(make.info)Options/Recursion


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Communicating Options to a Sub-`make'
-------------------------------------

   Flags such as `-s' and `-k' are passed automatically to the
sub-`make' through the variable `MAKEFLAGS'.  This variable is set up
automatically by `make' to contain the flag letters that `make'
received.  Thus, if you do `make -ks' then `MAKEFLAGS' gets the value
`ks'.

   As a consequence, every sub-`make' gets a value for `MAKEFLAGS' in
its environment.  In response, it takes the flags from that value and
processes them as if they had been given as arguments.  Note: Summary
of Options.

   The options `-C', `-f', `-I', `-o', and `-W' are not put into
`MAKEFLAGS'; these options are not passed down.

   The `-j' option is a special case (*note Parallel Execution:
Parallel.).  If you set it to some numeric value, `-j 1' is always put
into `MAKEFLAGS' instead of the value you specified.  This is because if
the `-j' option were passed down to sub-`make's, you would get many
more jobs running in parallel than you asked for.  If you give `-j'
with no numeric argument, meaning to run as many jobs as possible in
parallel, this is passed down, since multiple infinities are no more
than one.

   If you do not want to pass the other flags down, you must change the
value of `MAKEFLAGS', like this:

     MAKEFLAGS=
     subsystem:
             cd subdir; $(MAKE)

   or like this:

     subsystem:
             cd subdir; $(MAKE) MAKEFLAGS=

   A similar variable `MFLAGS' exists also, for historical
compatibility.  It has the same value as `MAKEFLAGS' except that it
always begins with a hyphen unless it is empty (`MAKEFLAGS' begins with
a hyphen only when it begins with an option that has no single-letter
version, such as `--warn-undefined-variables').  `MFLAGS' was
traditionally used explicitly in the recursive `make' command, like
this:

     subsystem:
             cd subdir; $(MAKE) $(MFLAGS)

but now `MAKEFLAGS' makes this usage redundant.  If you want your
makefiles to be compatible with old `make' programs, use this
technique; it will work fine with more modern `make' versions too.

   The `MAKEFLAGS' variable can also be useful if you want to have
certain options, such as `-k' (Note: Summary of Options
Summary.), set each time you run `make'.  You simply put a value for
`MAKEFLAGS' in your environment.  You can also set `MAKEFLAGS' in a
makefile, to specify additional flags that should also be in effect for
that makefile.  (Note that you cannot use `MFLAGS' this way.  That
variable is set only for compatibility; `make' does not interpret a
value you set for it in any way.)

   When `make' interprets the value of `MAKEFLAGS' (either from the
environment or from a makefile), it first prepends a hyphen if the value
does not already begin with one.  Then it chops the value into words
separated by blanks, and parses these words as if they were options
given on the command line (except that `-C', `-f', `-h', `-o', `-W',
and their long-named versions are ignored; and there is no error for an
invalid option).

   If you do put `MAKEFLAGS' in your environment, you should be sure not
to include any options that will drastically affect the actions of
`make' and undermine the purpose of makefiles and of `make' itself.
For instance, the `-t', `-n', and `-q' options, if put in one of these
variables, could have disastrous consequences and would certainly have
at least surprising and probably annoying effects.


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