What does the GNU ld --undefined option do?

Question

Can somebody explain what the GNU ld option --undefined does?

Working on a LiteOS project. The app is linked with many -u options. For example -utask_shellcmd.

The GNU linker manual for --undefined=symbol simply says:

Force symbol to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries.

So the symbol will be included in the output file as an undefined. What if the symbol is already defined in one of the linked obj files? If it is really undefined, when the linking of additional modules will happen and how does that happen?

Answer 1

The -u option is only relevant when archive (.a) libraries are involved (maybe also .so libraries with --as-needed in effect).

Unlike individual object files (.o) on the linking command line, which are all linked in the order in which they appear, object files from an archive library are only linked when they satisfy one or more undefined symbol references at the point they appear in the link command line order. Once once .o file from the archive is pulled into the link, the process is repeated recursively, so that if it introduces more undefined symbol references, other object files from the same (or later) archives will be pulled in to satisfy them.

Using -u allows you to cause a particular symbol (and, indirectly, all dependencies of the object file it was defined in) to be pulled into the link. Of course you could just put all .o files on the command line directly, without using any archive libraries, but by using libraries you can avoid linking unused object files (this is especially useful if large parts of the code may be unused depending on build-time-configurable settings in other files!) while getting the ones you need.