|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
|
|
|
set_thread_area(2) System Calls Manual set_thread_area(2)
get_thread_area, set_thread_area - manipulate thread-local storage
information
Standard C library (libc, -lc)
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
#if defined __i386__ || defined __x86_64__
# include <asm/ldt.h> /* Definition of struct user_desc */
int syscall(SYS_get_thread_area, struct user_desc *u_info);
int syscall(SYS_set_thread_area, struct user_desc *u_info);
#elif defined __m68k__
int syscall(SYS_get_thread_area);
int syscall(SYS_set_thread_area, unsigned long tp);
#elif defined __mips__ || defined __csky__
int syscall(SYS_set_thread_area, unsigned long addr);
#endif
Note: glibc provides no wrappers for these system calls,
necessitating the use of syscall(2).
These calls provide architecture-specific support for a thread-
local storage implementation. At the moment, set_thread_area() is
available on m68k, MIPS, C-SKY, and x86 (both 32-bit and 64-bit
variants); get_thread_area() is available on m68k and x86.
On m68k, MIPS and C-SKY, set_thread_area() allows storing an
arbitrary pointer (provided in the tp argument on m68k and in the
addr argument on MIPS and C-SKY) in the kernel data structure
associated with the calling thread; this pointer can later be
retrieved using get_thread_area() (see also NOTES for information
regarding obtaining the thread pointer on MIPS).
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.
Both of these system calls take an argument that is a pointer to a
structure of the following type:
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
};
get_thread_area() reads the GDT entry indicated by
u_info->entry_number and fills in the rest of the fields in
u_info.
set_thread_area() sets a TLS entry in the GDT.
The TLS array entry set by set_thread_area() corresponds to the
value of u_info->entry_number passed in by the user. If this
value is in bounds, set_thread_area() writes the TLS descriptor
pointed to by u_info into the thread's TLS array.
When set_thread_area() is passed an entry_number of -1, it
searches for a free TLS entry. If set_thread_area() finds a free
TLS entry, the value of u_info->entry_number is set upon return to
show which entry was changed.
A user_desc is considered "empty" if read_exec_only and
seg_not_present are set to 1 and all of the other fields are 0.
If an "empty" descriptor is passed to set_thread_area(), the
corresponding TLS entry will be cleared. See BUGS for additional
details.
Since Linux 3.19, set_thread_area() cannot be used to write non-
present segments, 16-bit segments, or code segments, although
clearing a segment is still acceptable.
On x86, these system calls return 0 on success, and -1 on failure,
with errno set to indicate the error.
On C-SKY, MIPS and m68k, set_thread_area() always returns 0. On
m68k, get_thread_area() returns the thread area pointer value
(previously set via set_thread_area()).
EFAULT u_info is an invalid pointer.
EINVAL u_info->entry_number is out of bounds.
ENOSYS get_thread_area() or set_thread_area() was invoked as a
64-bit system call.
ESRCH (set_thread_area()) A free TLS entry could not be located.
Linux.
set_thread_area()
Linux 2.5.29.
get_thread_area()
Linux 2.5.32.
These system calls are generally intended for use only by
threading libraries.
arch_prctl(2) can interfere with set_thread_area() on x86. See
arch_prctl(2) for more details. This is not normally a problem,
as arch_prctl(2) is normally used only by 64-bit programs.
On MIPS, the current value of the thread area pointer can be
obtained using the instruction:
rdhwr dest, $29
This instruction traps and is handled by kernel.
On 64-bit kernels before Linux 3.19, one of the padding bits in
user_desc, if set, would prevent the descriptor from being
considered empty (see modify_ldt(2)). As a result, the only
reliable way to clear a TLS entry is to use memset(3) to zero the
entire user_desc structure, including padding bits, and then to
set the read_exec_only and seg_not_present bits. On Linux 3.19, a
user_desc consisting entirely of zeros except for entry_number
will also be interpreted as a request to clear a TLS entry, but
this behaved differently on older kernels.
Prior to Linux 3.19, the DS and ES segment registers must not
reference TLS entries.
arch_prctl(2), modify_ldt(2), ptrace(2) (PTRACE_GET_THREAD_AREA
and PTRACE_SET_THREAD_AREA)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 set_thread_area(2)
Pages that refer to this page: strace(1), arch_prctl(2), clone(2), modify_ldt(2), ptrace(2), syscalls(2)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|