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.