.\"	$OpenBSD: amd64_get_fsbase.2,v 1.5 2016/03/30 06:58:06 jmc Exp $
.\"	$NetBSD: i386_get_ioperm.2,v 1.3 1996/02/27 22:57:17 jtc Exp $
.\"
.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by John T. Kohl and Charles M. Hannum.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
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.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE
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.\"
.Dd $Mdocdate: March 30 2016 $
.Dt AMD64_GET_FSBASE 2 amd64
.Os
.Sh NAME
.Nm amd64_get_fsbase ,
.Nm amd64_set_fsbase
.Nd manage amd64 per-thread %fs base address
.Sh SYNOPSIS
.In sys/types.h
.In machine/sysarch.h
.Ft int
.Fn amd64_get_fsbase "void **base"
.Ft int
.Fn amd64_set_fsbase "void *base"
.Sh DESCRIPTION
.Fn amd64_get_fsbase
copies the current base address of the %fs segment into the memory
referenced by
.Fa base .
.Pp
.Fn amd64_set_fsbase
sets the base address of the %fs segment to the address
.Fa base .
.Pp
The segment base address is local to each thread.
The initial thread of a new process inherits its segment base address
from the parent thread.
.Xr __tfork 3
sets the initial segment base address for threads that it creates.
.Pp
.Sy Note:
Code using the
.Fn amd64_get_fsbase
and
.Fn amd64_set_fsbase
functions must be compiled using
.Cm -lamd64 .
.Sh RETURN VALUES
Upon successful completion,
.Fn amd64_get_fsbase
and
.Fn amd64_set_fsbase
return 0.
Otherwise, a value of \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn amd64_get_fsbase
will fail if:
.Bl -tag -width [EINVAL]
.It Bq Er EFAULT
.Fa base
points outside the process's allocated address space.
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr __tfork 3
.Sh WARNING
The ELF Thread-Local Storage ABI reserves %fs for its own use and
requires that the dynamic linker and thread library set it to
reference data-structures internal to and shared between them.
Programs should use the __thread storage class keyword instead of
using these calls.
To be maximally portable,
programs that require per-thread data should use the
.Fn pthread_key_create
interface.