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GETNAMEINFO(3)             Linux Programmer's ManualGETNAMEINFO(3)NAME       getnameinfo  - address-to-name translation in protocol-independent man-       nerSYNOPSIS       #include <sys/socket.h>       #include <netdb.h>       int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,                       char *host, socklen_t hostlen,                       char *serv, socklen_t servlen, int flags);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):       getnameinfo():           Since glibc 2.22: _POSIX_C_SOURCE >= 201112L           Glibc 2.21 and earlier: _POSIX_C_SOURCEDESCRIPTION       The getnameinfo() function is the inverse ofgetaddrinfo(3):  it  con-       verts a socket address to a corresponding host and service, in a proto-       col-independent manner.  It  combines  the  functionality  of  gethost-byaddr(3)  andgetservbyport(3),  but unlike those functions, getname-       info() is reentrant and allows programs to  eliminate  IPv4-versus-IPv6       dependencies.       The  addr  argument  is a pointer to a generic socket address structure       (of type sockaddr_in or sockaddr_in6) of size addrlen  that  holds  the       input  IP  address  and  port  number.  The arguments host and serv are       pointers to caller-allocated buffers (of size hostlen and  servlen  re-       spectively)  into  which  getnameinfo()  places null-terminated strings       containing the host and service names respectively.       The caller can specify that no hostname (or no  service  name)  is  re-       quired  by  providing  a NULL host (or serv) argument or a zero hostlen       (or servlen) argument.  However, at least one of  hostname  or  service       name must be requested.       The flags argument modifies the behavior of getnameinfo() as follows:       NI_NAMEREQD              If  set, then an error is returned if the hostname cannot be de-              termined.       NI_DGRAM              If set, then the service is datagram  (UDP)  based  rather  than              stream  (TCP)  based.   This  is  required  for  the  few  ports              (512-514) that have different services for UDP and TCP.       NI_NOFQDN              If set, return only the hostname part of the fully qualified do-              main name for local hosts.       NI_NUMERICHOST              If  set,  then  the  numeric  form  of the hostname is returned.              (When not set, this will still happen in case  the  node's  name              cannot be determined.)       NI_NUMERICSERV              If  set,  then  the  numeric  form of the service address is re-              turned.  (When not set, this will still happen in case the  ser-              vice's name cannot be determined.)   Extensions to getnameinfo() for Internationalized Domain Names       Starting  with  glibc  2.3.4, getnameinfo() has been extended to selec-       tively allow hostnames to be transparently converted to  and  from  the       Internationalized  Domain Name (IDN) format (see RFC 3490, Internation-       alizing Domain Names in Applications (IDNA)).  Three new flags are  de-       fined:       NI_IDN If  this flag is used, then the name found in the lookup process              is converted from IDN format to the locale's encoding if  neces-              sary.   ASCII-only  names  are  not  affected by the conversion,              which makes this flag usable in existing programs  and  environ-              ments.       NI_IDN_ALLOW_UNASSIGNED, NI_IDN_USE_STD3_ASCII_RULES              Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow              unassigned Unicode code  points)  and  IDNA_USE_STD3_ASCII_RULES              (check  output  to  make  sure it is a STD3 conforming hostname)              flags respectively to be used in the IDNA handling.RETURN VALUE       On success, 0 is returned, and node and service  names,  if  requested,       are  filled with null-terminated strings, possibly truncated to fit the       specified buffer lengths.  On error, one of the following nonzero error       codes is returned:       EAI_AGAIN              The name could not be resolved at this time.  Try again later.       EAI_BADFLAGS              The flags argument has an invalid value.       EAI_FAIL              A nonrecoverable error occurred.       EAI_FAMILY              The address family was not recognized, or the address length was              invalid for the specified family.       EAI_MEMORY              Out of memory.       EAI_NONAME              The  name  does  not  resolve  for   the   supplied   arguments.              NI_NAMEREQD  is  set  and  the host's name cannot be located, or              neither hostname nor service name were requested.       EAI_OVERFLOW              The buffer pointed to by host or serv was too small.       EAI_SYSTEM              A system error occurred.  The error code can be found in errno.       Thegai_strerror(3) function translates these error codes  to  a  human       readable string, suitable for error reporting.FILES       /etc/hosts       /etc/nsswitch.conf       /etc/resolv.confVERSIONS       getnameinfo() is provided in glibc since version 2.1.ATTRIBUTES       For  an  explanation  of  the  terms  used  in  this  section,  see at-tributes(7).       +--------------+---------------+--------------------+       |Interface     | Attribute     | Value              |       +--------------+---------------+--------------------+       |getnameinfo() | Thread safety | MT-Safe env locale |       +--------------+---------------+--------------------+CONFORMING TO       POSIX.1-2001, POSIX.1-2008, RFC 2553.NOTES       In order to assist the programmer in choosing reasonable sizes for  the       supplied buffers, <netdb.h> defines the constants           #define NI_MAXHOST      1025           #define NI_MAXSERV      32       Since glibc 2.8, these definitions are exposed only if suitable feature       test macros are defined, namely:  _GNU_SOURCE,  _DEFAULT_SOURCE  (since       glibc   2.19),  or  (in  glibc  versions  up  to  and  including  2.19)       _BSD_SOURCE or _SVID_SOURCE.       The former is the  constant  MAXDNAME  in  recent  versions  of  BIND's       <arpa/nameser.h>  header file.  The latter is a guess based on the ser-       vices listed in the current Assigned Numbers RFC.       Before glibc version 2.2, the hostlen and servlen arguments were  typed       as size_t.EXAMPLE       The  following code tries to get the numeric hostname and service name,       for a given socket address.  Note that there is no hardcoded  reference       to a particular address family.           struct sockaddr *addr;     /* input */           socklen_t addrlen;         /* input */           char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];           if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf,                       sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)               printf("host=%s, serv=%s\n", hbuf, sbuf);       The  following  version  checks if the socket address has a reverse ad-       dress mapping.           struct sockaddr *addr;     /* input */           socklen_t addrlen;         /* input */           char hbuf[NI_MAXHOST];           if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf),                       NULL, 0, NI_NAMEREQD))               printf("could not resolve hostname");           else               printf("host=%s\n", hbuf);       An example program using getnameinfo() can be found ingetaddrinfo(3).SEE ALSOaccept(2),getpeername(2),getsockname(2),recvfrom(2),socket(2),getaddrinfo(3),gethostbyaddr(3),getservbyname(3),getservbyport(3),inet_ntop(3),hosts(5),services(5),hostname(7),named(8)       R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic  Socket  Inter-       face Extensions for IPv6, RFC 2553, March 1999.       Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped       Addresses,  internet  draft,  work  in   progress   <ftp://ftp.ietf.org       /internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt>.       Craig Metz, Protocol Independence Using the Sockets API, Proceedings of       the freenix track: 2000 USENIX annual technical conference, June 2000       <http://www.usenix.org/publications/library/proceedings/usenix2000       /freenix/metzprotocol.html>.COLOPHON       This page is part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs, and the       latest version of this page, can be found at       https://www.kernel.org/doc/man-pages/.GNU                               2019-03-06GETNAMEINFO(3)