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Re: question about Generating Prerequisites Automatically

From: Paul D. Smith
Subject: Re: question about Generating Prerequisites Automatically
Date: Wed, 3 May 2006 08:24:55 -0400

This section tells you pretty much everything there is to know about
intermediate files.

The definition:

     However, even if `N.c' does not exist and is not mentioned [in the
  makefile], `make' knows how to envision it as the missing link between
  `N.o' and `N.y'!  In this case, `N.c' is called an "intermediate

And how they behave:

  Once `make' has decided to use the intermediate file, it is entered in
  the data base as if it had been mentioned in the makefile, along with
  the implicit rule that says how to create it.

     Intermediate files are remade using their rules just like all other
  files.  But intermediate files are treated differently in two ways.

     The first difference is what happens if the intermediate file does
  not exist.  If an ordinary file B does not exist, and `make' considers
  a target that depends on B, it invariably creates B and then updates
  the target from B.  But if B is an intermediate file, then `make' can
  leave well enough alone.  It won't bother updating B, or the ultimate
  target, unless some prerequisite of B is newer than that target or
  there is some other reason to update that target.

     The second difference is that if `make' _does_ create B in order to
  update something else, it deletes B later on after it is no longer
  needed.  Therefore, an intermediate file which did not exist before
  `make' also does not exist after `make'.  `make' reports the deletion
  to you by printing a `rm -f' command showing which file it is deleting.

     Ordinarily, a file cannot be intermediate if it is mentioned in the
  makefile as a target or prerequisite.  However, you can explicitly mark
  a file as intermediate by listing it as a prerequisite of the special
  target `.INTERMEDIATE'.  This takes effect even if the file is mentioned
  explicitly in some other way.

     You can prevent automatic deletion of an intermediate file by
  marking it as a "secondary" file.  To do this, list it as a
  prerequisite of the special target `.SECONDARY'.  When a file is
  secondary, `make' will not create the file merely because it does not
  already exist, but `make' does not automatically delete the file.
  Marking a file as secondary also marks it as intermediate.

 Paul D. Smith <address@hidden>          Find some GNU make tips at:            
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist

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