Linker and Libraries Guide

Thread-Local Storage Section

Separate copies of thread-local data that have been 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 that are identified with the SHF_TLS flag. These sections provide initialized TLS and uninitialized TLS based on how the storage is declared.

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 that are generated as a result of initialized thread-local variables are applied to this template. The relocated values are 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 that is 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 TLS offset for defined symbols. This field contains zero for undefined symbols.

Several relocations are defined to support access to TLS. See SPARC: Thread-Local Storage Relocation Types, 32-bit x86: Thread-Local Storage Relocation Types and x64: Thread-Local Storage Relocation Types. TLS relocations typically reference symbols of type STT_TLS. TLS relocations can also reference local section symbols in association with a GOT entry. In this case, the assigned TLS offset is stored in the associated GOT entry.

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

Table 8–1 ELF PT_TLS Program Header Entry




File offset of the TLS initialization image


Virtual memory address of the TLS initialization image




Size of the TLS initialization image


Total size of the TLS template




Alignment of the TLS template