Re: can't always get make to auto create build directory

[LMH]

Paul Smith wrote:
> On Wed, 2016-06-29 at 14:03 -0400, LMH wrote:
> > #create build directory if it doesn't exist
> > $(BDIR):
> >          @mkdir -p $(BDIR)
> >
> > all: $(BDIR)/SMD2_i386.exe
>
> Here you've created a target $(BDIR), but your "all" target depends
> only on the object file $(BDIR)/SMD2_i386.exe.
>
> Since nothing depends on the actual target $(BDIR), make will never try
> to build that target.
>
> In order for this to work you'd need to define the directory as a
> prerequisite of the target which needs it, which in this case is
> $(BDIR)/SMD2_i386.exe, so you'd need to write:
>
>    $(BDIR)/SMD2_i386.exe : <other prereqs> $(BDIR)
>
> However, you should generally not have targets depend on directories,
> because make treats them just like any other file when it checks
> modification times; however directories do not act like normal files
> when it comes to modification times.
>
> I usually just use:
>
>    $(shell mkdir -p $(BDIR))
>
> but other people prefer order-only prerequisites:
>
>    $(BDIR)/SMD2_i386.exe : <other prereqs> | $(BDIR)
>
> (note the extra "|" there).  Check the GNU make manual for details.
>
> Note these are all specific to GNU make and not portable to other versions of 
> make.


Going back through older versions of some of my makefiles, I find that I used to have 
an "archdir" target defined along with the code to create the build directory.


archdir: ${BDIR}
${BDIR}:
        @mkdir -p $(BDIR)


The archdir target was included in the all target like,


all: archdir $(BDIR)/SMD2_i386.exe


Somewhere along the line, this archdir target seems to have been left out and I guess 
this is where I started having problems in cases where the build directory didn't 
already exist. The build directory likely did exist in many cases, so I probably 
didn't notice right away or thought it was a problem esoteric to the OS I was booted 
into.

If you don't mind my asking, how does the above "archdir" solution compare with what 
you suggested as far as defining $(BDIR) as a prerequisite for $(BDIR)/SMD2_i386.exe? 
Is there any difference?

Your shell solution is simpler than mine in that it doesn't need to actually check if 
the directory exists and just suppresses the error message if it does. I am wondering 
if there is any value to actually making the check, but I can't think of any off hand.

LMH