set_thread_area(2) System Calls Manual set_thread_area(2) NAME get_thread_area, set_thread_area - manipulate thread-local storage information LIBRARY Standard C library (libc, -lc) SYNOPSIS #include /* Definition of SYS_* constants */ #include #if defined __i386__ || defined __x86_64__ # include /* Definition of struct user_desc */ int syscall(SYS_get_thread_area, struct user_desc *u_info); int syscall(SYS_set_thread_area, struct user_desc *u_info); #elif defined __m68k__ int syscall(SYS_get_thread_area); int syscall(SYS_set_thread_area, unsigned long tp); #elif defined __mips__ || defined __csky__ int syscall(SYS_set_thread_area, unsigned long addr); #endif Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2). DESCRIPTION These calls provide architecture-specific support for a thread-local storage implementation. At the moment, set_thread_area() is available on m68k, MIPS, C-SKY, and x86 (both 32-bit and 64-bit variants); get_thread_area() is available on m68k and x86. On m68k, MIPS and C-SKY, set_thread_area() allows storing an arbitrary pointer (provided in the tp argument on m68k and in the addr argument on MIPS and C-SKY) in the kernel data structure associated with the calling thread; this pointer can later be retrieved using get_thread_area() (see also NOTES for information regarding obtaining the thread pointer on MIPS). On x86, 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 int 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; #ifdef __x86_64__ unsigned int lm:1; #endif }; 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 On x86, these system calls return 0 on success, and -1 on failure, with errno set to indicate the error. On C-SKY, MIPS and m68k, set_thread_area() always returns 0. On m68k, get_thread_area() returns the thread area pointer value (previously set via set_thread_area()). ERRORS EFAULT u_info is an invalid pointer. EINVAL u_info->entry_number is out of bounds. ENOSYS get_thread_area() or set_thread_area() was invoked as a 64-bit system call. ESRCH (set_thread_area()) A free TLS entry could not be located. STANDARDS Linux. HISTORY set_thread_area() Linux 2.5.29. get_thread_area() Linux 2.5.32. NOTES These system calls are generally intended for use only by threading libraries. arch_prctl(2) can interfere with set_thread_area() on x86. 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 MIPS, the current value of the thread area pointer can be obtained using the instruction: rdhwr dest, $29 This instruction traps and is handled by kernel. 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), ptrace(2) (PTRACE_GET_THREAD_AREA and PTRACE_SET_THREAD_AREA) Linux man-pages 6.9.1 2024-05-02 set_thread_area(2)