Limitations on Makefiles

Concurrent building of multiple targets places some restrictions on makefiles. Makefiles that depend on the implicit ordering of dependencies may fail when built concurrently. Targets in makefiles that modify the same files may fail if those files are modified concurrently by two different targets. Some examples of possible problems are discussed in this section.

Dependency Lists

When building targets concurrently, it is important that dependency lists be accurate. For example, if two executables use the same object file but only one specifies the dependency, then the build may cause errors when done concurrently. For example, consider the following makefile fragment:

all: prog1 prog2 
prog1: prog1.o aux.o 
	$(LINK.c) prog1.o aux.o -o prog1 
prog2: prog2.o 
	$(LINK.c) prog2.o aux.o -o prog2 

When built serially, the target aux.o is built as a dependent of prog1 and is up-to-date for the build of prog2. If built in parallel, the link of prog2 may begin before aux.o is built, and is therefore incorrect. The .KEEP_STATE feature of make detects some dependencies, but not the one shown above.

Explicit Ordering of Dependency Lists

Other examples of implicit ordering dependencies are more difficult to fix. For example, if all of the headers for a system must be constructed before anything else is built, then everything must be dependent on this construction. This causes the makefile to be more complex and increases the potential for error when new targets are added to the makefile. The user can specify the special target .WAIT in a makefile to indicate this implicit ordering of dependents. When dmake encounters the .WAIT target in a dependency list, it finishes processing all prior dependents before proceeding with the following dependents. More than one .WAIT target can be used in a dependency list. The following example shows how to use .WAIT to indicate that the headers must be constructed before anything else.

You can add an empty rule for the .WAIT target to the makefile so that the makefile is backward-compatible.