Inference rules are templates that define how a file with one extension is created from a file with another extension. NMAKE uses inference rules to supply commands for updating targets and to infer dependents for targets. In the dependency tree, inference rules cause targets to have inferred dependents as well as explicitly specified dependents; see “Inferred Dependents”. The .SUFFIXES list determines priorities for applying inference rules; see “Dot Directives”.
Inference rules provide a convenient shorthand for common operations. For instance, you can use an inference rule to avoid repeating the same command in several description blocks. You can define your own inference rules or use predefined inference rules. Inference rules can be specified in the makefile or in TOOLS.INI.
Inference rules can be used in the following situations:
If NMAKE encounters a description block that has no commands, it checks the .SUFFIXES list and the files in the current or specified directory and then searches for an inference rule that matches the extensions of the target and an existing dependent file with the highest possible .SUFFIXES priority.
If a dependent file doesn't exist and is not listed as a target in another description block, NMAKE looks for an inference rule that shows how to create the missing dependent from another file with the same base name.
If a target has no dependents and its description block has no commands, NMAKE can use an inference rule to create the target.
If a target is specified on the command line and there is no makefile (or no mention of the target in the makefile), inference rules are used to build the target.
If a target is used in more than one single-colon dependency, an inference rule might not be applied as expected; see “Accumulating Targets in Dependencies”.