gethostid(3)

1gethostid(3)               Library Functions Manual               gethostid(3)
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NAME

6       gethostid,  sethostid - get or set the unique identifier of the current
7       host
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LIBRARY

10       Standard C library (libc, -lc)
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SYNOPSIS

13       #include <unistd.h>
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15       long gethostid(void);
16       int sethostid(long hostid);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       gethostid():
21           Since glibc 2.20:
22               _DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
23           Up to and including glibc 2.19:
24               _BSD_SOURCE || _XOPEN_SOURCE >= 500
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26       sethostid():
27           Since glibc 2.21:
28               _DEFAULT_SOURCE
29           In glibc 2.19 and 2.20:
30               _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
31           Up to and including glibc 2.19:
32               _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
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DESCRIPTION

35       gethostid() and sethostid() respectively get or  set  a  unique  32-bit
36       identifier for the current machine.  The 32-bit identifier was intended
37       to be unique among all UNIX systems in existence.  This normally resem‐
38       bles  the  Internet address for the local machine, as returned by geth‐
39       ostbyname(3), and thus usually never needs to be set.
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41       The sethostid() call is restricted to the superuser.
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RETURN VALUE

44       gethostid() returns the 32-bit identifier for the current host  as  set
45       by sethostid().
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47       On  success, sethostid() returns 0; on error, -1 is returned, and errno
48       is set to indicate the error.
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ERRORS

51       sethostid() can fail with the following errors:
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53       EACCES The caller did not have permission to write to the file used  to
54              store the host ID.
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56       EPERM  The calling process's effective user or group ID is not the same
57              as its corresponding real ID.
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ATTRIBUTES

60       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
61       tributes(7).
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63       ┌────────────┬───────────────┬─────────────────────────────────────────┐
64       │Interface   │ Attribute     │ Value                                   │
65       ├────────────┼───────────────┼─────────────────────────────────────────┤
66       │gethostid() │ Thread safety │ MT-Safe hostid env locale               │
67       ├────────────┼───────────────┼─────────────────────────────────────────┤
68       │sethostid() │ Thread safety │ MT-Unsafe const:hostid                  │
69       └────────────┴───────────────┴─────────────────────────────────────────┘
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VERSIONS

72       In  the  glibc  implementation,  the  hostid  is  stored  in  the  file
73       /etc/hostid.  (Before glibc 2.2, the file /var/adm/hostid was used.)
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75       In the glibc implementation, if gethostid() cannot open the  file  con‐
76       taining the host ID, then it obtains the hostname using gethostname(2),
77       passes that hostname to  gethostbyname_r(3)  in  order  to  obtain  the
78       host's  IPv4 address, and returns a value obtained by bit-twiddling the
79       IPv4 address.  (This value may not be unique.)
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STANDARDS

82       gethostid()
83              POSIX.1-2008.
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85       sethostid()
86              None.
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HISTORY

89       4.2BSD; dropped in 4.4BSD.  SVr4 and POSIX.1-2001  include  gethostid()
90       but not sethostid().
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BUGS

93       It is impossible to ensure that the identifier is globally unique.
94