Introduction

1

This manual describes the operations of the SunOS operating system link-editor and runtime linker, together with the objects on which they operate. The basic operation of the SunOS linkers involves the combination of objects and the connection of symbolic references from one object to the symbolic definitions within another. This operation is often referred to as binding.

The main areas this manual expands upon are:

The Link-Editor
The link-editor, ld(1), is responsible for concatenating one or more input files (either relocatable objects, shared objects, or archive libraries) to produce one output file (either a relocatable object, an executable application, or a shared object). The link-editor is most commonly invoked as part of the compilation environment (see cc(1)).
The Runtime Linker
The runtime linker, ld.so.1¹,is responsible for processing dynamic executables and shared objects at runtime, and binding them to create a runable process.

1. ld.so.1 is a special case of a shared object and thus allows itself to be versioned. Here we use a version number of 1, however later releases of the SunOS operating system may provide higher version numbers.

Shared Objects (sometimes also referred to as Shared Libraries)
Shared objects are one form of output from the link-edit phase. However, their importance in creating a powerful, flexible runtime environment warrants a section of its own.
Object Files
The SunOS linkers work with files that conform to the executable and linking format (ELF).

These four areas, although separable into individual topics, have a great deal of overlap. While explaining each area, this document brings together the connecting principles and designs.

Link-Editing

Link-editing takes a variety of input modules, from cc(1), as(1) or ld(1), and performs concatenation and interpretation of the data within these input modules to form a single output module. Although the link-editor provides numerous options, the output module produced is one of four basic kinds shown in Figure 1-1 on page 3.

Relocatable object - a concatenation of input relocatable objects, which may be used in subsequent link-edit phases.
Static executable -a concatenation of input relocatable objects that has all symbolic references bound to the executable, and thus represents a ready to run process.
Dynamic executable - a concatenation of input relocatable objects that requires intervention by the run-time linker to produce a runable process. Its symbolic references may still need to be bound at run-time, and it may have one or more dependencies in the form of shared objects.
Shared object - a concatenation of input relocatable objects that provides services that may be bound to a dynamic executable at runtime. The shared object may also have dependencies on other shared objects.

Of the above four types of link-editor output, the last two, dynamic executables and shared objects, are the main focus of this document.

Runtime Linking

Runtime linking involves the binding of objects, normally generated from one or more previous link-edits, to generate a runable process. During the generation of these objects by the link-editor, the binding requirements are verified and appropriate bookkeeping information is added to each object to allow the runtime linker to map, relocate, and complete the binding process. During the execution of the process, the facilities of the runtime linker are also made available and may be used to extend the process' address space by adding additional shared libraries on demand. The two most common components involved in runtime linking are dynamic executables and shared objects.

Dynamic Executables

Dynamic executables are applications that are executed under the control of a runtime linker. These applications normally have dependencies in the form of shared objects, which are located and bound by the runtime linker to create a runable process. Dynamic executables are the default output module generated by the link-editor.

Shared Objects

Shared objects provide the key building block to a dynamically linked system. Basically, a shared object is similar to a dynamic executable, however shared objects normally have no entry point and they have not yet been assigned a virtual address. Dynamic executables normally have dependencies on one or more shared objects. That is, the shared object(s) must be bound to the dynamic executable to produce a runable process. Because shared objects may be used by many applications, many of the aspects of their construction directly affect shareability, versioning and performance.

It is useful to distinguish the processing of shared objects by either the link-editor or the runtime linker by referring to the environments in which the shared objects are being used:

The compilation environment. Here, shared objects are processed by the link-editor to generate dynamic executables or other shared objects. The shared objects become dependencies of the output file being generated.
The runtime environment. Here, shared objects are processed by the runtime linker, together with a dynamic executable, to produce a runable process.

Related Topics

Dynamic Linking

Dynamic linking is a term often used to embrace those portions of the link-editing process that generate dynamic executables and shared objects, together with the runtime linking of these objects to generate a runable process. Dynamic linking allows multiple applications to use the code provided by a shared object by enabling the application to bind to the shared object at runtime.

By separating an application from the services of standard libraries, dynamic linking also increases the portability and extensibility of an application. This separation between the interface of a service and its implementation enables the system to evolve while maintaining application stability, and is a crucial factor in providing an application binary interface (ABI). Dynamic linking is the preferred compilation method for SunOS applications.

Application Binary Interfaces

To enable the asynchronous evolution of system and application components, binary interfaces between these facilities are defined. The SunOS linkers operate upon these interfaces to assemble applications for execution. Although all components handled by the SunOS linkers have binary interfaces, one family of such interfaces of particular interest to applications writers is the System V Application Binary Interface.

The System V Application Binary Interface, or ABI, defines a system interface for compiled application programs. Its purpose is to document a standard binary interface for application programs on systems that implement the System V Interface Definition, Third Edition. The SunOS operating system provides for the generation and execution of ABI-conformant applications. On SPARC systems, the ABI is contained as a subset of the SPARC^(R) Compliance Definition (SCD).

Many of the topics covered in the following chapters are influenced by the ABI. For more detailed information refer to the appropriate ABI manuals.

Support Tools

Together with the objects mentioned in the previous sections come a number of support tools and libraries. These tools provide for the analysis and inspection of these objects and the linking processes. Among these tools are: nm(1), dump(1), ldd(1), elf(3E), and a linker debugging support library. Throughout this document we augment any discussions with examples of the use of these tools.