Linker and Libraries Guide

Shared Object Processing

Shared objects are indivisible whole units that have been generated by a previous link-edit of one or more input files. When the link-editor processes a shared object, the entire contents of the shared object become a logical part of the resulting output file image. This logical inclusion means that all symbol entries defined in the shared object are made available to the link-editing process.

The shared object's program data sections and most of the link-editing information sections are unused by the link-editor. These sections are interpreted by the runtime linker when the shared object is bound to generate a runnable process. However, the occurrence of a shared object is remembered. Information is stored in the output file image to indicate that this object is a dependency that must be made available at runtime.

By default, all shared objects specified as part of a link-edit are recorded as dependencies in the object being built. This recording is made regardless of whether the object being built actually references symbols offered by the shared object. To minimize the overhead of runtime linking, only specify those dependencies that resolve symbol references from the object being built. The link-editor's debugging capabilities, and ldd(1) with the -u option, can be used to determine unused dependencies. The link-editor's -z ignore option can be used to suppress the dependency recording of any unused shared objects.

If a shared object has dependencies on other shared objects, these dependencies can also be processed. This processing occurs after all command line input files have been processed, to complete the symbol resolution process. However, the shared object names are not recorded as dependencies in the output file image being generated.

Although the position of a shared object on the command line has less significance than archive processing, the position can have a global effect. Multiple symbols of the same name are allowed to occur between relocatable objects and shared objects, and between multiple shared objects. See Symbol Resolution.

The order of shared objects processed by the link-editor is maintained in the dependency information that is stored in the output file image. In the absence of lazy loading, the runtime linker loads the specified shared objects in the same order. Therefore, the link-editor and the runtime linker select the first occurrence of a symbol of a multiply-defined series of symbols.

Note –

Multiple symbol definitions, are reported in the load map output generated using the -m option.