.KEEP_STATE and Command Dependency Checking

In addition to the normal dependency checking, you can use the special target .KEEP_STATE to activate command dependency checking. When activated, make not only checks each target file against its dependency files, it compares each command line in the rule with those it ran the last time the target was built. This information is stored in the .make.state file in the current directory (see "State File").

With the makefile:

CFLAGS= -O 
.KEEP_STATE:

functions: functions.c 
        	cc -o functions functions.c

the following commands work as shown:

$ make 
cc -O -o functions functions.c
$ make CFLAGS=-g 
cc -g -o functions functions.c 
$ make "CFLAGS= -O -pg" 
cc -O -pg -o functions functions.c

This ensures that make compiles a program with the options you want, even if a different variant is present and otherwise up to date.

The first make run with .KEEP_STATE in effect recompiles all targets in order to gather and record the necessary information. The KEEP_STATE variable, when imported from the environment, has the same effect as the .KEEP_STATE target.

Suppressing or Forcing Command Dependency Checking for Selected Lines

To suppress command dependency checking for a given command line, insert a question mark as the first character after the TAB.

Command dependency checking is automatically suppressed for lines containing the dynamic macro $?. This macro stands for the list of dependencies that are newer than the current target, and can be expected to differ between any two make runs.

To force make to perform command dependency checking on a line containing this macro, prefix the command line with a ! character (following the TAB).

State File

When .KEEP_STATE is in effect, make writes out a state file named .make.state, in the current directory. This file lists all targets that have ever been processed while .KEEP_STATE has been in effect, along with the rules to build them, in makefile format. In order to ensure that this state file is maintained consistently, after you have added .KEEP_STATE to a makefile, it is recommended that you leave it in effect.

Since this target is ignored in earlier versions of make, it does not introduce any compatibility problems. Other versions treat it as a superfluous target that no targets depend on, with an empty rule and no dependencies of its own. Because it starts with a dot, it is not used as the starting target.