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GETHOSTBYNAME   (3) manpage
GETHOSTBYNAME
3
2002-08-05
Linux Programmer's Manual
  • NAME
      gethostbyname, gethostbyaddr, sethostent, endhostent, herror, hstrerror - get network host entry
  • SYNOPSIS 
      
 #include <netdb.h>
 extern int h_errno;

 struct hostent *gethostbyname(const char * name );


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

          void sethostent(int  stayopen ); 

          void endhostent(void); 

          void herror(const char * s ); 

          const char *hstrerror(int  err ); 

        /* GNU extensions */
      


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

          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 );
  • DESCRIPTION
      The gethostbyname() function returns a structure of type hostent
      for the given host name.  Here name is either a host name, or
      an IPv4 address in standard dot notation, 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 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.  (See
      resolv+ (8)).
       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.
      h_aliases
      A zero-terminated array of alternative names for the host.
      h_addrtype
      The type of address; always
      AF_INET or
      AF_INET6 at present.
      h_length
      The length of the address in bytes.
      h_addr_list
      A zero-terminated array of network addresses for the host in network byte
      order.
      h_addr
      The first address in h_addr_list for backward compatibility.
  • RETURN VALUE
      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.
  • ERRORS
      The variable h_errno can have the following values:
      HOST_NOT_FOUND
      The specified host is unknown.
      NO_ADDRESS " or " NO_DATA
      The requested name is valid but does not have an IP address.
      NO_RECOVERY
      A non-recoverable name server error occurred.
      TRY_AGAIN
      A temporary error occurred on an authoritative name server.  Try again
      later.
  • FILES
      /etc/host.conf
      resolver configuration file
      /etc/hosts
      host database file
  • CONFORMING TO
      BSD 4.3.
  • NOTES
      The SUS-v2 standard is buggy and declares the
      len
       parameter of
      gethostbyaddr() to be of type
      size_t .
       (That is wrong, because it has to be
      int ,
       and
      size_t
       is not. POSIX 1003.1-2001 makes it
      socklen_t ,
       which is OK.)

      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.

      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
      gethostbyname_r() and
      gethostbyname2_r() .
       These return 0 on success and nonzero on error. The result of the call
      is now stored in the struct with address
      ret .
       After the call,
      * result will be NULL on error or point to the result on success.
      Auxiliary data is stored in the buffer
      buf
       of length
      buflen .
       (If the buffer is too small, these functions will return
      ERANGE .)
       No global variable
      h_errno
       is modified, but the address of a variable in which to store error numbers
      is passed in
      h_errnop .


      POSIX 1003.1-2001 marks
      gethostbyaddr() and
      gethostbyname() obsolescent. See
      getaddrinfo(3) ,
       getnameinfo(3) ,
       gai_strerror(3) .
  • SEE ALSO
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