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


     gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,
     h_errno, 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
     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  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
     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  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
     allbox;  lbw18 lb lbw29 l l l.  Interface Attribute Value T{ gethostby-
     name() T}   Thread safety  T{ MT-Unsafe race:hostbyname env
     locale  T}  T{   gethostbyaddr()   T}   Thread   safety  T{   MT-Unsafe
     race:hostbyaddr env
     locale T} T{ sethostent(),
     gethostent_r() T}   Thread safety  T{ MT-Unsafe race:hostent env
     locale T} T{ herror(),
     hstrerror()  T}   Thread  safety  MT-Safe  T{  gethostent() T}   Thread
     safety  T{ MT-Unsafe race:hostent
     race:hostentbuf  env  locale   T}   T{   gethostbyname2()   T}   Thread
     safety  T{ MT-Unsafe race:hostbyname2
     env  locale  T}  T{  gethostbyaddr_r(),  gethostbyname_r(),  gethostby-
     name2_r() T}   Thread safety  MT-Safe env locale
     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
     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  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
     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 4.16 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)
/data/webs/external/dokuwiki/data/pages/man/gethostent.txt · Last modified: 2019/05/17 09:32 by

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