nil {
 		return cgoSockaddrInet6(ip6, zoneCache.index(zone)), _C_socklen_t(syscall.SizeofSockaddrInet6)
 	}
 	return nil, 0
 }

 func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error, completed bool) {
 	resources, err := resSearch(ctx, name, int(dnsmessage.TypeCNAME), int(dnsmessage.ClassINET))
 	if err != nil {
 		return
 	}
 	cname, err = parseCNAMEFromResources(resources)
 	if err != nil {
 		return "", err, false
 	}
 	return cname, nil, true
 }

 // resSearch will make a call to the 'res_nsearch' routine in the C library
 // and parse the output as a slice of DNS resources.
 func resSearch(ctx context.Context, hostname string, rtype, class int) ([]dnsmessage.Resource, error) {
 	return doBlockingWithCtx(ctx, hostname, func() ([]dnsmessage.Resource, error) {
 		return cgoResSearch(hostname, rtype, class)
 	})
 }

 func cgoResSearch(hostname string, rtype, class int) ([]dnsmessage.Resource, error) {
 	resStateSize := unsafe.Sizeof(_C_struct___res_state{})
 	var state *_C_struct___res_state
 	if resStateSize > 0 {
 		mem := _C_malloc(resStateSize)
 		defer _C_free(mem)
 		memSlice := unsafe.Slice((*byte)(mem), resStateSize)
 		clear(memSlice)
 		state = (*_C_struct___res_state)(unsafe.Pointer(&memSlice[0]))
 	}
 	if err := _C_res_ninit(state); err != nil {
 		return nil, errors.New("res_ninit failure: " + err.Error())
 	}
 	defer _C_res_nclose(state)

 	// Some res_nsearch implementations (like macOS) do not set errno.
 	// They set h_errno, which is not per-thread and useless to us.
 	// res_nsearch returns the size of the DNS response packet.
 	// But if the DNS response packet contains failure-like response codes,
 	// res_search returns -1 even though it has copied the packet into buf,
 	// giving us no way to find out how big the packet is.
 	// For now, we are willing to take res_search's word that there's nothing
 	// useful in the response, even though there *is* a response.
 	bufSize := maxDNSPacketSize
 	buf := (*_C_uchar)(_C_malloc(uintptr(bufSize)))
 	defer _C_free(unsafe.Pointer(buf))

 	s, err := syscall.BytePtrFromString(hostname)
 	if err != nil {
 		return nil, err
 	}

 	var size int
 	for {
 		size := _C_res_nsearch(state, (*_C_char)(unsafe.Pointer(s)), class, rtype, buf, bufSize)
 		if size <= 0 || size > 0xffff {
 			return nil, errors.New("res_nsearch failure")
 		}
 		if size <= bufSize {
 			break
 		}

 		// Allocate a bigger buffer to fit the entire msg.
 		_C_free(unsafe.Pointer(buf))
 		bufSize = size
 		buf = (*_C_uchar)(_C_malloc(uintptr(bufSize)))
 	}

 	var p dnsmessage.Parser
 	if _, err := p.Start(unsafe.Slice((*byte)(unsafe.Pointer(buf)), size)); err != nil {
 		return nil, err
 	}
 	p.SkipAllQuestions()
 	resources, err := p.AllAnswers()
 	if err != nil {
 		return nil, err
 	}
 	return resources, nil
 }
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// This file is called cgo_unix.go, but to allow syscalls-to-libc-based
	// implementations to share the code, it does not use cgo directly.
	// Instead of C.foo it uses _C_foo, which is defined in either
	// cgo_unix_cgo.go or cgo_unix_syscall.go

	//go:build !netgo && ((cgo && unix) \|\| darwin)

	package net

	import (
	"context"
	"errors"
	"internal/bytealg"
	"net/netip"
	"runtime"
	"syscall"
	"unsafe"

	"golang.org/x/net/dns/dnsmessage"
	)

	// cgoAvailable set to true to indicate that the cgo resolver
	// is available on this system.
	const cgoAvailable = true

	// An addrinfoErrno represents a getaddrinfo, getnameinfo-specific
	// error number. It's a signed number and a zero value is a non-error
	// by convention.
	type addrinfoErrno int

	func (eai addrinfoErrno) Error() string { return _C_gai_strerror(_C_int(eai)) }
	func (eai addrinfoErrno) Temporary() bool { return eai == _C_EAI_AGAIN }
	func (eai addrinfoErrno) Timeout() bool { return false }

	// isAddrinfoErrno is just for testing purposes.
	func (eai addrinfoErrno) isAddrinfoErrno() {}

	// doBlockingWithCtx executes a blocking function in a separate goroutine when the provided
	// context is cancellable. It is intended for use with calls that don't support context
	// cancellation (cgo, syscalls). blocking func may still be running after this function finishes.
	// For the duration of the execution of the blocking function, the thread is 'acquired' using [acquireThread],
	// blocking might not be executed when the context gets canceled early.
	func doBlockingWithCtx[T any](ctx context.Context, lookupName string, blocking func() (T, error)) (T, error) {
	if err := acquireThread(ctx); err != nil {
	var zero T
	return zero, &DNSError{
	Name: lookupName,
	Err: mapErr(err).Error(),
	IsTimeout: err == context.DeadlineExceeded,
	}
	}

	if ctx.Done() == nil {
	defer releaseThread()
	return blocking()
	}

	type result struct {
	res T
	err error
	}

	res := make(chan result, 1)
	go func() {
	defer releaseThread()
	var r result
	r.res, r.err = blocking()
	res <- r
	}()

	select {
	case r := <-res:
	return r.res, r.err
	case <-ctx.Done():
	var zero T
	return zero, &DNSError{
	Name: lookupName,
	Err: mapErr(ctx.Err()).Error(),
	IsTimeout: ctx.Err() == context.DeadlineExceeded,
	}
	}
	}

	func cgoLookupHost(ctx context.Context, name string) (hosts []string, err error) {
	addrs, err := cgoLookupIP(ctx, "ip", name)
	if err != nil {
	return nil, err
	}
	for _, addr := range addrs {
	hosts = append(hosts, addr.String())
	}
	return hosts, nil
	}

	func cgoLookupPort(ctx context.Context, network, service string) (port int, err error) {
	var hints _C_struct_addrinfo
	switch network {
	case "ip": // no hints
	case "tcp", "tcp4", "tcp6":
	*_C_ai_socktype(&hints) = _C_SOCK_STREAM
	*_C_ai_protocol(&hints) = _C_IPPROTO_TCP
	case "udp", "udp4", "udp6":
	*_C_ai_socktype(&hints) = _C_SOCK_DGRAM
	*_C_ai_protocol(&hints) = _C_IPPROTO_UDP
	default:
	return 0, &DNSError{Err: "unknown network", Name: network + "/" + service}
	}
	switch ipVersion(network) {
	case '4':
	*_C_ai_family(&hints) = _C_AF_INET
	case '6':
	*_C_ai_family(&hints) = _C_AF_INET6
	}

	return doBlockingWithCtx(ctx, network+"/"+service, func() (int, error) {
	return cgoLookupServicePort(&hints, network, service)
	})
	}

	func cgoLookupServicePort(hints *_C_struct_addrinfo, network, service string) (port int, err error) {
	cservice, err := syscall.ByteSliceFromString(service)
	if err != nil {
	return 0, &DNSError{Err: err.Error(), Name: network + "/" + service}
	}
	// Lowercase the C service name.
	for i, b := range cservice[:len(service)] {
	cservice[i] = lowerASCII(b)
	}
	var res *_C_struct_addrinfo
	gerrno, err := _C_getaddrinfo(nil, (*_C_char)(unsafe.Pointer(&cservice[0])), hints, &res)
	if gerrno != 0 {
	switch gerrno {
	case _C_EAI_SYSTEM:
	if err == nil { // see golang.org/issue/6232
	err = syscall.EMFILE
	}
	return 0, newDNSError(err, network+"/"+service, "")
	case _C_EAI_SERVICE, _C_EAI_NONAME: // Darwin returns EAI_NONAME.
	return 0, newDNSError(errUnknownPort, network+"/"+service, "")
	default:
	return 0, newDNSError(addrinfoErrno(gerrno), network+"/"+service, "")
	}
	}
	defer _C_freeaddrinfo(res)

	for r := res; r != nil; r = *_C_ai_next(r) {
	switch *_C_ai_family(r) {
	case _C_AF_INET:
	sa := (syscall.RawSockaddrInet4)(unsafe.Pointer(_C_ai_addr(r)))
	p := (*[2]byte)(unsafe.Pointer(&sa.Port))
	return int(p[0])<<8 \| int(p[1]), nil
	case _C_AF_INET6:
	sa := (syscall.RawSockaddrInet6)(unsafe.Pointer(_C_ai_addr(r)))
	p := (*[2]byte)(unsafe.Pointer(&sa.Port))
	return int(p[0])<<8 \| int(p[1]), nil
	}
	}
	return 0, newDNSError(errUnknownPort, network+"/"+service, "")
	}

	func cgoLookupHostIP(network, name string) (addrs []IPAddr, err error) {
	var hints _C_struct_addrinfo
	*_C_ai_flags(&hints) = cgoAddrInfoFlags
	*_C_ai_socktype(&hints) = _C_SOCK_STREAM
	*_C_ai_family(&hints) = _C_AF_UNSPEC
	switch ipVersion(network) {
	case '4':
	*_C_ai_family(&hints) = _C_AF_INET
	case '6':
	*_C_ai_family(&hints) = _C_AF_INET6
	}

	h, err := syscall.BytePtrFromString(name)
	if err != nil {
	return nil, &DNSError{Err: err.Error(), Name: name}
	}
	var res *_C_struct_addrinfo
	gerrno, err := _C_getaddrinfo((*_C_char)(unsafe.Pointer(h)), nil, &hints, &res)
	if gerrno != 0 {
	switch gerrno {
	case _C_EAI_SYSTEM:
	if err == nil {
	// err should not be nil, but sometimes getaddrinfo returns
	// gerrno == _C_EAI_SYSTEM with err == nil on Linux.
	// The report claims that it happens when we have too many
	// open files, so use syscall.EMFILE (too many open files in system).
	// Most system calls would return ENFILE (too many open files),
	// so at the least EMFILE should be easy to recognize if this
	// comes up again. golang.org/issue/6232.
	err = syscall.EMFILE
	}
	return nil, newDNSError(err, name, "")
	case _C_EAI_NONAME, _C_EAI_NODATA:
	return nil, newDNSError(errNoSuchHost, name, "")
	case _C_EAI_ADDRFAMILY:
	if runtime.GOOS == "freebsd" {
	// FreeBSD began returning EAI_ADDRFAMILY for valid hosts without
	// an A record in 13.2. We previously returned "no such host" for
	// this case.
	//
	// https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273912
	return nil, newDNSError(errNoSuchHost, name, "")
	}
	fallthrough
	default:
	return nil, newDNSError(addrinfoErrno(gerrno), name, "")
	}

	}
	defer _C_freeaddrinfo(res)

	for r := res; r != nil; r = *_C_ai_next(r) {
	// We only asked for SOCK_STREAM, but check anyhow.
	if *_C_ai_socktype(r) != _C_SOCK_STREAM {
	continue
	}
	switch *_C_ai_family(r) {
	case _C_AF_INET:
	sa := (syscall.RawSockaddrInet4)(unsafe.Pointer(_C_ai_addr(r)))
	addr := IPAddr{IP: copyIP(sa.Addr[:])}
	addrs = append(addrs, addr)
	case _C_AF_INET6:
	sa := (syscall.RawSockaddrInet6)(unsafe.Pointer(_C_ai_addr(r)))
	addr := IPAddr{IP: copyIP(sa.Addr[:]), Zone: zoneCache.name(int(sa.Scope_id))}
	addrs = append(addrs, addr)
	}
	}
	return addrs, nil
	}

	func cgoLookupIP(ctx context.Context, network, name string) (addrs []IPAddr, err error) {
	return doBlockingWithCtx(ctx, name, func() ([]IPAddr, error) {
	return cgoLookupHostIP(network, name)
	})
	}

	// These are roughly enough for the following:
	//
	// Source Encoding Maximum length of single name entry
	// Unicast DNS ASCII or <=253 + a NUL terminator
	// Unicode in RFC 5892 252 * total number of labels + delimiters + a NUL terminator
	// Multicast DNS UTF-8 in RFC 5198 or <=253 + a NUL terminator
	// the same as unicast DNS ASCII <=253 + a NUL terminator
	// Local database various depends on implementation
	const (
	nameinfoLen = 64
	maxNameinfoLen = 4096
	)

	func cgoLookupPTR(ctx context.Context, addr string) (names []string, err error) {
	ip, err := netip.ParseAddr(addr)
	if err != nil {
	return nil, &DNSError{Err: "invalid address", Name: addr}
	}
	sa, salen := cgoSockaddr(IP(ip.AsSlice()), ip.Zone())
	if sa == nil {
	return nil, &DNSError{Err: "invalid address " + ip.String(), Name: addr}
	}

	return doBlockingWithCtx(ctx, addr, func() ([]string, error) {
	return cgoLookupAddrPTR(addr, sa, salen)
	})
	}

	func cgoLookupAddrPTR(addr string, sa *_C_struct_sockaddr, salen _C_socklen_t) (names []string, err error) {
	var gerrno int
	var b []byte
	for l := nameinfoLen; l <= maxNameinfoLen; l *= 2 {
	b = make([]byte, l)
	gerrno, err = cgoNameinfoPTR(b, sa, salen)
	if gerrno == 0 \|\| gerrno != _C_EAI_OVERFLOW {
	break
	}
	}
	if gerrno != 0 {
	switch gerrno {
	case _C_EAI_SYSTEM:
	if err == nil { // see golang.org/issue/6232
	err = syscall.EMFILE
	}
	return nil, newDNSError(err, addr, "")
	case _C_EAI_NONAME:
	return nil, newDNSError(errNoSuchHost, addr, "")
	default:
	return nil, newDNSError(addrinfoErrno(gerrno), addr, "")
	}
	}
	if i := bytealg.IndexByte(b, 0); i != -1 {
	b = b[:i]
	}
	return []string{absDomainName(string(b))}, nil
	}

	func cgoSockaddr(ip IP, zone string) (*_C_struct_sockaddr, _C_socklen_t) {
	if ip4 := ip.To4(); ip4 != nil {
	return cgoSockaddrInet4(ip4), _C_socklen_t(syscall.SizeofSockaddrInet4)
	}
	if ip6 := ip.To16(); ip6 != nil {
	return cgoSockaddrInet6(ip6, zoneCache.index(zone)), _C_socklen_t(syscall.SizeofSockaddrInet6)
	}
	return nil, 0
	}

	func cgoLookupCNAME(ctx context.Context, name string) (cname string, err error, completed bool) {
	resources, err := resSearch(ctx, name, int(dnsmessage.TypeCNAME), int(dnsmessage.ClassINET))
	if err != nil {
	return
	}
	cname, err = parseCNAMEFromResources(resources)
	if err != nil {
	return "", err, false
	}
	return cname, nil, true
	}

	// resSearch will make a call to the 'res_nsearch' routine in the C library
	// and parse the output as a slice of DNS resources.
	func resSearch(ctx context.Context, hostname string, rtype, class int) ([]dnsmessage.Resource, error) {
	return doBlockingWithCtx(ctx, hostname, func() ([]dnsmessage.Resource, error) {
	return cgoResSearch(hostname, rtype, class)
	})
	}

	func cgoResSearch(hostname string, rtype, class int) ([]dnsmessage.Resource, error) {
	resStateSize := unsafe.Sizeof(_C_struct___res_state{})
	var state *_C_struct___res_state
	if resStateSize > 0 {
	mem := _C_malloc(resStateSize)
	defer _C_free(mem)
	memSlice := unsafe.Slice((*byte)(mem), resStateSize)
	clear(memSlice)
	state = (*_C_struct___res_state)(unsafe.Pointer(&memSlice[0]))
	}
	if err := _C_res_ninit(state); err != nil {
	return nil, errors.New("res_ninit failure: " + err.Error())
	}
	defer _C_res_nclose(state)

	// Some res_nsearch implementations (like macOS) do not set errno.
	// They set h_errno, which is not per-thread and useless to us.
	// res_nsearch returns the size of the DNS response packet.
	// But if the DNS response packet contains failure-like response codes,
	// res_search returns -1 even though it has copied the packet into buf,
	// giving us no way to find out how big the packet is.
	// For now, we are willing to take res_search's word that there's nothing
	// useful in the response, even though there is a response.
	bufSize := maxDNSPacketSize
	buf := (*_C_uchar)(_C_malloc(uintptr(bufSize)))
	defer _C_free(unsafe.Pointer(buf))

	s, err := syscall.BytePtrFromString(hostname)
	if err != nil {
	return nil, err
	}

	var size int
	for {
	size := _C_res_nsearch(state, (*_C_char)(unsafe.Pointer(s)), class, rtype, buf, bufSize)
	if size <= 0 \|\| size > 0xffff {
	return nil, errors.New("res_nsearch failure")
	}
	if size <= bufSize {
	break
	}

	// Allocate a bigger buffer to fit the entire msg.
	_C_free(unsafe.Pointer(buf))
	bufSize = size
	buf = (*_C_uchar)(_C_malloc(uintptr(bufSize)))
	}

	var p dnsmessage.Parser
	if _, err := p.Start(unsafe.Slice((*byte)(unsafe.Pointer(buf)), size)); err != nil {
	return nil, err
	}
	p.SkipAllQuestions()
	resources, err := p.AllAnswers()
	if err != nil {
	return nil, err
	}
	return resources, nil
	}