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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |BUGS |SEE ALSO |COLOPHON | |
gethostid(3) Library Functions Manualgethostid(3)gethostid, sethostid - get or set the unique identifier of the current host
Standard C library (libc,-lc)
#include <unistd.h>long gethostid(void);int sethostid(longhostid); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):gethostid(): Since glibc 2.20: _DEFAULT_SOURCE || _XOPEN_SOURCE >= 500 Up to and including glibc 2.19: _BSD_SOURCE || _XOPEN_SOURCE >= 500sethostid(): Since glibc 2.21: _DEFAULT_SOURCE In glibc 2.19 and 2.20: _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500) Up to and including glibc 2.19: _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
gethostid() andsethostid() respectively get or set a unique 32-bit identifier for the current machine. The 32-bit identifier was intended to be unique among all UNIX systems in existence. This normally resembles the Internet address for the local machine, as returned bygethostbyname(3), and thus usually never needs to be set. Thesethostid() call is restricted to the superuser.
gethostid() returns the 32-bit identifier for the current host as set bysethostid(). On success,sethostid() returns 0; on error, -1 is returned, anderrno is set to indicate the error.
sethostid() can fail with the following errors:EACCESThe caller did not have permission to write to the file used to store the host ID.EPERMThe calling process's effective user or group ID is not the same as its corresponding real ID.
For an explanation of the terms used in this section, seeattributes(7). ┌─────────────┬───────────────┬──────────────────────────────────┐ │Interface│Attribute│Value│ ├─────────────┼───────────────┼──────────────────────────────────┤ │gethostid() │ Thread safety │ MT-Safe hostid env locale │ ├─────────────┼───────────────┼──────────────────────────────────┤ │sethostid() │ Thread safety │ MT-Unsafe const:hostid │ └─────────────┴───────────────┴──────────────────────────────────┘
In the glibc implementation, thehostid is stored in the file/etc/hostid. (Before glibc 2.2, the file/var/adm/hostid was used.) In the glibc implementation, ifgethostid() cannot open the file containing the host ID, then it obtains the hostname usinggethostname(2), passes that hostname togethostbyname_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.)
gethostid() POSIX.1-2008.sethostid() None.
4.2BSD; dropped in 4.4BSD. SVr4 and POSIX.1-2001 includegethostid() but notsethostid().
It is impossible to ensure that the identifier is globally unique.
hostid(1),gethostbyname(3)
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HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |