Very Computer

The make man page is not clear to me at all.

TIA

Patrick

Let's say some of the images are bitmaps:

%.ps: %.pnm

%.pnm: %.gif

This is a dependency chain; if foo.ps is older than foo.gif,
make will regenerate it even if foo.gif doesn't exist.
(This is why I use % instead of *.)

Some images are xfig files:

%.ps: %.fig

If you put this together into one Makefile, make will complain:
it now has two ways to create foo.ps.

With standard : rules, like these, you can only provide one set
of commands to generate any given target.  Additional empty rules,
that just list dependencies, are fine:

%.ps: .remake-ps-files-please

but only one of the rules for .ps can specify commands to create them.

The :: rules is for cases in which we do have two different ways
to create the same type of target.  The following is a valid Makefile:

%.ps:: %.fig

%.ps:: %.pnm

%.pnm: %.gif

This will give make two different ways to create any .ps file.
Now, when you type

  make foo.ps

it will use the first rule if foo.fig exists,
and the second one if foo.pnm exists.

The problem is that we don't have the .pnm file, we have a .gif file,
so if foo.pnm is missing, 'make foo.ps' won't recreate it from foo.gif:
dependency chains aren't followed across :: rules, says the manpage.

Another problem is that GNU make and /usr/ccs/bin/make behave differently,
and I don't remember the details.

This makes sense in general: if it happened, it would be nontrivial
to find the chain to use.

Fortunately, :: and : rules can be mixed:

%.ps:: %.fig

%.ps: %.pnm

%.pnm: %.gif

This will make both makes do what we want.

--
Reinier

Let's say some of the images are bitmaps:

%.ps: %.pnm

%.pnm: %.gif

This is a dependency chain; if foo.ps is older than foo.gif,
make will regenerate it even if foo.gif doesn't exist.
(This is why I use % instead of *.)

Some images are xfig files:

%.ps: %.fig

If you put this together into one Makefile, make will complain:
it now has two ways to create foo.ps.

With standard : rules, like these, you can only provide one set
of commands to generate any given target.  Additional empty rules,
that just list dependencies, are fine:

%.ps: .remake-ps-files-please

but only one of the rules for .ps can specify commands to create them.

The :: rules is for cases in which we do have two different ways
to create the same type of target.  The following is a valid Makefile:

%.ps:: %.fig

%.ps:: %.pnm

%.pnm: %.gif

This will give make two different ways to create any .ps file.
Now, when you type

  make foo.ps

it will use the first rule if foo.fig exists,
and the second one if foo.pnm exists.

The problem is that we don't have the .pnm file, we have a .gif file.
If foo.pnm is missing, '/usr/ccs/bin/make foo.ps' will recreate it
from foo.gif, but '/usr/gnu/bin/make foo.ps' will not, and I like
my Makefiles to be portable.

Fortunately, :: and : rules can be mixed:

%.ps:: %.fig

%.ps: %.pnm

%.pnm: %.gif

This will make even GNU make do what we want.

--
Reinier