The -u option provides a mechanism for generating a global symbol reference from the link-edit command line. This option can be used to perform a link-edit entirely from archives. This option can also provide additional flexibility in selecting the objects to extract from multiple archives. See section Archive Processing for an overview of archive extraction.
For example, perhaps you want to generate a dynamic executable from the relocatable object main.o, which refers to the symbols foo and bar. You want to obtain the symbol definition foo from the relocatable object foo.o contained in lib1.a, and the symbol definition bar from the relocatable object bar.o, contained in lib2.a.
However, the archive lib1.a also contains a relocatable object that defines the symbol bar. This relocatable object is presumably of differing functionality to the relocatable object that is provided in lib2.a. To specify the required archive extraction, you can use the following link-edit.
$ cc -o prog -L. -u foo -l1 main.o -l2
The -u option generates a reference to the symbol foo. This reference causes extraction of the relocatable object foo.o from the archive lib1.a. The first reference to the symbol bar occurs in main.o, which is encountered after lib1.a has been processed. Therefore, the relocatable object bar.o is obtained from the archive lib2.a.
This simple example assumes that the relocatable object foo.o from lib1.a does not directly or indirectly reference the symbol bar. If lib1.a does reference bar, then the relocatable object bar.o is also extracted from lib1.a during its processing. See Archive Processing for a discussion of the link-editor's multi-pass processing of an archive.