Q1: How to link object files from a different subdirectory?
Let's say your program prog is a C program that will be linked from object file0.o, file1.o which
are to be compiled into subdirectory obj. Here is the sort of thing you typically need in your
makefile to perform that linkage.
$(OBJS) = $(patsubst %.o,obj/%.o,file0.o file1.o)
prog: $(OBJS)
gcc -o $@ $(CFLAGS) $(OBJS) $(LDFLAGS) $(LDLIBS)
This makes $(OBJS) = obj/file0.o, obj/file1.o and you simply pass the object
files like that to the link command. Documentation of patsubst
N.B. This is not sufficient to create the obj subdirectory if it doesn't exist when you want to
compile an object file into it. You'll have to create it yourself or study how to make make do it.
Q2: How to include different search paths?
This is an ambiguous question - the ambiguity is confusing you - and we must break it down into Q2.a, Q2.b, Q2.c:
Q2.a: How to specify different search paths where the preprocessor will look for header files that are #include-ed in the source code?
By default, the preprocessor will look for header files using a built-in a list of standard search paths. You can see them by
running the preprocessor in verbose mode, e.g.cpp -v (CTRL-C to terminate). The output will contain something like:
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/4.8/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
Let's suppose you have some header files of your own in subdirectories
inc and inc/foo/bar and want the preprocessor to search these directories
as well. Then you need to pass the preprocessor options:
-I inc -I inc/foo/bar
to your compile command. Preprocessor options are conventionally assigned to
the make variable CPPFLAGS, e.g.
CPPFLAGS = -I inc -I inc/foo/bar
(along with any other preprocessor options you require), and passed via this
variable in the compile command recipe, e.g.
gcc -c -o $@ $(CPPFLAGS) $(CFLAGS) $<
N.B. It is a common mistake to think that CPPFLAGS is the conventional make variable for C++ compiler flags.
The conventional make variable for C++ compiler flags is CXXFLAGS.
You can see the effect of the -I option by running:
mkdir -p inc/foo/bar # Just to create the path
cpp -v -I inc -I inc/foo/bar
(CTRL-C to terminate). Now the output will contain the like of:
#include "..." search starts here:
#include <...> search starts here:
inc
inc/foo/bar
/usr/lib/gcc/x86_64-linux-gnu/4.8/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
Q2.b: How to specify different search paths where the the linker will look for libraries?
Let's suppose you are have a library, libfoobar.a that you need to link with prog and
that it resides in a directory lib that is 2 levels up from your makefile. Then you
need to pass the linker options:
-L ../../lib
and
-lfoobar
to your link command. The first of these will tell the linker that ../../lib is one of the places to look
for libraries. Conventionally, you pass this option in the linker command recipe via LDFLAGS. The second tells
the linker to search for some library called libfoobar.a (a static library) or libfoobar.so
(a dynamic library). Conventionally, you pass this option in the linker command recipe via LDLIBS
Just as there is a default list of search paths for the preprocessor, there is a default
list of search paths for the linker. You can see them by running:
gcc -v -Wl,--verbose 2>&1 | grep 'LIBRARY_PATH'
The output will be something like:
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/:
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:
/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../:/lib/:/usr/lib/
When you need to link one of the standard libraries, e.g. libm (the math library), that resides in one of the
the default library search paths, you don't need to pass any -L option. -lm alone will do.
Q2.c: How to specify different search paths where make will look for the prerequisites
of targets?
N.B. This is a question about make, not a question about the preprocessor, compiler, or
linker.
We have assumed that all of your object files will be compiled into the subdirectory obj.
To get them compiled there, it would be nice and simple just to use the pattern rule:
obj/%.o:%.c
gcc -c -o $@ $(CPPFLAGS) $(CFLAGS) $<
which tell make that, e.g. obj/file0.o is made from file0.c by the recipe:
gcc -c -o obj/file0.o $(CPPFLAGS) $(CFLAGS) file0.c
and similarly for any file obj/*.o and matching file *.c
This is fine as long file0.c resides in the same directory as the makefile, but
suppose you have your *.c files somewhere else? Say your source files are organised
in subdirectories, foo/file0.c and bar/file1.c. Then make will be unable to
satisfy that pattern rule and will say there is "no rule to make target obj/file0.o",
etc.
To solve this problem use VPATH, a make variable that has a special meaning.
If you assign a list of directory names, punctuated by ':', to VPATH, then
make will search for a prerequisite in each of the listed directories whenever
it can't find it in the current directory. So:
VPATH = foo:bar
will cause make to look in the current directory, and then foo and bar when
it tries to find .c files to match that pattern rule. It will succeed in satisfying
the rule and will compile the necessary source files.
N.B. You have used VPATH wrongly in your posted code:
VPATH = -L/home/dkumar/libtsnnls-2.3.3/tsnnls
You have asigned a linker search path to it, with the linker option -L, which has no
business there.
Bottom line:
The preprecessor search paths, for locating header files, are specified with
the preprocessor's -I<dirname> option. Pass these options to the compile recipe in CPPFLAGS
The linker search paths, for locating libraries, are specified with the
linker's -L<dirname> option. Pass these options to the linkage recipe in LDFLAGS
The search paths for the preprequisities of make rules are specified in
the make variable VPATH, as a ':-punctuated list of directory names.
VPATHdocumentation. – Catechism*.c/*.cpp) that correspond to the object files? E.g. where is the source file that corresponds tolibtsnnls_la-taucs_malloc.o? – Coeternal