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GETHOSTBYADDR(3P)          POSIX Programmer's Manual         GETHOSTBYADDR(3P)

       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

       gethostbyaddr, gethostbyname - network host database functions

       #include <netdb.h>

       struct hostent *gethostbyaddr(const void *addr, socklen_t len,
              int type);
       struct hostent *gethostbyname(const char *name);

       These  functions  shall  retrieve  information  about  hosts.  This information is considered to be stored in a
       database that can be accessed sequentially or randomly. Implementation of this database is unspecified.

       Note:  In many cases it is implemented by the Domain Name System, as  documented  in  RFC 1034,  RFC 1035,  and
              RFC 1886.

       Entries shall be returned in hostent structures.

       The  gethostbyaddr()  function  shall  return an entry containing addresses of address family type for the host
       with address addr. The len argument contains the length of the address pointed to by addr. The  gethostbyaddr()
       function  need  not be reentrant. A function that is not required to be reentrant is not required to be thread-

       The gethostbyname() function shall return an entry containing addresses of address family AF_INET for the  host
       with  name  name.  The  gethostbyname()  function need not be reentrant.  A function that is not required to be
       reentrant is not required to be thread-safe.

       The addr argument of gethostbyaddr() shall be an in_addr structure when type is AF_INET. It contains  a  binary
       format  (that is, not null-terminated) address in network byte order. The gethostbyaddr() function is not guar-
       anteed to return addresses of address families other than AF_INET,  even  when  such  addresses  exist  in  the

       If  gethostbyaddr() returns successfully, then the h_addrtype field in the result shall be the same as the type
       argument that was passed to the function, and the h_addr_list field shall list a single address that is a  copy
       of the addr argument that was passed to the function.

       The  name  argument  of  gethostbyname()  shall  be  a node name; the behavior of gethostbyname() when passed a
       numeric address string is unspecified. For IPv4, a numeric address string shall be in the dotted-decimal  nota-
       tion described in inet_addr() .

       If  name  is  not  a  numeric  address string and is an alias for a valid host name, then gethostbyname() shall
       return information about the host name to which the alias refers, and name shall be included  in  the  list  of
       aliases returned.

       Upon  successful  completion,  these  functions  shall return a pointer to a hostent structure if the requested
       entry was found, and a null pointer if the end of the database was reached  or  the  requested  entry  was  not

       Upon unsuccessful completion, gethostbyaddr() and gethostbyname() shall set h_errno to indicate the error.

       These  functions shall fail in the following cases. The gethostbyaddr() and gethostbyname() functions shall set
       h_errno to the value shown in the list below. Any changes to errno are unspecified.


              No such host is known.

              The server recognized the request and the name, but no address is available. Another type of request  to
              the name server for the domain might return an answer.


              An unexpected server failure occurred which cannot be recovered.

              A temporary and possibly transient error occurred, such as a failure of a server to respond.

       The following sections are informative.


       The  gethostbyaddr() and gethostbyname() functions may return pointers to static data, which may be overwritten
       by subsequent calls to any of these functions.

       The getaddrinfo() and getnameinfo() functions are preferred over the gethostbyaddr() and gethostbyname()  func-


       The gethostbyaddr() and gethostbyname() functions may be withdrawn in a future version.

       endhostent(),  endservent(), gai_strerror(), getaddrinfo(), h_errno(), inet_addr(), the Base Definitions volume
       of IEEE Std 1003.1-2001, <netdb.h>

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,  Inc  and  The
       Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Stan-
       dard, the original IEEE and The Open Group Standard is the referee  document.  The  original  Standard  can  be
       obtained online at .

IEEE/The Open Group                  2003                    GETHOSTBYADDR(3P)