Archives are built using ar(1), and usually consist of a collection of relocatable objects with an archive symbol table. This symbol table provides an association of symbol definitions with the objects that supply these definitions. By default, the link-editor provides selective extraction of archive members. When the link-editor reads an archive, it uses information within the internal symbol table it is creating to select only the objects from the archive it requires to complete the binding process. You can also explicitly extract all members of an archive.
The archive member contains a data symbol definition that satisfies a tentative symbol definition presently held in the link-editor's internal symbol table. An example of this is a
FORTRAN COMMON block definition, which causes the extraction of a relocatable object that defines the same DATA symbol.
Under selective archive extraction, a weak symbol reference does not extract an object from an archive unless the -z weakextract option is in effect. See Simple Resolutions for more information.
With selective archive extraction, the link-editor makes multiple passes through an archive to extract relocatable objects as needed to satisfy the symbol information being accumulated in the link-editor internal symbol table. After the link-editor has made a complete pass through the archive without extracting any relocatable objects, it moves on to process the next input file.
By extracting from the archive only the relocatable objects needed at the time the archive was encountered, the position of the archive within the input file list can be significant. See Position of an Archive on the Command Line.
Although the link-editor makes multiple passes through an archive to resolve symbols, this mechanism can be quite costly for large archives containing random organizations of relocatable objects. In these cases, you should use tools like lorder(1) and tsort(1) to order the relocatable objects within the archive and so reduce the number of passes the link-editor must carry out.