.\" Copyright (C) 2003 Free Software Foundation, Inc.
.\" Copyright (C) 2015 Andrew Lutomirski
.\" Author: Kent Yoder
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.TH set_thread_area 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
get_thread_area, set_thread_area \- manipulate thread-local storage information
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <sys/syscall.h>" "     /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.B #if defined __i386__ || defined __x86_64__
.BR "# include <asm/ldt.h>" "        /* Definition of " "struct user_desc" " */"
.PP
.BI "int syscall(SYS_get_thread_area, struct user_desc *" u_info );
.BI "int syscall(SYS_set_thread_area, struct user_desc *" u_info );
.PP
.B #elif defined __m68k__
.PP
.B "int syscall(SYS_get_thread_area);"
.BI "int syscall(SYS_set_thread_area, unsigned long " tp );
.PP
.B #elif defined __mips__
.PP
.BI "int syscall(SYS_set_thread_area, unsigned long " addr );
.PP
.B #endif
.fi
.PP
.IR Note :
glibc provides no wrappers for these system calls,
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
These calls provide architecture-specific support for a thread-local storage
implementation.
At the moment,
.BR set_thread_area ()
is available on m68k, MIPS, and x86 (both 32-bit and 64-bit variants);
.BR get_thread_area ()
is available on m68k and x86.
.PP
On m68k and MIPS,
.BR set_thread_area ()
allows storing an arbitrary pointer (provided in the
.B tp
argument on m68k and in the
.B addr
argument on MIPS)
in the kernel data structure associated with the calling thread;
this pointer can later be retrieved using
.BR get_thread_area ()
(see also NOTES
for information regarding obtaining the thread pointer on MIPS).
.PP
On x86, Linux dedicates three global descriptor table (GDT) entries for
thread-local storage.
For more information about the GDT, see the
Intel Software Developer's Manual or the AMD Architecture Programming Manual.
.PP
Both of these system calls take an argument that is a pointer
to a structure of the following type:
.PP
.in +4n
.EX
struct user_desc {
    unsigned int  entry_number;
    unsigned int  base_addr;
    unsigned int  limit;
    unsigned int  seg_32bit:1;
    unsigned int  contents:2;
    unsigned int  read_exec_only:1;
    unsigned int  limit_in_pages:1;
    unsigned int  seg_not_present:1;
    unsigned int  useable:1;
#ifdef __x86_64__
    unsigned int  lm:1;
#endif
};
.EE
.in
.PP
.BR get_thread_area ()
reads the GDT entry indicated by
.I u_info\->entry_number
and fills in the rest of the fields in
.IR u_info .
.PP
.BR set_thread_area ()
sets a TLS entry in the GDT.
.PP
The TLS array entry set by
.BR set_thread_area ()
corresponds to the value of
.I u_info\->entry_number
passed in by the user.
If this value is in bounds,
.BR set_thread_area ()
writes the TLS descriptor pointed to by
.I u_info
into the thread's TLS array.
.PP
When
.BR set_thread_area ()
is passed an
.I entry_number
of \-1, it searches for a free TLS entry.
If
.BR set_thread_area ()
finds a free TLS entry, the value of
.I u_info\->entry_number
is set upon return to show which entry was changed.
.PP
A
.I user_desc
is considered "empty" if
.I read_exec_only
and
.I seg_not_present
are set to 1 and all of the other fields are 0.
If an "empty" descriptor is passed to
.BR set_thread_area (),
the corresponding TLS entry will be cleared.
See BUGS for additional details.
.PP
Since Linux 3.19,
.BR set_thread_area ()
cannot be used to write non-present segments, 16-bit segments, or code
segments, although clearing a segment is still acceptable.
.SH RETURN VALUE
On x86, these system calls
return 0 on success, and \-1 on failure, with
.I errno
set to indicate the error.
.PP
On MIPS and m68k,
.BR set_thread_area ()
always returns 0.
On m68k,
.BR get_thread_area ()
returns the thread area pointer value
(previously set via
.BR set_thread_area ()).
.SH ERRORS
.TP
.B EFAULT
\fIu_info\fP is an invalid pointer.
.TP
.B EINVAL
\fIu_info\->entry_number\fP is out of bounds.
.TP
.B ENOSYS
.BR get_thread_area ()
or
.BR set_thread_area ()
was invoked as a 64-bit system call.
.TP
.B ESRCH
.RB ( set_thread_area ())
A free TLS entry could not be located.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.BR set_thread_area ()
Linux 2.5.29.
.TP
.BR get_thread_area ()
Linux 2.5.32.
.SH NOTES
These system calls are generally intended for use only by threading libraries.
.PP
.BR arch_prctl (2)
can interfere with
.BR set_thread_area ()
on x86.
See
.BR arch_prctl (2)
for more details.
This is not normally a problem, as
.BR arch_prctl (2)
is normally used only by 64-bit programs.
.PP
On MIPS, the current value of the thread area pointer can be obtained
using the instruction:
.PP
.in +4n
.EX
rdhwr dest, $29
.EE
.in
.PP
This instruction traps and is handled by kernel.
.SH BUGS
On 64-bit kernels before Linux 3.19,
.\" commit e30ab185c490e9a9381385529e0fd32f0a399495
one of the padding bits in
.IR user_desc ,
if set, would prevent the descriptor from being considered empty (see
.BR modify_ldt (2)).
As a result, the only reliable way to clear a TLS entry is to use
.BR memset (3)
to zero the entire
.I user_desc
structure, including padding bits, and then to set the
.I read_exec_only
and
.I seg_not_present
bits.
On Linux 3.19, a
.I user_desc
consisting entirely of zeros except for
.I entry_number
will also be interpreted as a request to clear a TLS entry, but this
behaved differently on older kernels.
.PP
Prior to Linux 3.19, the DS and ES segment registers must not reference
TLS entries.
.SH SEE ALSO
.BR arch_prctl (2),
.BR modify_ldt (2),
.BR ptrace (2)
.RB ( PTRACE_GET_THREAD_AREA " and " PTRACE_SET_THREAD_AREA )