.\" $OpenBSD: __get_tcb.2,v 1.7 2023/02/22 06:31:51 guenther Exp $
.\"
.\" Copyright (c) 2011 Philip Guenther <guenther@openbsd.org>
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.Dd $Mdocdate: February 22 2023 $
.Dt __GET_TCB 2
.Os
.Sh NAME
.Nm __get_tcb ,
.Nm __set_tcb
.Nd get or set the address of the thread control block of the current thread
.Sh SYNOPSIS
.Ft void *
.Fn __get_tcb "void"
.Ft void
.Fn __set_tcb "void *"
.Sh DESCRIPTION
The
.Fn __get_tcb
and
.Fn __set_tcb
functions are for use by libpthread and other parts of the C
runtime to retrieve and set the address of the current thread's
thread control block (TCB).
This is used to locate per-thread data such as
.Va errno .
Each kernel-level thread in a process has a separate value for this
address, which can be obtained and changed via these system calls.
.Pp
The initial thread of a new process created using
.Xr fork 2
or
.Xr vfork 2
inherits the TCB address of the thread that created it.
A thread created using
.Xr __tfork 3
starts with the TCB address specified in that call.
.Xr execve 2
resets the thread's TCB address to zero.
.Pp
On some platforms, this address is also directly mapped to a CPU
register which can be accessed from userspace.
.Sh RETURN VALUES
.Fn __get_tcb
returns the address of the thread control block of the current thread.
.Sh SEE ALSO
.Xr __tfork 3
.Sh HISTORY
The
.Fn __get_tcb
and
.Fn __set_tcb
system calls appeared in
.Ox 5.1 .