GETHOSTBYNAME(3)           Linux Programmer's Manual          GETHOSTBYNAME(3)

       gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_er-
       rno, herror,  hstrerror,  gethostbyaddr_r,  gethostbyname2,  gethostby-
       name2_r, gethostbyname_r, gethostent_r - get network host entry

       #include <netdb.h>
       extern int h_errno;

       struct hostent *gethostbyname(const char *name);

       #include <sys/socket.h>       /* for AF_INET */
       struct hostent *gethostbyaddr(const void *addr,
                                     socklen_t len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char *hstrerror(int err);

       /* System V/POSIX extension */
       struct hostent *gethostent(void);

       /* GNU extensions */
       struct hostent *gethostbyname2(const char *name, int af);

       int gethostent_r(
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

       int gethostbyaddr_r(const void *addr, socklen_t len, int type,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

       int gethostbyname_r(const char *name,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

       int gethostbyname2_r(const char *name, int af,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(),
           Since glibc 2.19:
           Glibc versions up to and including 2.19:
               _BSD_SOURCE || _SVID_SOURCE

       herror(), hstrerror():
           Since glibc 2.19:
           Glibc 2.8 to 2.19:
               _BSD_SOURCE || _SVID_SOURCE
           Before glibc 2.8:

           Since glibc 2.19
               _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
           Glibc 2.12 to 2.19:
               _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
           Before glibc 2.12:

       The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() func-
       tions  are  obsolete.  Applications should use getaddrinfo(3), getname-
       info(3), and gai_strerror(3) instead.

       The gethostbyname() function returns a structure of  type  hostent  for
       the given host name.  Here name is either a hostname or an IPv4 address
       in standard dot notation (as for inet_addr(3)).  If name is an IPv4 ad-
       dress,  no  lookup  is performed and gethostbyname() simply copies name
       into the h_name field  and  its  struct  in_addr  equivalent  into  the
       h_addr_list[0]  field  of  the  returned  hostent  structure.   If name
       doesn't end in a dot and the environment variable HOSTALIASES  is  set,
       the  alias  file  pointed  to by HOSTALIASES will first be searched for
       name (see hostname(7) for the file format).  The current domain and its
       parents are searched unless name ends in a dot.

       The  gethostbyaddr()  function  returns a structure of type hostent for
       the given host address addr of length len and address type type.  Valid
       address types are AF_INET and AF_INET6.  The host address argument is a
       pointer to a struct of a type depending on the address type, for  exam-
       ple  a  struct in_addr * (probably obtained via a call to inet_addr(3))
       for address type AF_INET.

       The sethostent() function specifies, if stayopen is true  (1),  that  a
       connected  TCP  socket  should  be used for the name server queries and
       that the connection should remain open during successive queries.  Oth-
       erwise, name server queries will use UDP datagrams.

       The  endhostent()  function  ends  the use of a TCP connection for name
       server queries.

       The (obsolete) herror() function prints the  error  message  associated
       with the current value of h_errno on stderr.

       The  (obsolete)  hstrerror()  function takes an error number (typically
       h_errno) and returns the corresponding message string.

       The domain name queries carried out  by  gethostbyname()  and  gethost-
       byaddr()  rely on the Name Service Switch (nsswitch.conf(5)) configured
       sources or a local name server (named(8)).  The default  action  is  to
       query  the  Name  Service Switch (nsswitch.conf(5)) configured sources,
       failing that, a local name server (named(8)).

       The nsswitch.conf(5) file is the modern way of controlling the order of
       host lookups.

       In glibc 2.4 and earlier, the order keyword was used to control the or-
       der of host lookups as defined in /etc/host.conf (host.conf(5)).

       The hostent structure is defined in <netdb.h> as follows:

           struct hostent {
               char  *h_name;            /* official name of host */
               char **h_aliases;         /* alias list */
               int    h_addrtype;        /* host address type */
               int    h_length;          /* length of address */
               char **h_addr_list;       /* list of addresses */
           #define h_addr h_addr_list[0] /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name of the host.

              An array of alternative names for the host, terminated by a null

              The type of address; always AF_INET or AF_INET6 at present.

              The length of the address in bytes.

              An  array of pointers to network addresses for the host (in net-
              work byte order), terminated by a null pointer.

       h_addr The first address in h_addr_list for backward compatibility.

       The gethostbyname() and gethostbyaddr() functions  return  the  hostent
       structure  or a null pointer if an error occurs.  On error, the h_errno
       variable holds an error number.  When non-NULL, the  return  value  may
       point at static data, see the notes below.

       The variable h_errno can have the following values:

              The specified host is unknown.

              The  requested  name  is  valid but does not have an IP address.
              Another type of request to the name server for this  domain  may
              return  an  answer.   The  constant  NO_ADDRESS is a synonym for

              A nonrecoverable name server error occurred.

              A temporary error occurred on an authoritative name server.  Try
              again later.

              resolver configuration file

              host database file

              name service switch configuration

       For  an  explanation  of  the  terms  used  in  this  section,  see at-

       |Interface          | Attribute     | Value                         |
       |gethostbyname()    | Thread safety | MT-Unsafe race:hostbyname env |
       |                   |               | locale                        |
       |gethostbyaddr()    | Thread safety | MT-Unsafe race:hostbyaddr env |
       |                   |               | locale                        |
       |sethostent(),      | Thread safety | MT-Unsafe race:hostent env    |
       |endhostent(),      |               | locale                        |
       |gethostent_r()     |               |                               |
       |herror(),          | Thread safety | MT-Safe                       |
       |hstrerror()        |               |                               |
       |gethostent()       | Thread safety | MT-Unsafe race:hostent        |
       |                   |               | race:hostentbuf env locale    |
       |gethostbyname2()   | Thread safety | MT-Unsafe race:hostbyname2    |
       |                   |               | env locale                    |
       |gethostbyaddr_r(), | Thread safety | MT-Safe env locale            |
       |gethostbyname_r(), |               |                               |
       |gethostbyname2_r() |               |                               |
       In the above table, hostent in race:hostent signifies that  if  any  of
       the  functions  sethostent(),  gethostent(), gethostent_r(), or endhos-
       tent() are used in parallel in different threads  of  a  program,  then
       data races could occur.

       POSIX.1-2001  specifies gethostbyname(), gethostbyaddr(), sethostent(),
       endhostent(),  gethostent(),  and  h_errno;  gethostbyname(),  gethost-
       byaddr(),   and  h_errno  are  marked  obsolescent  in  that  standard.
       POSIX.1-2008 removes the specifications  of  gethostbyname(),  gethost-
       byaddr(),  and h_errno, recommending the use of getaddrinfo(3) and get-
       nameinfo(3) instead.

       The functions gethostbyname() and gethostbyaddr() may  return  pointers
       to  static  data, which may be overwritten by later calls.  Copying the
       struct hostent does not suffice, since it  contains  pointers;  a  deep
       copy is required.

       In  the original BSD implementation the len argument of gethostbyname()
       was an int.  The SUSv2 standard is buggy and declares the len  argument
       of  gethostbyaddr()  to  be of type size_t.  (That is wrong, because it
       has to be int, and size_t is not.   POSIX.1-2001  makes  it  socklen_t,
       which is OK.)  See also accept(2).

       The  BSD  prototype for gethostbyaddr() uses const char * for the first

   System V/POSIX extension
       POSIX requires the gethostent() call, which should return the next  en-
       try in the host data base.  When using DNS/BIND this does not make much
       sense, but it may be reasonable if the host data base is  a  file  that
       can  be  read  line  by  line.  On many systems, a routine of this name
       reads from the file /etc/hosts.  It may be available only when the  li-
       brary  was  built  without  DNS support.  The glibc version will ignore
       ipv6 entries.  This function is not reentrant, and glibc adds  a  reen-
       trant version gethostent_r().

   GNU extensions
       Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
       permits to specify the address family to which the address must belong.

       Glibc2 also has reentrant versions  gethostent_r(),  gethostbyaddr_r(),
       gethostbyname_r()  and  gethostbyname2_r().  The caller supplies a hos-
       tent structure ret which will be filled in on success, and a  temporary
       work  buffer  buf of size buflen.  After the call, result will point to
       the result on success.  In case of an error or if no entry is found re-
       sult will be NULL.  The functions return 0 on success and a nonzero er-
       ror number on failure.  In addition to the errors returned by the  non-
       reentrant  versions  of these functions, if buf is too small, the func-
       tions will return ERANGE, and the call should be retried with a  larger
       buffer.   The  global variable h_errno is not modified, but the address
       of a variable in which to store error numbers is passed in h_errnop.

       gethostbyname() does not recognize components of a dotted IPv4  address
       string that are expressed in hexadecimal.

       getaddrinfo(3),  getnameinfo(3),  inet(3),  inet_ntop(3), inet_pton(3),
       resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

       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

                                  2017-09-15                  GETHOSTBYNAME(3)
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