Support for parallel execution in @command{make} implementation varies.
Generally, using GNU make is your best bet.
-Some make implementations (among them, FreeBSD @command{make}, NetBSD
-@command{make}, and Solaris @command{dmake}), when invoked with a
-@option{-j@var{N}} 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 behaviour
-of the spawned processes. For example, even if the @command{make}
-standard output is connected to a tty, the recipe command will not be:
-
-@example
-$ @kbd{cat Makefile}
-all:
- @@test -t 1 && echo "Is a tty" || echo "Is not a tty"
-$ @kbd{make -j 2} # FreeBSD make
-Is not a tty
-$ @kbd{make -j 2} # NetBSD make
---- all ---
-Is not a tty
-$ @kbd{dmake -j 2} # Solaris dmake
-@var{hostname} --> 1 job
-@var{hostname} --> Job output
-Is not a tty
-@end example
-
-@noindent
-On the other hand:
-
-@example
-$ @kbd{make -j 2} # GNU make, Heirloom make
-Is a tty
-@end example
-
-Furthermore, parallel runs of those @command{make} implementations will
-route standard error from commands that they spawn into their own
-standard output, and may remove leading whitespace from output lines.
-
When NetBSD or FreeBSD @command{make} are run in parallel mode, they will
reuse the same shell for multiple commands within one recipe. This can
-have unexpected consequences. For example, change of directories or
-variables persist between commands:
+have various unexpected consequences. For example, changes of directories
+or variables persist between recipes, so that:
@example
all:
@end example
@noindent
-may output the following with @code{make -j1}:
+may output the following with @code{make -j1}, at least on NetBSD up to
+5.1 and FreeBSD up to 8.2:
@example
---- all ---
/
value
32235
32235
@end example
+@noindent
while without @option{-j1}, or with @option{-B}, the output looks less
surprising:
32239
@end example
-Another consequence of this is that, if one command in a recipe uses
-@code{exit 0} to indicate a successful exit, the shell will be gone
-and the remaining commands of this recipe will not be executed.
+@noindent
+Another consequence is that, if one command in a recipe uses @code{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 @command{make} implementations, when run in parallel mode,
+will also pass the @command{Makefile} recipes to the shell through
+its standard input, thus making it unusable from the recipes:
+
+@example
+$ @kbd{cat Makefile}
+read:
+ @@read line; echo LINE: $$line
+@c $$ @c restore font-lock
+$ @kbd{echo foo | make read}
+LINE: foo
+$ @kbd{echo foo | make -j1 read} # NetBSD 5.1 and FreeBSD 8.2
+LINE:
+@end example
+
+@noindent
+Moreover, when FreeBSD @command{make} (up at least to 8.2) is run in
+parallel mode, it implements the @code{@@} and @code{-} ``recipe
+modifiers'' by dynamically modifying the active shell flags. This
+behaviour has the effects of potentially clobbering the exit status
+of recipes silenced with the @code{@@} modifier if they also unset
+the @option{errexit} shell flag, and of mangling the output in
+unexpected ways:
-The above example also shows additional status output produced by NetBSD
-@command{make} (but @emph{not} by FreeBSD @command{make}) in parallel
-mode for targets being updated.
+@example
+$ @kbd{cat Makefile}
+a:
+ @@echo $$-; set +e; false
+b:
+ -echo $$-; false; echo set -
+$ @kbd{make a; echo status: $?}
+ehBc
+*** Error code 1
+status: 1
+$ @kbd{make -j1 a; echo status: $?}
+ehB
+status: 0
+$ @kbd{make b}
+echo $-; echo set -
+hBc
+set -
+$ @kbd{make -j1 b}
+echo $-; echo hvB
+@end example
-You can avoid these issues by using the @option{-B} option to enable
+@noindent
+You can avoid all these issues by using the @option{-B} option to enable
compatibility semantics. However, that will effectively also disable
all parallelism as that will cause prerequisites to be updated in the
order they are listed in a rule.
+Some make implementations (among them, FreeBSD @command{make}, NetBSD
+@command{make}, and Solaris @command{dmake}), when invoked with a
+@option{-j@var{N}} option, connect the standard output and standard
+error of all their child processes to pipes or temporary regular
+files. This can lead to subtly different semantics in the behaviour
+of the spawned processes. For example, even if the @command{make}
+standard output is connected to a tty, the recipe command will not be:
+
+@example
+$ @kbd{cat Makefile}
+all:
+ @@test -t 1 && echo "Is a tty" || echo "Is not a tty"
+$ @kbd{make -j 2} # FreeBSD 8.2 make
+Is not a tty
+$ @kbd{make -j 2} # NetBSD 5.1 make
+--- all ---
+Is not a tty
+$ @kbd{dmake -j 2} # Solaris 10 dmake
+@var{hostname} --> 1 job
+@var{hostname} --> Job output
+Is not a tty
+@end example
+
+@noindent
+On the other hand:
+
+@example
+$ @kbd{make -j 2} # GNU make, Heirloom make
+Is a tty
+@end example
+
+@noindent
+The above examples also show additional status output produced in parallel
+mode for targets being updated by Solaris @command{dmake} and NetBSD
+@command{make} (but @emph{not} by FreeBSD @command{make}).
+
+Furthermore, parallel runs of those @command{make} implementations will
+route standard error from commands that they spawn into their own
+standard output, and may remove leading whitespace from output lines.
+
+
@node Comments in Make Rules
@section Comments in Make Rules
@cindex Comments in @file{Makefile} rules
projects / thirdparty / autoconf.git / commitdiff
