Man Linux: Main Page and Category List


       gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,
       h_errno,   herror,    hstrerror,    gethostbyaddr_r,    gethostbyname2,
       gethostbyname2_r,  gethostbyname_r,  gethostent_r  -  get  network host


       #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(),
       gethostbyname2_r(): _BSD_SOURCE || _SVID_SOURCE

       herror(), hstrerror() (since glibc 2.8): _BSD_SOURCE || _SVID_SOURCE ||


       The  gethostbyname*()  and  gethostbyaddr*()  functions  are  obsolete.
       Applications should use getaddrinfo(3) and getnameinfo(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)), or an IPv6
       address in colon (and possibly dot) notation.  (See  RFC 1884  for  the
       description of IPv6 addresses.)  If name is an IPv4 or IPv6 address, 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
       example  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.
       Otherwise, 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
       gethostbyaddr() use a combination of any or  all  of  the  name  server
       named(8),   a   broken  out  line  from  /etc/hosts,  and  the  Network
       Information Service (NIS or YP), depending upon  the  contents  of  the
       order line in /etc/host.conf.  The default action is to query named(8),
       followed by /etc/hosts.

       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
              network 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.

              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


       POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(),  sethostent(),
       endhostent(),     gethostent(),     and    h_errno;    gethostbyname(),
       gethostbyaddr(), and h_errno are marked obsolescent in  that  standard.
       POSIX.1-2008    removes    the   specifications   of   gethostbyname(),
       gethostbyaddr(), and h_errno, recommending the  use  of  getaddrinfo(3)
       and getnameinfo(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, that should return the next entry
       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 library
       was built without DNS support.  The  glibc  version  will  ignore  ipv6
       entries.   This  function  is not reentrant, and glibc adds a reentrant
       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
       hostent 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  result  will  be  NULL.  The functions return 0 on success and a
       nonzero error number on failure.  In addition to the errors returned by
       the  nonreentrant versions of these functions, if buf is too small, the
       functions 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


       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 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

                                  2009-12-03                  GETHOSTBYNAME(3)