set_thread_area(2) System Calls Manual set_thread_area(2)
NAME
get_thread_area, set_thread_area - manipulate thread-local storage in-
formation
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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__
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).
DESCRIPTION
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 In-
tel 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" descrip-
tor 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 seg-
ment is still acceptable.
RETURN VALUE
On x86, these system calls return 0 on success, and -1 on failure, with
errno set to indicate the error.
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()).
ERRORS
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.
VERSIONS
set_thread_area() first appeared in Linux 2.5.29. get_thread_area()
first appeared in Linux 2.5.32.
STANDARDS
set_thread_area() and get_thread_area() are Linux-specific and should
not be used in programs that are intended to be portable.
NOTES
These system calls are generally intended for use only by threading li-
braries.
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.
BUGS
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.
SEE ALSO
arch_prctl(2), modify_ldt(2), ptrace(2) (PTRACE_GET_THREAD_AREA and
PTRACE_SET_THREAD_AREA)
Linux man-pages 6.03 2022-10-30 set_thread_area(2)
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