man-pages/man2/gethostname.2

.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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.\" Modified 1995-07-22 by Michael Chastain <mec@duracef.shout.net>:
.\"   'gethostname' is real system call on Linux/Alpha.
.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2000-06-04, 2001-12-15 by aeb
.\" Modified 2004-06-17 by mtk
.\" Modified 2008-11-27 by mtk
.\"
.TH GETHOSTNAME 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
gethostname, sethostname \- get/set hostname
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int gethostname(char *" name ", size_t " len );
.BI "int sethostname(const char *" name ", size_t " len );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR gethostname ():
.nf
    _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
        || /* Glibc 2.19 and earlier */ _BSD_SOURCE
.\" The above is something of a simplification
.\" also in glibc before 2.3 there was a bit churn
.fi
.PP
.BR sethostname ():
.nf
    Since glibc 2.21:
.\"		commit 266865c0e7b79d4196e2cc393693463f03c90bd8
        _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)
.fi
.SH DESCRIPTION
These system calls are used to access or to change the system hostname.
More precisely, they operate on the hostname associated with the calling
process's UTS namespace.
.PP
.BR sethostname ()
sets the hostname to the value given in the character array
.IR name .
The
.I len
argument specifies the number of bytes in
.IR name .
(Thus,
.I name
does not require a terminating null byte.)
.PP
.BR gethostname ()
returns the null-terminated hostname in the character array
.IR name ,
which has a length of
.I len
bytes.
If the null-terminated hostname is too large to fit,
then the name is truncated, and no error is returned (but see NOTES below).
POSIX.1 says that if such truncation occurs,
then it is unspecified whether the returned buffer
includes a terminating null byte.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
.I name
is an invalid address.
.TP
.B EINVAL
.I len
is negative
.\" Can't occur for gethostbyname() wrapper, since 'len' has an
.\" unsigned type; can occur for the underlying system call.
or, for
.BR sethostname (),
.I len
is larger than the maximum allowed size.
.TP
.B ENAMETOOLONG
.RB "(glibc " gethostname ())
.I len
is smaller than the actual size.
(Before version 2.1, glibc uses
.BR EINVAL
for this case.)
.TP
.B EPERM
For
.BR sethostname (),
the caller did not have the
.B CAP_SYS_ADMIN
capability in the user namespace associated with its UTS namespace (see
.BR namespaces (7)).
.SH CONFORMING TO
SVr4, 4.4BSD  (these interfaces first appeared in 4.2BSD).
POSIX.1-2001 and POSIX.1-2008 specify
.BR gethostname ()
but not
.BR sethostname ().
.SH NOTES
SUSv2 guarantees that "Host names are limited to 255 bytes".
POSIX.1 guarantees that "Host names (not including
the terminating null byte) are limited to
.B HOST_NAME_MAX
bytes".
On Linux,
.B HOST_NAME_MAX
is defined with the value 64, which has been the limit since Linux 1.0
(earlier kernels imposed a limit of 8 bytes).
.SS C library/kernel differences
The GNU C library does not employ the
.BR gethostname ()
system call; instead, it implements
.BR gethostname ()
as a library function that calls
.BR uname (2)
and copies up to
.I len
bytes from the returned
.I nodename
field into
.IR name .
Having performed the copy, the function then checks if the length of the
.I nodename
was greater than or equal to
.IR len ,
and if it is, then the function returns \-1 with
.I errno
set to
.BR ENAMETOOLONG ;
in this case, a terminating null byte is not included in the returned
.IR name .
.PP
Versions of glibc before 2.2
.\" At least glibc 2.0 and 2.1, older versions not checked
handle the case where the length of the
.I nodename
was greater than or equal to
.I len
differently: nothing is copied into
.I name
and the function returns \-1 with
.I errno
set to
.BR ENAMETOOLONG .
.SH SEE ALSO
.BR hostname (1),
.BR getdomainname (2),
.BR setdomainname (2),
.BR uname (2),
.BR uts_namespaces (7)