get_thread_area, set_thread_area - manipulate thread-local storage information
#include <linux/unistd.h>
#if defined __i386__ || defined __x86_64__
# include <asm/ldt.h>
int get_thread_area(struct user_desc *u_info);
int set_thread_area(struct user_desc *u_info);
#elif defined __m68k__
int get_thread_area(void);
int set_thread_area(unsigned long tp);
#elif defined __mips__
int set_thread_area(unsigned long addr);
#endif
Note
: There are no glibc wrappers for these system calls;
see NOTES.
These calls provide architecture-specific support for a thread-local storage implementation. At the moment, set_thread_area() is available on m68k, MIPS, and x86 (both 32-bit and 64-bit variants); get_thread_area() is available on m68k and x86.
On m68k and MIPS, set_thread_area() allows storing an arbitrary pointer (provided in the tp argument on m68k and in the addr argument on MIPS) 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 appropriately.
On 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()).
u_info
is an invalid pointer.
u_info->entry_number
is out of bounds.
get_thread_area() or set_thread_area() was invoked as a 64-bit system call.
(set_thread_area()) A free TLS entry could not be located.
set_thread_area() first appeared in Linux 2.5.29. get_thread_area() first appeared in Linux 2.5.32.
set_thread_area() and get_thread_area() are Linux-specific and should not be used in programs that are intended to be portable.
Glibc does not provide wrappers for these system calls, since they are generally intended for use only by threading libraries. In the unlikely event that you want to call them directly, use syscall(2).
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 release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.