Next: Comments in Make Rules, Previous: The Make Macro SHELL, Up: Portable 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 $$$$
may output the following with
make -j1, at least on NetBSD up to
5.1 and FreeBSD up to 8.2:
/ value 32235 / value 32235
while without -j1, or with -B, the output looks less surprising:
/ value 32238 /tmp 32239
Another consequence is that, if one command in a recipe uses
to indicate a successful exit, the shell will be gone and the remaining
commands of this recipe will not be executed.
The BSD make implementations, when run in parallel mode, will also pass the Makefile recipes to the shell through its standard input, thus making it unusable from the recipes:
$ cat Makefile read: @read line; echo LINE: $$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
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
$ 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.