Thread-Local Storage Section

Separate copies of thread-local data, allocated at compile-time, must be associated with individual threads of execution. To provide this data, TLS sections are used to specify the size and initial contents.

The compilation environment allocates TLS in sections identified with the SHF_TLS flag. These sections provide initialized and uninitialized TLS based on how the storage is declared:

• An initialized thread-local variable is allocated in a .tdata, or .tdata1 section. This initialization may require relocation.

• An uninitialized thread-local variable is defined as a COMMON symbol. The resulting allocation is made in a .tbss section.

The uninitialized section is allocated immediately following any initialized sections, subject to padding for proper alignment. Together, the combined sections form a TLS template that is used to allocate TLS whenever a new thread is created.

The initialized portion of this template is called the TLS initialization image. All relocations generated as a result of initialized thread-local variables are applied to this template. These relocated values are then used when a new thread requires the initial values.

TLS symbols have the symbol type STT_TLS. These symbols are assigned offsets relative to the beginning of the TLS template. The actual virtual address associated with these symbols is irrelevant. The address refers only to the template, and not to the per-thread copy of each data item.

In dynamic executables and shared objects, the st_value field of a STT_TLS symbol contains the assigned offset for defined symbols, or zero for undefined symbols.

Several relocations are defined to support access to TLS. See SPARC: Thread-Local Storage Relocation Types and x86: Thread-Local Storage Relocation Types. Symbols of type STT_TLS are only referenced by TLS relocations. TLS relocations only reference symbols of type STT_TLS.

In dynamic executables and shared objects, a PT_TLS program entry describes a TLS template, and has the following members:

Table 8–1 ELF PT_TLS Program Header Entry

Member	Value
p_offset	File offset of the TLS initialization image
p_vaddr	Virtual memory address of the TLS initialization image
p_paddr	Reserved
p_filesz	Size of the TLS initialization image
p_memsz	Total size of the TLS template
p_flags	PF_R
p_align	Alignment of the TLS template