Re: Limitations of Make: VPATH target lookup (Was: Re: Can't `make install' Autoconf 2.53b if makeinfo is missing + an OSF make issue)

[Paul Eggert]

> From: Alexandre Duret-Lutz <address@hidden>
> Date: 30 Jul 2002 23:33:14 +0200
> 
> Here it is.  Ok to commit?

Looks good to me, except for the minor glitches noted below.  Thanks.


> 2002-07-30  Alexandre Duret-Lutz  <address@hidden>
> 
>       * doc/autoconf.texi (Limitations of Make): Add a 'target lookup'
>       subentry in the 'VPATH' entry.
> 
> Index: doc/autoconf.texi
> ===================================================================
> RCS file: /cvsroot/autoconf/autoconf/doc/autoconf.texi,v
> retrieving revision 1.653
> diff -u -r1.653 autoconf.texi
> --- doc/autoconf.texi 30 Jul 2002 00:42:58 -0000      1.653
> +++ doc/autoconf.texi 30 Jul 2002 21:32:19 -0000
> @@ -7405,6 +7405,8 @@
>  Prepend directory @var{dir} to the search path.  This is used to include
>  the language-specific files before any third-party macros.
>  
> address@hidden table
> +

What is this insertion doing?  Is it fixing some other bug?

>  @cindex @file{autom4te.cfg}
>  As an example, if Autoconf is installed in its default location,
>  @file{/usr/local}, running @samp{autom4te -l m4sugar foo.m4} is
> @@ -10705,6 +10707,99 @@
>  
>  The above @command{command} will be run on the empty @file{foo/bar}
>  directory that was created in the current directory.
> +
> address@hidden target lookup
> address@hidden @code{VPATH}, resolving target pathnames
> +
> +GNU make uses a rather complex algortihm to decide when it should use

make ->
@command{make}
[and similarly for other instances of "make"]

rather complex algortihm ->
       complex algorithm

> +files found via a @code{VPATH} search.  @xref{Search Algorithm,,
> +How Directory Searches are Performed, make, The GNU Make Manual}.
> +
> +If a target need to be rebuild, GNU make discards the filename found

need to be rebuild ->
needs to be rebuilt

> +during the @code{VPATH} search for this target, and build the file

build -> builds

> +locally using the filename given in the Makefile.  If a target does
> +not need to be rebuild, GNU make uses the filename found during the

rebuild -> rebuilt

> address@hidden search.
> +
> +Other make implementations, like BSD make, are easier to describe: the
> +filename found during the @code{VPATH} search will be used whether the
> +target needs to be rebuilt or not.  Therefore new files are created
> +locally, but existing files are updated at their @code{VPATH} location.
> +
> +When attempting a @code{VPATH} build for an autoconfiscated package
> +(e.g, @code{mkdir build; ../configure}), this means the GNU make will
> +build everything locally in the @file{build} directory, while BSD make
> +will build new files locally and update existing files in the source
> +directory.
> +
> address@hidden
> +% @kbd{cat Makefile}
> +VPATH = ..
> +all: foo.x bar.x
> +foo.x bar.x: newer.x
> +        @@echo Building $@@
> +% @kbd{touch ../bar.x}
> +% @kbd{touch ../newer.x}
> +% @kbd{make}        # GNU make
> +Building foo.x
> +Building bar.x
> +% @kbd{pmake}       # BSD make
> +Building foo.x
> +Building ../bar.x
> address@hidden example
> +
> +Another point worth to mention is that once GNU make has decided to

to mention ->
mentioning

> +ignore a @code{VPATH} filename (e.g. it ignored @file{../bar.x} in the
> +above example) it will continue to ignore it when the target occurs as a
> +prerequisite of another rule.
> +
> +The following example shows that GNU make with not lookup @file{bar.x}

with ->
does

lookup ->
look up

> +in @code{VPATH} before performing the @code{.x.y} rule, because it
> +ignored the @code{VPATH} result of @file{bar.x} while running the
> address@hidden: newer.x} rule.
> +
> address@hidden
> +% @kbd{cat Makefile}
> +VPATH = ..
> +all: bar.y
> +bar.x: newer.x
> +        @@echo Building $@@
> +.SUFFIXES: .x .y
> +.x.y:
> +        cp $< $@@
> +% @kbd{touch ../bar.x}
> +% @kbd{touch ../newer.x}
> +% @kbd{make}        # GNU make
> +Building bar.x
> +cp bar.x bar.y
> +cp: cannot stat `bar.x': No such file or directory
> +make: *** [bar.y] Error 1
> +% @kbd{pmake}       # BSD make
> +Building ../bar.x
> +cp ../bar.x bar.y
> address@hidden example
> +
> +Note that if you drop away the command from the @code{bar.x: newer.x}
> +rule, things will magically start to work: GNU make knows that
> address@hidden hasn't been updated, therefore it doesn't discard the
> +result from @code{VPATH} (@file{../bar.x}) in succeedding uses.

succeedding ->
succeeding

> +
> address@hidden
> +% @kbd{cat Makefile}
> +VPATH = ..
> +all: bar.y
> +bar.x: newer.x
> +.SUFFIXES: .x .y
> +.x.y:
> +        cp $< $@@
> +% @kbd{touch ../bar.x}
> +% @kbd{touch ../newer.x}
> +% @kbd{make}        # GNU make
> +cp ../bar.x bar.y
> +% @kbd{rm bar.y}
> +% @kbd{pmake}       # BSD make
> +cp ../bar.x bar.y
> address@hidden example
>  
>  @end table
>  @end table