Archive Processing

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. It is also possible to explicitly extract all members of an archive.

The link-editor will extract a relocatable object from an archive if:

• The archive contains a symbol definition that satisfies a symbol reference (sometimes referred to as an undefined symbol) presently held in the link-editor's internal symbol table.

• The archive 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 will cause the extraction of a relocatable object that defines the same DATA symbol.

• The link-editors -zallextract is in effect. This option suspends selective archive extraction and causes all archive members to be extracted from the archive being processed.

Under selective archive extraction, a weak symbol reference will not cause the extraction of an object from an archive unless the -zweakextract option is in effect. Weak symbols are expanded upon in section "Simple Resolutions".

The options -zweakextract, -zallextract, and -zdefaultextract provide a means to toggle the archive extraction mechanism among multiple archives.

Under selective archive extraction the link-editor makes multiple passes through an archive extracting 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 means that the position of the archive within the input file list can be significant (see "Position of an Archive on the Command-Line" for more details).

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 can 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.