Chapter 6. Symbol Versioning

Table of Contents
Symbol Version Table
Version Definitions
Version Requirements
Startup Sequence
Symbol Resolution

This chapter describes the Symbol Versioning mechanism. All ELF objects may provide or depend on versioned symbols. Symbol Versioning is implemented by 3 section types: SHT_GNU_versym, SHT_GNU_verdef, and SHT_GNU_verneed.

The term "Elfxx" means "Elf32" or "Elf64" depending on the architecture.

Versions are described by strings. The structures that are used for symbol versions also contain a member that holds the ELF hashing values of the strings. This allows for more efficient processing.

Symbol Version Table

The Symbol Version Table is contained in the special section .gnu.version which has a section type of SHT_GNU_versym. This section has the same number of entries as the Dynamic Symbol Table.

This section contains an array of elements of type Elfxx_Half. Each entry specifies the version defined for or required by the corresponding symbol in the Dynamic Symbol Table.

The values in the Symbol Version Table are unique to the object in which they are located. These values are identifiers that are provided by the the vna_other member of the Elfxx_Vernaux structure or the vd_ndx member of the Elfxx_Verdef structure.

The values 0 and 1 are reserved.

All other values are used to identify version strings located in one of the other Symbol Version sections. The value itself is not the version associated with the symbol. The string identified by the value defines the version of the symbol.