Thread: How do I make shared libs on Linux?

Hi,
I'm building static and shared libs (.a & .sl) with these make options:

Code:

SHLIB=libname.sl
CC=cc
AR=ld
ARFLAGS=-Bdynamic -shared -warn-common -warn-constructors -warn-multiple-gp -warn-once -o
INC=../src
DEFS=-DPOSIX -DANSI
CFLAGS=-Wall -Wno-unused -fpic -shared -I. -I$(INC) $(DEFS)

$(SHLIB): $(OBJS)
	rm -f $(SHLIB)
	$(AR) $(ARFLAGS) $(SHLIB) $(OBJS)

debug.o: $(SRC)/debug.c $(HEADERS)
	$(CC) -c $(CFLAGS) $(SRC)/debug.c
...

But after I build my test program and run it, I get a segmentation fault. The program works fine with the static libs.

On Solaris, the LD_LIBRARY_PATH environment variable is used to specify where the shared libs are. As far as I can tell, Linux uses LIBRARY_PATH. Is that right, or do I need something else?

Do I need any other link options, or should I get rid of any that I have...?

This is how my test program is built:

Code:

CC=cc
CFLAGS=-Wall -DPOSIX -DANSI
SHLIBS=-L$$SHLIB_PATH

ivpslibs: scp.o rproc.o sstubs.o smain.o
	$(CC) -shared -o ivpslibs scp.o rproc.o sstubs.o smain.o $(SHLIBS)

sstubs.o: sstubs.c ivp.h
	$(CC) $(CFLAGS) -c $(INCS) sstubs.c
...

I don't know if it's a problem with how I build the shared lib or the test program or both?

Originally Posted by cpjust

On Solaris, the LD_LIBRARY_PATH environment variable is used to specify where the shared libs are. As far as I can tell, Linux uses LIBRARY_PATH. Is that right, or do I need something else?

No, it is also LD_LIBRARY_PATH.

You don't need "-shared" in your CFLAGS. Also, why name the library with .sl instead of .so? The dynamic linker is probably confused by that.

Originally Posted by matsp

You do not create shared libraries with "ar" - you create them with gcc or ld - I'm not quite sure about which switches you throw to make it create a .so file [which is the correct name, by the way].

He IS using ld, he just sticks it in a variable called $(AR). I thought it was weird but didn't say anything.

OK thanks, I'll try those suggestions.

The .sl looked weird to me too. I ported this makefile from HPUX...

Originally Posted by cpjust

OK thanks, I'll try those suggestions.

The .sl looked weird to me too. I ported this makefile from HPUX...

I should have known.

Yes, on Linux use .so not .sl

Great! It's working now!

I didn't change anything in the first makefile for the shared libs (should I?). I just changed the test program makefile to use:

Code:

SHLIBS=$$SHLIB_PATH/libnwrpc.so $$SHLIB_PATH/libnwstcp.so

and removed the -shared flag.

One strange thing I just noticed though... I changed the LD_LIBRARY_PATH, LIBRARY_PATH & SHLIB_PATH environment variables to /lib and the program still runs. Did the paths get hard coded using the compile options I gave it?

Originally Posted by cpjust

One strange thing I just noticed though... I changed the LD_LIBRARY_PATH, LIBRARY_PATH & SHLIB_PATH environment variables to /lib and the program still runs. Did the paths get hard coded using the compile options I gave it?

Yes, it is possible for a shared library path to be hard-coded into the shared binary so that it doesn't have to occur in LD_LIBRARY_PATH. Run "ldd" on the binary to see a dump of where it thinks the libraries are.

Originally Posted by brewbuck

Yes, it is possible for a shared library path to be hard-coded into the shared binary so that it doesn't have to occur in LD_LIBRARY_PATH. Run "ldd" on the binary to see a dump of where it thinks the libraries are.

OK, that reports:

Code:

/home/test/slib/libname.so => /home/test/slib/libname.so (0xb75e2000)

I'm guessing that means it's hard coded.

How can I make it look for that .so file in the LD_LIBRARY_PATH variable?

Originally Posted by brewbuck

What exactly is your link command?

EDIT: I see the problem, anyway. You can't just list the full path to the .so file -- as you found out, that causes the path to be hard-coded. Instead, use "-L/home/test/slib -lname"

Thanks, that did it.

BTW, do you know when it's necessary to use the -shared flag?
I'm still using it for building the .so library. Should I remove it there too?

Originally Posted by cpjust

Thanks, that did it.

BTW, do you know when it's necessary to use the -shared flag?
I'm still using it for building the .so library. Should I remove it there too?

-shared is how .so libraries are created. So yes, it's necessary there. You don't need it elsewhere.

Also remember that shared library code must be compiled with -fpic or -fPIC.

Originally Posted by brewbuck

-shared is how .so libraries are created. So yes, it's necessary there. You don't need it elsewhere.

Also remember that shared library code must be compiled with -fpic or -fPIC.

Great. and the -Bdynamic flag for ld? Is that useful, or does -shared already imply that?