Overview

The link-editing process builds an output file from one or more input files. The building of the output file is directed by the options supplied to the link-editor together with the input sections provided by the input files.

All files are represented in the executable and linking format (ELF). For a complete description of the ELF format see Chapter 7, Object Files. For this introduction, however, it is first necessary to introduce two ELF structures, sections and segments.

Sections are the smallest indivisible units that can be processed within an ELF file. Segments are a collection of sections that represent the smallest individual units that can be mapped to a memory image by exec(2) or by the runtime linker ld.so.1(1).

Although there are many types of ELF sections, they all fall into two categories with respect to the link-editing phase:

• Sections that contain program data, whose interpretation is meaningful only to the application itself (such as the program instructions .text and the associated data .data and .bss).

• Sections that contain link-editing information, such as the symbol table information found from .symtab and .strtab, and relocation information such as .rela.text.

Basically, the link-editor concatenates the program data sections into the output file. The link-editing information sections are interpreted by the link-editor to modify other sections or to generate new output information sections used in later processing of the output file.

The following simple breakdown of link-editor functionality introduces the topics covered in this chapter:

• It verifies and checks for consistency all the options passed to it.

• It concatenates sections of the same characteristics (for example, type, attributes, and name) from the input relocatable objects to form new sections within the output file. These concatenated sections can in turn be associated to output segments.

• It reads symbol table information from both relocatable objects and shared objects to verify and unite references with definitions, and usually generates a new symbol table, or tables, within the output file.

• It reads relocation information from the input relocatable objects and applies this information to the output file by updating other input sections. In addition, output relocation sections might be generated for use by the runtime linker.

• It generates program headers that describe all segments created.

• It generates a dynamic linking information section if necessary, which provides information such as shared object dependencies to the runtime linker.

The process of concatenating like sections and associating sections to segments is carried out using default information within the link-editor. The default section and segment handling provided by the link-editor is usually sufficient for most link-edits. However, these defaults can be manipulated using the -M option with an associated mapfile (see Chapter 8, Mapfile Option for more details).