362 lines
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362 lines
8.2 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SET_THREAD_AREA</TITLE>
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</HEAD><BODY>
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<H1>SET_THREAD_AREA</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2020-02-09<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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get_thread_area, set_thread_area - manipulate thread-local storage information
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/linux/unistd.h">linux/unistd.h</A>></B>
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<B>#if defined __i386__ || defined __x86_64__</B>
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<B># include <<A HREF="file:///usr/include/asm/ldt.h">asm/ldt.h</A>></B>
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<B>int get_thread_area(struct user_desc *</B><I>u_info</I><B>);</B>
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<B>int set_thread_area(struct user_desc *</B><I>u_info</I><B>);</B>
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<B>#elif defined __m68k__</B>
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<B>int get_thread_area(void);</B>
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<B>int set_thread_area(unsigned long </B><I>tp</I><B>);</B>
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<B>#elif defined __mips__</B>
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<B>int set_thread_area(unsigned long </B><I>addr</I><B>);</B>
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<B>#endif</B>
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</PRE>
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<P>
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<I>Note</I>:
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There are no glibc wrappers for these system calls; see NOTES.
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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These calls provide architecture-specific support for a thread-local storage
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implementation.
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At the moment,
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<B>set_thread_area</B>()
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is available on m68k, MIPS, and x86 (both 32-bit and 64-bit variants);
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<B>get_thread_area</B>()
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is available on m68k and x86.
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<P>
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On m68k and MIPS,
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<B>set_thread_area</B>()
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allows storing an arbitrary pointer (provided in the
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<B>tp</B>
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argument on m68k and in the
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<B>addr</B>
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argument on MIPS)
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in the kernel data structure associated with the calling thread;
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this pointer can later be retrieved using
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<B>get_thread_area</B>()
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(see also NOTES
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for information regarding obtaining the thread pointer on MIPS).
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<P>
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On x86, Linux dedicates three global descriptor table (GDT) entries for
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thread-local storage.
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For more information about the GDT, see the
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Intel Software Developer's Manual or the AMD Architecture Programming Manual.
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<P>
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Both of these system calls take an argument that is a pointer
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to a structure of the following type:
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<P>
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struct user_desc {
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<BR> unsigned int entry_number;
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<BR> unsigned int base_addr;
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<BR> unsigned int limit;
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<BR> unsigned int seg_32bit:1;
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<BR> unsigned int contents:2;
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<BR> unsigned int read_exec_only:1;
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<BR> unsigned int limit_in_pages:1;
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<BR> unsigned int seg_not_present:1;
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<BR> unsigned int useable:1;
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#ifdef __x86_64__
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<BR> unsigned int lm:1;
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#endif
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};
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<P>
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<B>get_thread_area</B>()
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reads the GDT entry indicated by
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<I>u_info->entry_number</I>
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and fills in the rest of the fields in
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<I>u_info</I>.
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<P>
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<B>set_thread_area</B>()
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sets a TLS entry in the GDT.
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<P>
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The TLS array entry set by
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<B>set_thread_area</B>()
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corresponds to the value of
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<I>u_info->entry_number</I>
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passed in by the user.
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If this value is in bounds,
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<B>set_thread_area</B>()
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writes the TLS descriptor pointed to by
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<I>u_info</I>
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into the thread's TLS array.
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<P>
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When
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<B>set_thread_area</B>()
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is passed an
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<I>entry_number</I>
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of -1, it searches for a free TLS entry.
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If
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<B>set_thread_area</B>()
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finds a free TLS entry, the value of
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<I>u_info->entry_number</I>
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is set upon return to show which entry was changed.
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<P>
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A
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<I>user_desc</I>
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is considered "empty" if
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<I>read_exec_only</I>
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and
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<I>seg_not_present</I>
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are set to 1 and all of the other fields are 0.
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If an "empty" descriptor is passed to
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<B>set_thread_area</B>(),
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the corresponding TLS entry will be cleared.
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See BUGS for additional details.
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<P>
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Since Linux 3.19,
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<B>set_thread_area</B>()
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cannot be used to write non-present segments, 16-bit segments, or code
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segments, although clearing a segment is still acceptable.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On x86, these system calls
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return 0 on success, and -1 on failure, with
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<I>errno</I>
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set appropriately.
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<P>
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On MIPS and m68k,
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<B>set_thread_area</B>()
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always returns 0.
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On m68k,
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<B>get_thread_area</B>()
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returns the thread area pointer value
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(previously set via
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<B>set_thread_area</B>()).
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="1"><B>EFAULT</B>
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<DD>
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<I>u_info</I> is an invalid pointer.
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<DT id="2"><B>EINVAL</B>
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<DD>
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<I>u_info->entry_number</I> is out of bounds.
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<DT id="3"><B>ENOSYS</B>
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<DD>
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<B>get_thread_area</B>()
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or
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<B>set_thread_area</B>()
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was invoked as a 64-bit system call.
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<DT id="4"><B>ESRCH</B>
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<DD>
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(<B>set_thread_area</B>())
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A free TLS entry could not be located.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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<B>set_thread_area</B>()
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first appeared in Linux 2.5.29.
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<B>get_thread_area</B>()
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first appeared in Linux 2.5.32.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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<B>set_thread_area</B>()
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and
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<B>get_thread_area</B>()
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are Linux-specific and should not be used in programs that are intended
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to be portable.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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Glibc does not provide wrappers for these system calls,
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since they are generally intended for use only by threading libraries.
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In the unlikely event that you want to call them directly, use
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<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2).
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<P>
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<B><A HREF="/cgi-bin/man/man2html?2+arch_prctl">arch_prctl</A></B>(2)
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can interfere with
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<B>set_thread_area</B>()
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on x86.
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See
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<B><A HREF="/cgi-bin/man/man2html?2+arch_prctl">arch_prctl</A></B>(2)
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for more details.
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This is not normally a problem, as
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<B><A HREF="/cgi-bin/man/man2html?2+arch_prctl">arch_prctl</A></B>(2)
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is normally used only by 64-bit programs.
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<P>
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On MIPS, the current value of the thread area pointer can be obtained
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using the instruction:
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<P>
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rdhwr dest, $29
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<P>
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This instruction traps and is handled by kernel.
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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On 64-bit kernels before Linux 3.19,
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one of the padding bits in
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<I>user_desc</I>,
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if set, would prevent the descriptor from being considered empty (see
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<B><A HREF="/cgi-bin/man/man2html?2+modify_ldt">modify_ldt</A></B>(2)).
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As a result, the only reliable way to clear a TLS entry is to use
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<B><A HREF="/cgi-bin/man/man2html?3+memset">memset</A></B>(3)
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to zero the entire
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<I>user_desc</I>
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structure, including padding bits, and then to set the
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<I>read_exec_only</I>
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and
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<I>seg_not_present</I>
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bits.
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On Linux 3.19, a
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<I>user_desc</I>
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consisting entirely of zeros except for
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<I>entry_number</I>
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will also be interpreted as a request to clear a TLS entry, but this
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behaved differently on older kernels.
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<P>
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Prior to Linux 3.19, the DS and ES segment registers must not reference
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TLS entries.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+arch_prctl">arch_prctl</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+modify_ldt">modify_ldt</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+ptrace">ptrace</A></B>(2)
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(<B>PTRACE_GET_THREAD_AREA</B> and <B>PTRACE_SET_THREAD_AREA</B>)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="5"><A HREF="#lbAB">NAME</A><DD>
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<DT id="6"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="7"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="8"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="9"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="10"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="11"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="12"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="13"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="14"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="15"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:34 GMT, March 31, 2021
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