Re: creating directories through dependency

[Philip Guenther]

On 1/9/07, Michal Nowikowski <address@hidden> wrote:
> I have a problem of chicken-and-egg type. Makefile:
> ---
> BUILDDIR=build
>
> $(BUILDDIR):
>    mkdir -p $(BUILDDIR)
>
> $(objs):%.o:%.c $(BUILDDIR)
>    $(CC) -c $< -o $(BUILDDIR)/$@
> ---
>
> In this situation BUILDDIR is created before sources are compiled.
> But when everything is compiled and I re-run make it tries to recompile
> some source files because make claims that BUILDDIR is newer then
> some object files. It is like that because BUILDDIR had been changing when
> new object file were generated and put into it.
>
> How to solve this problem i.e. avoid recompiling files during subsequent
> make run?

There are a number of ways to solve this.  Here are the (IMHO) best two:

1) use an order-only dependency (requires GNU make version 3.80 or later):
   put the dependency on $(BUILDDIR) after a '|', like this:

$(objs):%.o:%.c | $(BUILDDIR)


2) just create the directory at the start of the run and don't put any
dependencies on
   it at all.  The only difference is that $(BUILDDIR) will be
created even when it wasn't
   really needed for the particular target(s) being requested.  On
the plus side, it
   requires less memory and work from make.

$(shell mkdir -p $(BUILDDIR))


Philip Guenther