Provided by: manpages-dev_4.04-2_all bug


       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(),
           _BSD_SOURCE || _SVID_SOURCE

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

           Since glibc 2.12:
               _BSD_SOURCE || _SVID_SOURCE ||
                   (_POSIX_C_SOURCE < 200809L && _XOPEN_SOURCE < 700)
           Before glibc 2.12:


       The   gethostbyname*(),  gethostbyaddr*(),  herror(),  and  hstrerror()
       functions  are  obsolete.   Applications  should  use   getaddrinfo(3),
       getnameinfo(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
       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() 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
       order 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
              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.
              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

       │InterfaceAttributeValue                         │
       │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(3),   gethostent(3),   gethostent_r(3),   or
       endhostent(3)  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(),
       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,  which 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 4.04 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

                                  2015-07-23                  GETHOSTBYNAME(3)