(automake-1.16.info)Automake Silent Rules
21.3 How Automake can help in silencing make
============================================
The tricks and idioms for silencing ‘make’ described in the previous
section can be useful from time to time, but we’ve seen that they all
have their serious drawbacks and limitations. That’s why automake
provides support for a more advanced and flexible way of obtaining
quieter output from ‘make’ (for most rules at least).
To give the gist of what Automake can do in this respect, here is a
simple comparison between a typical ‘make’ output (where silent rules
are disabled) and one with silent rules enabled:
% cat Makefile.am
bin_PROGRAMS = foo
foo_SOURCES = main.c func.c
% cat main.c
int main (void) { return func (); } /* func used undeclared */
% cat func.c
int func (void) { int i; return i; } /* i used uninitialized */
The make output is by default very verbose. This causes warnings
from the compiler to be somewhat hidden, and not immediate to spot.
% make CFLAGS=-Wall
gcc -DPACKAGE_NAME=\"foo\" -DPACKAGE_TARNAME=\"foo\" ...
-DPACKAGE_STRING=\"foo\ 1.0\" -DPACKAGE_BUGREPORT=\"\" ...
-DPACKAGE=\"foo\" -DVERSION=\"1.0\" -I. -Wall -MT main.o
-MD -MP -MF .deps/main.Tpo -c -o main.o main.c
main.c: In function ‘main’:
main.c:3:3: warning: implicit declaration of function ‘func’
mv -f .deps/main.Tpo .deps/main.Po
gcc -DPACKAGE_NAME=\"foo\" -DPACKAGE_TARNAME=\"foo\" ...
-DPACKAGE_STRING=\"foo\ 1.0\" -DPACKAGE_BUGREPORT=\"\" ...
-DPACKAGE=\"foo\" -DVERSION=\"1.0\" -I. -Wall -MT func.o
-MD -MP -MF .deps/func.Tpo -c -o func.o func.c
func.c: In function ‘func’:
func.c:4:3: warning: ‘i’ used uninitialized in this function
mv -f .deps/func.Tpo .deps/func.Po
gcc -Wall -o foo main.o func.o
Clean up, so that we we can rebuild everything from scratch.
% make clean
test -z "foo" || rm -f foo
rm -f *.o
Silent rules enabled: the output is minimal but informative. In
particular, the warnings from the compiler stick out very clearly.
% make V=0 CFLAGS=-Wall
CC main.o
main.c: In function ‘main’:
main.c:3:3: warning: implicit declaration of function ‘func’
CC func.o
func.c: In function ‘func’:
func.c:4:3: warning: ‘i’ used uninitialized in this function
CCLD foo
Also, in projects using ‘libtool’, the use of silent rules can
automatically enable the ‘libtool’’s ‘--silent’ option:
% cat Makefile.am
lib_LTLIBRARIES = libx.la
% make # Both make and libtool are verbose by default.
...
libtool: compile: gcc -DPACKAGE_NAME=\"foo\" ... -DLT_OBJDIR=\".libs/\"
-I. -g -O2 -MT libx.lo -MD -MP -MF .deps/libx.Tpo -c libx.c -fPIC
-DPIC -o .libs/libx.o
mv -f .deps/libx.Tpo .deps/libx.Plo
/bin/sh ./libtool --tag=CC --mode=link gcc -g -O2 -o libx.la -rpath
/usr/local/lib libx.lo
libtool: link: gcc -shared .libs/libx.o -Wl,-soname -Wl,libx.so.0
-o .libs/libx.so.0.0.0
libtool: link: cd .libs && rm -f libx.so && ln -s libx.so.0.0.0 libx.so
...
% make V=0
CC libx.lo
CCLD libx.la
For Automake-generated ‘Makefile’s, the user may influence the
verbosity at ‘configure’ run time as well as at ‘make’ run time:
• Passing ‘--enable-silent-rules’ to ‘configure’ will cause build
rules to be less verbose; the option ‘--disable-silent-rules’ will
cause normal verbose output.
• At ‘make’ run time, the default chosen at ‘configure’ time may be
overridden: ‘make V=1’ will produce verbose output, ‘make V=0’ less
verbose output.
Note that silent rules are _disabled_ by default; the user must
enable them explicitly at either ‘configure’ run time or at ‘make’ run
time. We think that this is a good policy, since it provides the casual
user with enough information to prepare a good bug report in case
anything breaks.
Still, notwithstanding the rationales above, a developer who really
wants to make silent rules enabled by default in his own package can do
so by calling ‘AM_SILENT_RULES([yes])’ in ‘configure.ac’.
Users who prefer to have silent rules enabled by default can edit
their ‘config.site’ file to make the variable ‘enable_silent_rules’
default to ‘yes’. This should still allow disabling silent rules at
‘configure’ time and at ‘make’ time.
For portability to different ‘make’ implementations, package authors
are advised to not set the variable ‘V’ inside the ‘Makefile.am’ file,
to allow the user to override the value for subdirectories as well.
To work at its best, the current implementation of this feature
normally uses nested variable expansion ‘$(VAR1$(V))’, a ‘Makefile’
feature that is not required by POSIX 2008 but is widely supported in
practice. On the rare ‘make’ implementations that do not support nested
variable expansion, whether rules are silent is always determined at
configure time, and cannot be overridden at make time. Future versions
of POSIX are likely to require nested variable expansion, so this minor
limitation should go away with time.
To extend the silent mode to your own rules, you have few choices:
• You can use the predefined variable ‘AM_V_GEN’ as a prefix to
commands that should output a status line in silent mode, and
‘AM_V_at’ as a prefix to commands that should not output anything
in silent mode. When output is to be verbose, both of these
variables will expand to the empty string.
• You can silence a recipe unconditionally with ‘@’, and then use the
predefined variable ‘AM_V_P’ to know whether make is being run in
silent or verbose mode, adjust the verbose information your recipe
displays accordingly:
generate-headers:
... [commands defining a shell variable '$headers'] ...; \
if $(AM_V_P); then set -x; else echo " GEN [headers]"; fi; \
rm -f $$headers && generate-header --flags $$headers
• You can add your own variables, so strings of your own choice are
shown. The following snippet shows how you would define your own
equivalent of ‘AM_V_GEN’:
pkg_verbose = $(pkg_verbose_@AM_V@)
pkg_verbose_ = $(pkg_verbose_@AM_DEFAULT_V@)
pkg_verbose_0 = @echo PKG-GEN $@;
foo: foo.in
$(pkg_verbose)cp $(srcdir)/foo.in $@
As a final note, observe that, even when silent rules are enabled,
the ‘--no-print-directory’ option is still required with GNU ‘make’ if
the “Entering/Leaving directory ...” messages are to be disabled.
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