1SET_THREAD_AREA(2) Linux Programmer's Manual SET_THREAD_AREA(2)
2
6 get_thread_area, set_thread_area - manipulate thread-local storage in‐
7 formation
8
10 #include <linux/unistd.h>
11 #if defined __i386__ || defined __x86_64__
13 # include <asm/ldt.h>
14 int get_thread_area(struct user_desc *u_info);
16 int set_thread_area(struct user_desc *u_info);
17 #elif defined __m68k__
19 int get_thread_area(void);
21 int set_thread_area(unsigned long tp);
22 #elif defined __mips__
24 int set_thread_area(unsigned long addr);
26 #endif
28 Note: There are no glibc wrappers for these system calls; see NOTES.
30
32 These calls provide architecture-specific support for a thread-local
33 storage implementation. At the moment, set_thread_area() is available
34 on m68k, MIPS, and x86 (both 32-bit and 64-bit variants);
35 get_thread_area() is available on m68k and x86.
36 On m68k and MIPS, set_thread_area() allows storing an arbitrary pointer
38 (provided in the tp argument on m68k and in the addr argument on MIPS)
39 in the kernel data structure associated with the calling thread; this
40 pointer can later be retrieved using get_thread_area() (see also NOTES
41 for information regarding obtaining the thread pointer on MIPS).
42 On x86, Linux dedicates three global descriptor table (GDT) entries for
44 thread-local storage. For more information about the GDT, see the In‐
45 tel Software Developer's Manual or the AMD Architecture Programming
46 Manual.
47 Both of these system calls take an argument that is a pointer to a
49 structure of the following type:
50 struct user_desc {
52 unsigned int entry_number;
53 unsigned int base_addr;
54 unsigned int limit;
55 unsigned int seg_32bit:1;
56 unsigned int contents:2;
57 unsigned int read_exec_only:1;
58 unsigned int limit_in_pages:1;
59 unsigned int seg_not_present:1;
60 unsigned int useable:1;
61 #ifdef __x86_64__
62 unsigned int lm:1;
63 #endif
64 };
65 get_thread_area() reads the GDT entry indicated by u_info->entry_number
67 and fills in the rest of the fields in u_info.
68 set_thread_area() sets a TLS entry in the GDT.
70 The TLS array entry set by set_thread_area() corresponds to the value
72 of u_info->entry_number passed in by the user. If this value is in
73 bounds, set_thread_area() writes the TLS descriptor pointed to by
74 u_info into the thread's TLS array.
75 When set_thread_area() is passed an entry_number of -1, it searches for
77 a free TLS entry. If set_thread_area() finds a free TLS entry, the
78 value of u_info->entry_number is set upon return to show which entry
79 was changed.
80 A user_desc is considered "empty" if read_exec_only and seg_not_present
82 are set to 1 and all of the other fields are 0. If an "empty" descrip‐
83 tor is passed to set_thread_area(), the corresponding TLS entry will be
84 cleared. See BUGS for additional details.
85 Since Linux 3.19, set_thread_area() cannot be used to write non-present
87 segments, 16-bit segments, or code segments, although clearing a seg‐
88 ment is still acceptable.
89
91 On x86, these system calls return 0 on success, and -1 on failure, with
92 errno set appropriately.
93 On MIPS and m68k, set_thread_area() always returns 0. On m68k,
95 get_thread_area() returns the thread area pointer value (previously set
96 via set_thread_area()).
97
99 EFAULT u_info is an invalid pointer.
100 EINVAL u_info->entry_number is out of bounds.
102 ENOSYS get_thread_area() or set_thread_area() was invoked as a 64-bit
104 system call.
105 ESRCH (set_thread_area()) A free TLS entry could not be located.
107
109 set_thread_area() first appeared in Linux 2.5.29. get_thread_area()
110 first appeared in Linux 2.5.32.
111
113 set_thread_area() and get_thread_area() are Linux-specific and should
114 not be used in programs that are intended to be portable.
115
117 Glibc does not provide wrappers for these system calls, since they are
118 generally intended for use only by threading libraries. In the un‐
119 likely event that you want to call them directly, use syscall(2).
120 arch_prctl(2) can interfere with set_thread_area() on x86. See
122 arch_prctl(2) for more details. This is not normally a problem, as
123 arch_prctl(2) is normally used only by 64-bit programs.
124 On MIPS, the current value of the thread area pointer can be obtained
126 using the instruction:
127 rdhwr dest, $29
129 This instruction traps and is handled by kernel.
131
133 On 64-bit kernels before Linux 3.19, one of the padding bits in
134 user_desc, if set, would prevent the descriptor from being considered
135 empty (see modify_ldt(2)). As a result, the only reliable way to clear
136 a TLS entry is to use memset(3) to zero the entire user_desc structure,
137 including padding bits, and then to set the read_exec_only and
138 seg_not_present bits. On Linux 3.19, a user_desc consisting entirely
139 of zeros except for entry_number will also be interpreted as a request
140 to clear a TLS entry, but this behaved differently on older kernels.
141 Prior to Linux 3.19, the DS and ES segment registers must not reference
143 TLS entries.
144
146 arch_prctl(2), modify_ldt(2), ptrace(2) (PTRACE_GET_THREAD_AREA and
147 PTRACE_SET_THREAD_AREA)
148
150 This page is part of release 5.10 of the Linux man-pages project. A
151 description of the project, information about reporting bugs, and the
152 latest version of this page, can be found at
153 https://www.kernel.org/doc/man-pages/.
154Linux 2020-02-09 SET_THREAD_AREA(2)