управлять информацией о локальном хранилище потока (manipulate thread-local storage information)
Имя (Name)
get_thread_area, set_thread_area - manipulate thread-local
storage information
Синопсис (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).
Описание (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 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.
Возвращаемое значение (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
()).
Ошибки (Error)
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.
Версии (Versions)
set_thread_area
() first appeared in Linux 2.5.29.
get_thread_area
() first appeared in Linux 2.5.32.
Стандарты (Conforming to)
set_thread_area
() and get_thread_area
() are Linux-specific and
should not be used in programs that are intended to be portable.
Примечание (Note)
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.
Ошибки (баги) (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.
Смотри также (See also)
arch_prctl(2), modify_ldt(2), ptrace(2) (PTRACE_GET_THREAD_AREA
and PTRACE_SET_THREAD_AREA
)