File: autoconf.info,  Node: Parallel Make,  Next: Comments in Make Rules,  Prev: The Make Macro SHELL,  Up: Portable Make

12.10 Parallel Make
===================

Support for parallel execution in ‘make’ implementation varies.
Generally, using GNU make is your best bet.

   When NetBSD or FreeBSD ‘make’ are run in parallel mode, they will
reuse the same shell for multiple commands within one recipe.  This can
have various unexpected consequences.  For example, changes of
directories or variables persist between recipes, so that:

     all:
             @var=value; cd /; pwd; echo $$var; echo $$$$
             @pwd; echo $$var; echo $$$$

may output the following with ‘make -j1’, at least on NetBSD up to 5.1
and FreeBSD up to 8.2:

     /
     value
     32235
     /
     value
     32235

while without ‘-j1’, or with ‘-B’, the output looks less surprising:

     /
     value
     32238
     /tmp

     32239

Another consequence is that, if one command in a recipe uses ‘exit 0’ to
indicate a successful exit, the shell will be gone and the remaining
commands of this recipe will not be executed.

   The BSD ‘make’ implementations, when run in parallel mode, will also
pass the ‘Makefile’ recipes to the shell through its standard input,
thus making it unusable from the recipes:

     $ cat Makefile
     read:
             @read line; echo LINE: $$line
     $ echo foo | make read
     LINE: foo
     $ echo foo | make -j1 read # NetBSD 5.1 and FreeBSD 8.2
     LINE:

Moreover, when FreeBSD ‘make’ (up at least to 8.2) is run in parallel
mode, it implements the ‘@’ and ‘-’ "recipe modifiers" by dynamically
modifying the active shell flags.  This behavior has the effects of
potentially clobbering the exit status of recipes silenced with the ‘@’
modifier if they also unset the ‘errexit’ shell flag, and of mangling
the output in unexpected ways:

     $ cat Makefile
     a:
             @echo $$-; set +e; false
     b:
             -echo $$-; false; echo set -
     $ make a; echo status: $?
     ehBc
     *** Error code 1
     status: 1
     $ make -j1 a; echo status: $?
     ehB
     status: 0
     $ make b
     echo $-; echo set -
     hBc
     set -
     $ make -j1 b
     echo $-; echo hvB

You can avoid all these issues by using the ‘-B’ option to enable
compatibility semantics.  However, that will effectively also disable
all parallelism as that will cause prerequisites to be updated in the
order they are listed in a rule.

   Some make implementations (among them, FreeBSD ‘make’, NetBSD ‘make’,
and Solaris ‘dmake’), when invoked with a ‘-jN’ option, connect the
standard output and standard error of all their child processes to pipes
or temporary regular files.  This can lead to subtly different semantics
in the behavior of the spawned processes.  For example, even if the
‘make’ standard output is connected to a tty, the recipe command will
not be:

     $ cat Makefile
     all:
             @test -t 1 && echo "Is a tty" || echo "Is not a tty"
     $ make -j 2 # FreeBSD 8.2 make
     Is not a tty
     $ make -j 2 # NetBSD 5.1 make
     --- all ---
     Is not a tty
     $ dmake -j 2 # Solaris 10 dmake
     HOSTNAME --> 1 job
     HOSTNAME --> Job output
     Is not a tty

On the other hand:

     $ make -j 2 # GNU make, Heirloom make
     Is a tty

The above examples also show additional status output produced in
parallel mode for targets being updated by Solaris ‘dmake’ and NetBSD
‘make’ (but _not_ by FreeBSD ‘make’).

   Furthermore, parallel runs of those ‘make’ implementations will route
standard error from commands that they spawn into their own standard
output, and may remove leading whitespace from output lines.