add-action-targets.md

How do I add action targets similar to PHONY targets in make?

I am working on an embedded project and I would like to define the upload action that flashes the new system image to the device. In make I would use a PHONY target with an appropriate recipe. How can I do something similar in build2?

The equivalent of a PHONY target in build2 is a target with the alias{} target type. So to define the upload action we can do (using an executable in place of a system image):

exe{image}: {hxx cxx}{*}

alias{upload}: exe{image}
{{
  diag upload $<
  echo "uploading $path($<)"
}}

See Ad hoc recipes for background on build2 recipes.

To trigger the upload action we have to specify the target type (without the target type it will be treated as an operation name; see below for more on that):

$ b alias{upload}

We can, however, prettify it a bit by defining a shorter and more meaningful alias for the alias{} target type (no pun intended). For example:

define do: alias

do{upload}: exe{image}
{{
   diag upload $<
   echo "uploading $path($<)"
}}

$ b do{upload}

You might have noticed that this is not how built-in "actions" such as update, test, or install work. In particular, there is no target type and we can specify targets to perform the actions on, as in:

$ b test: exe{image}

The reason for this difference is that these built-in actions are not targets but rather operations, a concept that does not exist in make.

It is possible to define new operations in build2 and we could add the upload operation, which would allow us to do things like:

$ b upload: exe{image}

However, adding a new operation is quite involved and can only be done by a build system module (implemented in C++). As a result, doing this for a small project is most likely an overkill. The operation/module route would be appropriate if, for example, you were the vendor of an embedded C++ SDK who wished to make it easier for projects that use this SDK to setup their builds (and who also had the time and expertise to implement and maintain the build system module).