Question

In my project, modules are organized in subdirs for tidiness.

My project dir hierarchy:

$ ls -R .:         configure.in Makefile.am  Makefile.cvs  src ./src:     log          Makefile.am  main.cpp ./src/log: log.cpp      Makefile.am 

configure.in:

AC_INIT(configure.in) AM_CONFIG_HEADER(config.h) AM_INIT_AUTOMAKE(myapp, 0.1) AC_LANG_CPLUSPLUS AC_PROG_CXX AM_PROG_LIBTOOL AC_OUTPUT(Makefile src/Makefile src/log/Makefile) 

Makefile.am:

AUTOMAKE_OPTIONS = foreign SUBDIRS = src 

Makefile.cvs:

default:     aclocal     libtoolize --force --copy     autoheader     automake --add-missing --copy     autoconf 

src/Makefile.am

bin_PROGRAMS = myapp myapp_SOURCES = main.cpp SUBDIRS = log myapp_LDADD = $(top_builddir)/src/log/liblog.la 

src/log/Makefile.am:

INCLUDES = $(all_includes) METASOURCES = AUTO noinst_LTLIBRARIES = liblog.la liblog_la_SOURCES = log.cpp 

src/main.cpp: contains int main(){}, src/log/log.cpp contains void f(){}.

Invoking make produces (edited for brevity):

libtool --mode=compile g++     -MT log.lo -MD -MP -MF .deps/log.Tpo -c -o log.lo log.cpp libtool: compile:  g++ -MT log.lo -MD -MP -MF .deps/log.Tpo -c log.cpp  -fPIC -DPIC -o .libs/log.o libtool: compile:  g++ -MT log.lo -MD -MP -MF .deps/log.Tpo -c log.cpp -o log.o >/dev/null 2>&1 mv -f .deps/log.Tpo .deps/log.Plo libtool --mode=link g++    -o liblog.la  log.lo   libtool: link: ar cru .libs/liblog.a .libs/log.o  libtool: link: ranlib .libs/liblog.a libtool: link: ( cd '.libs' && rm -f 'liblog.la' && ln -s '../liblog.la' 'liblog.la' ) g++ -MT main.o -MD -MP -MF .deps/main.Tpo -c -o main.o main.cpp mv -f .deps/main.Tpo .deps/main.Po libtool --mode=link g++    -o myapp main.o ../src/log/liblog.la  libtool: link: g++ -o myapp main.o  ../src/log/.libs/liblog.a 

The problem is the first three lines: log.cpp is compiled twice!

The question: why isn’t it just compiled once — taking half the time?

note: I have no idea what I’m doing — autotools is black magic to me, but we have to use it in our place. The docs are incomprehensible to me.

Answer 1

By default Libtool creates two types of libraries: static and shared. (aka libfoo.a and libfoo.so)

Static and shard require different compilation flags. Dynamic libraries — shared objects use Position Independent Code with following gcc flags:

-fPIC -DPIC 

Static are not. You can force build only one type by specifing

./configure --disable-shared 

or

./configure --disable-static 

Rationale

Usually when library is provided for user it provide in two setups — static for development that allows create pure static builds and dynamic for use by most of programs that use this library. So, usually when you install library in the system you only install shared object. (aka libfoo_XYZ.deb)

When you add development version (aka libfoo-dev_XYZ.deb) you add headers and statically build version of the library that allows users make staic builds if they whant.

This is common practice for shipping libraries in UNIX. Thus libtool does this for you automatically.