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       gethostid,  sethostid - get or set the unique identifier of the current


       #include <unistd.h>

       long gethostid(void);
       int sethostid(long hostid);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sethostid(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)


       gethostid() and sethostid() respectively get or  set  a  unique  32-bit
       identifier  for the current machine.  The 32-bit identifier is intended
       to be unique among  all  Unix  systems  in  existence.   This  normally
       resembles  the  Internet  address for the local machine, as returned by
       gethostbyname(3), and thus usually never needs to be set.

       The sethostid() call is restricted to the superuser.


       gethostid() returns the 32-bit identifier for the current host  as  set
       by sethostid().

       On  success, sethostid() returns 0; on error, -1 is returned, and errno
       is set to indicate the error.


       sethostid() can fail with the following errors:

       EACCES The caller did not have permission to write to the file used  to
              store the host ID.

       EPERM  The calling process’s effective user or group ID is not the same
              as its corresponding real ID.


       4.2BSD;  these  functions  were  dropped  in  4.4BSD.   SVr4   includes
       gethostid()  but  not  sethostid().  POSIX.1-2001 specifies gethostid()
       but not sethostid().


       In  the  glibc  implementation,  the  hostid  is  stored  in  the  file
       /etc/hostid.   (In  glibc versions before 2.2, the file /var/adm/hostid
       was used.)

       In the glibc  implementation,  if  gethostid()  cannot  open  the  file
       containing   the   host   ID,   then  it  obtains  the  hostname  using
       gethostname(2), passes that hostname to gethostbyname_r(3) in order  to
       obtain  the  host’s  IPv4 address, and returns a value obtained by bit-
       twiddling the IPv4 address.  (This value may not be unique.)


       It is impossible to ensure that the identifier is globally unique.


       hostid(1), gethostbyname(3)


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