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Oracle Solaris 11.1 Linkers and Libraries Guide     Oracle Solaris 11.1 Information Library
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Part I Using the Link-Editor and Runtime Linker

1.  Introduction to the Oracle Solaris Link Editors

2.  Link-Editor

3.  Runtime Linker

4.  Shared Objects

Part II Quick Reference

5.  Link-Editor Quick Reference

Part III Advanced Topics

6.  Direct Bindings

7.  Building Objects to Optimize System Performance

8.  Mapfiles

9.  Interfaces and Versioning

10.  Establishing Dependencies with Dynamic String Tokens

11.  Extensibility Mechanisms

Part IV ELF Application Binary Interface

12.  Object File Format

13.  Program Loading and Dynamic Linking

14.  Thread-Local Storage

C/C++ Programming Interface

Thread-Local Storage Section

Runtime Allocation of Thread-Local Storage

Program Startup

Thread Creation

Post-Startup Dynamic Loading

Deferred Allocation of Thread-Local Storage Blocks

Thread-Local Storage Access Models

SPARC: Thread-Local Variable Access

SPARC: General Dynamic (GD)

SPARC: Local Dynamic (LD)

32-bit SPARC: Initial Executable (IE)

64-bit SPARC: Initial Executable (IE)

SPARC: Local Executable (LE)

SPARC: Thread-Local Storage Relocation Types

32-bit x86: Thread-Local Variable Access

32-bit x86: General Dynamic (GD)

x86: Local Dynamic (LD)

32-bit x86: Initial Executable (IE)

32-bit x86: Local Executable (LE)

32-bit x86: Thread-Local Storage Relocation Types

x64: Thread-Local Variable Access

x64: General Dynamic (GD)

x64: Local Dynamic (LD)

x64: Initial Executable (IE)

x64: Local Executable (LE)

x64: Thread-Local Storage Relocation Types

Part V Appendices

A.  Linker and Libraries Updates and New Features

B.  System V Release 4 (Version 1) Mapfiles


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.

For relocations against static TLS items, the relocation address is encoded as a negative offset from the end of the static TLS template. This offset is calculated by first rounding the template size to the nearest 8-byte boundary in a 32-bit object, and to the nearest 16-byte boundary in a 64-bit object. This rounding ensures that the static TLS template is suitably aligned for any use.

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

Table 14-1 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