set_thread_area(2)

1set_thread_area(2)            System Calls Manual           set_thread_area(2)
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4

NAME

6       get_thread_area,  set_thread_area - manipulate thread-local storage in‐
7       formation
8

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <sys/syscall.h>     /* Definition of SYS_* constants */
14       #include <unistd.h>
15
16       #if defined __i386__ || defined __x86_64__
17       # include <asm/ldt.h>        /* Definition of struct user_desc */
18
19       int syscall(SYS_get_thread_area, struct user_desc *u_info);
20       int syscall(SYS_set_thread_area, struct user_desc *u_info);
21
22       #elif defined __m68k__
23
24       int syscall(SYS_get_thread_area);
25       int syscall(SYS_set_thread_area, unsigned long tp);
26
27       #elif defined __mips__
28
29       int syscall(SYS_set_thread_area, unsigned long addr);
30
31       #endif
32
33       Note: glibc provides no wrappers for these system calls,  necessitating
34       the use of syscall(2).
35

DESCRIPTION

37       These  calls  provide  architecture-specific support for a thread-local
38       storage implementation.  At the moment, set_thread_area() is  available
39       on   m68k,   MIPS,   and   x86   (both  32-bit  and  64-bit  variants);
40       get_thread_area() is available on m68k and x86.
41
42       On m68k and MIPS, set_thread_area() allows storing an arbitrary pointer
43       (provided  in the tp argument on m68k and in the addr argument on MIPS)
44       in the kernel data structure associated with the calling  thread;  this
45       pointer  can later be retrieved using get_thread_area() (see also NOTES
46       for information regarding obtaining the thread pointer on MIPS).
47
48       On x86, Linux dedicates three global descriptor table (GDT) entries for
49       thread-local  storage.  For more information about the GDT, see the In‐
50       tel Software Developer's Manual or  the  AMD  Architecture  Programming
51       Manual.
52
53       Both  of  these  system  calls  take an argument that is a pointer to a
54       structure of the following type:
55
56           struct user_desc {
57               unsigned int  entry_number;
58               unsigned int  base_addr;
59               unsigned int  limit;
60               unsigned int  seg_32bit:1;
61               unsigned int  contents:2;
62               unsigned int  read_exec_only:1;
63               unsigned int  limit_in_pages:1;
64               unsigned int  seg_not_present:1;
65               unsigned int  useable:1;
66           #ifdef __x86_64__
67               unsigned int  lm:1;
68           #endif
69           };
70
71       get_thread_area() reads the GDT entry indicated by u_info->entry_number
72       and fills in the rest of the fields in u_info.
73
74       set_thread_area() sets a TLS entry in the GDT.
75
76       The  TLS  array entry set by set_thread_area() corresponds to the value
77       of u_info->entry_number passed in by the user.  If  this  value  is  in
78       bounds,  set_thread_area()  writes  the  TLS  descriptor  pointed to by
79       u_info into the thread's TLS array.
80
81       When set_thread_area() is passed an entry_number of -1, it searches for
82       a  free  TLS  entry.   If set_thread_area() finds a free TLS entry, the
83       value of u_info->entry_number is set upon return to  show  which  entry
84       was changed.
85
86       A user_desc is considered "empty" if read_exec_only and seg_not_present
87       are set to 1 and all of the other fields are 0.  If an "empty" descrip‐
88       tor is passed to set_thread_area(), the corresponding TLS entry will be
89       cleared.  See BUGS for additional details.
90
91       Since Linux 3.19, set_thread_area() cannot be used to write non-present
92       segments,  16-bit  segments, or code segments, although clearing a seg‐
93       ment is still acceptable.
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RETURN VALUE

96       On x86, these system calls return 0 on success, and -1 on failure, with
97       errno set to indicate the error.
98
99       On  MIPS  and  m68k,  set_thread_area()  always  returns  0.   On m68k,
100       get_thread_area() returns the thread area pointer value (previously set
101       via set_thread_area()).
102

ERRORS

104       EFAULT u_info is an invalid pointer.
105
106       EINVAL u_info->entry_number is out of bounds.
107
108       ENOSYS get_thread_area()  or  set_thread_area() was invoked as a 64-bit
109              system call.
110
111       ESRCH  (set_thread_area()) A free TLS entry could not be located.
112

STANDARDS

114       Linux.
115

HISTORY

117       set_thread_area()
118              Linux 2.5.29.
119
120       get_thread_area()
121              Linux 2.5.32.
122

NOTES

124       These system calls are generally intended for use only by threading li‐
125       braries.
126
127       arch_prctl(2)   can  interfere  with  set_thread_area()  on  x86.   See
128       arch_prctl(2) for more details.  This is not  normally  a  problem,  as
129       arch_prctl(2) is normally used only by 64-bit programs.
130
131       On  MIPS,  the current value of the thread area pointer can be obtained
132       using the instruction:
133
134           rdhwr dest, $29
135
136       This instruction traps and is handled by kernel.
137

BUGS

139       On 64-bit kernels before  Linux  3.19,  one  of  the  padding  bits  in
140       user_desc,  if  set, would prevent the descriptor from being considered
141       empty (see modify_ldt(2)).  As a result, the only reliable way to clear
142       a TLS entry is to use memset(3) to zero the entire user_desc structure,
143       including  padding  bits,  and  then  to  set  the  read_exec_only  and
144       seg_not_present  bits.   On Linux 3.19, a user_desc consisting entirely
145       of zeros except for entry_number will also be interpreted as a  request
146       to clear a TLS entry, but this behaved differently on older kernels.
147
148       Prior to Linux 3.19, the DS and ES segment registers must not reference
149       TLS entries.
150