How do I make shared libs on Linux?

This is a discussion on How do I make shared libs on Linux? within the C Programming forums, part of the General Programming Boards category; 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 ...

  1. #1
    and the hat of sweating
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    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?

  2. #2
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    Quote Originally Posted by cpjust View Post
    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.

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    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].

    If you want statically link with a .a file (which is the normal output name from a "ar" built archive) that would be fine too - but you need to specify -lname to make that happen in that case - you are only specifying -L<somepath> which tells the linker (as called by gcc) to look for libraries in this path, not actually to include any of the libraries that may be in there.

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    Quote Originally Posted by matsp View Post
    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.

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    Quote Originally Posted by brewbuck View Post
    He IS using ld, he just sticks it in a variable called $(AR). I thought it was weird but didn't say anything.
    Ah, ok, so it's just the two things then: strange name (which may be OK, but it's hardly helping) and not linking in the actual library.

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    OK thanks, I'll try those suggestions.

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

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    Quote Originally Posted by cpjust View Post
    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

  8. #8
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    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?

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    Quote Originally Posted by cpjust View Post
    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.

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    Quote Originally Posted by brewbuck View Post
    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?

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    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"
    Last edited by brewbuck; 11-05-2007 at 01:48 PM.

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    Quote Originally Posted by brewbuck View Post
    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?

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    Quote Originally Posted by cpjust View Post
    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.

  14. #14
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    Quote Originally Posted by brewbuck View Post
    -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?

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