DESCRIPTION

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:

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.

RETURN VALUE

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

ERRORS

VERSIONS

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

CONFORMING TO

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

NOTES

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.

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.

SEE ALSO

arch_prctl(2), modify_ldt(2)