[erlehmann_]

A makefile consists of rules. Each rule contains a dependency line which defines a target and an enumeration of prerequisites. This means that the dependencies have to be known before the target is built. By design, it is impossible for a single-pass make(1) invocation to derive dependencies for a C program, as dependencies are output by the compiler.

By contrast, redo builds the target first and then records what was used to build it. For example, when compiling a C file with “gcc -M”, gcc will output dependency information. With redo, you normally record those dependencies after the target has been built. With make(1), that information has to end up in the makefile somehow, possibly leading to further builds.

[shakna]

I mentioned m4 before as a way of using more passes, and is how the Linux kernel approaches this, but looking deeper at make, I'm not even sure you need it.

From the manual, on implicit rules:

foo : foo.o bar.o cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)

Because you mention `foo.o' but do not give a rule for it, make will automatically look for an implicit rule that tells how to update it. This happens whether or not the file `foo.o' currently exists.

[erlehmann_]

I do not understand. I know that make(1) has many implicit rules – but which of the problems I mentioned does this solve and how? Please be specific about your solution.