set_thread_area(2) set a GDT entry for thread-local storage


#include <linux/unistd.h>
#include <asm/ldt.h>
int get_thread_area(struct user_desc *u_info);
int set_thread_area(struct user_desc *u_info);

Note: There are no glibc wrappers for these system calls; see NOTES.


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 long 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; };

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.


These system calls return 0 on success, and -1 on failure, with errno set appropriately.


u_info is an invalid pointer.
u_info->entry_number is out of bounds.
get_thread_area(2) or set_thread_area(2) was invoked as a 64-bit system call.
(set_thread_area()) A free TLS entry could not be located.


set_thread_area() first appeared in Linux 2.5.29. get_thread_area() first appeared in Linux 2.5.32.


set_thread_area() is Linux-specific and should not be used in programs that are intended to be portable.


Glibc does not provide wrappers for these system calls, since they are generally intended for use only by threading libraries. In the unlikely event that you want to call them directly, use syscall(2).

arch_prctl(2) can interfere with set_thread_area(2). 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 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.


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