Unix in a Nutshell, 4th Edition

Special Dependencies

GNU make has two special features for working with dependencies.

Library dependencies

A dependency of the form -l NAME causes make to search for a library file whose name is either lib NAME .so or lib NAME.a in the standard library directories. This is customizable with the .LIBPATTERNS variable; see the later section "Macros with Special Handling" for more information.

Order-only prerequisites

When a normal prerequisite of a target is out of date, two things happen. First, the prerequisite (and its prerequisites, recursively) are rebuilt as needed. This imposes an ordering on the building of targets and prerequisites. Second, after the prerequisites are updated, the target itself is rebuilt using the accompanying commands. Normally, both of these are what’s desired.

Sometimes, you just wish to impose an ordering, such that the prerequisites are themselves updated, but the target is not rebuilt by running its rules. Such order-only prerequisites are specified in a dependency line by placing them to the right of a vertical bar or pipe symbol, |:

    target: normal-dep1 normal-dep2 | order-dep1 order-dep2
            command

Dependency lines need not contain both. I.e., you do not have to provide regular dependencies if there are order-only dependencies as well; just place the | right after the colon.

Here is an annotated example of an order-only dependency:

    $ cat Makefile
    all: target                       First target is default, point to real target

    prereq0:                          How to make prereq0 @echo making prereq0 ...