Provided by: manpages-dev_5.05-1_all bug


       gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,  h_errno,  herror,
       hstrerror,    gethostbyaddr_r,    gethostbyname2,    gethostbyname2_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() 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 pointer.

              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 NO_DATA.

              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 attributes(7).

       │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(), gethostent(), gethostent_r(),  or  endhostent()  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)


       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 argument.

   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 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)