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 Makefiles, 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 a 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.