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Linux Dynamic library command help



i have created a program in c language to use dynamic libraries with it.

i have a main.c file in which i have created two functions(i.e. just calling those function from the lib.c file where they are fully defined) and lib.c which contain the full function implementation and have a iib.h which contain those two function declarations.

Now i have type the following command

gcc -fPIC -shared -o lib.so lib.c

gcc -o main main.c lib.so

After that i have copied the lib.so file to the /usr/lib directory

But now i am confused how do execute my program in a way that it calls the lib.so file in the /usr/lib at runtime and then my program works.

please help :) .




  • woboyle
    woboyle Posts: 501
    Two problem:

    1. You need to name your shared library something other than lib.so. Try libxyzzy.so where xyzzy is the name of your choice.
    2. Your main problem is that you directly linked the .so file into your program, so it will look for it where it was when you linked the executable. You need to first copy the shared library to /usr/lib, and then use the option "-l xyzzy" to link the executable. IE: gcc -o main main.c -l xyzzy
  • usmangt

    Thanks for the reply, well its working now after renaming the file

    here the steps i used to make it executable,

    gcc -fPIC -shared -o libelectronic.so libelectornic.c
    gcc -o main main.c -L. -lelectronic -Wl, -rpath <path to the libraray>
    chmod 777 main
  • usmangt
    Hi, again,

    I am trying to do the makefile,

    the files i currently have are main.c libelectronic.c libelectronic.so libelectronic.h(this one is included in both main.c and libelectronic.c )

    now i am not sure about the libelectronic.so i.e. how do i define this shared library it for the make file.

    please help me out.


  • woboyle
    woboyle Posts: 501
    Here is a quick example of a Makefile:
    CC = gcc
    HDRDIR = path-to-headers
    LIBDIR = path-to-libraries
    LIBLDFLAGS = -shared
    LDFLAGS = -L $(LIBDIR) -l electronic
    all: main
    main: main.c libelectronic.so
    	$(CC) $(CFLAGS) -o main main.c $(LDFLAGS)
    libelectronic.so: libelectronic.h libelectronic.c
    	$(CC) $(CFLAGS) $(LIBLDFLAGS) -o libelectronic.so libelectronic.c

    You don't need to run chmod on 'main' since the linker will make it executable for you. You don't need rpath. If you move libelectronic.so, then you will need to put its location in the LD_LIBRARY_PATH environment variable, but you don't need to do that either if you put it in one of the standard library directories such as /usr/lib. Usually, for this sort of stuff you might want to put it in /usr/local/lib and set that in your LD_LIBRARY_PATH.

    One final note. The indentations in the make file in the $(CC) lines HAS to be a tab character, not spaces, at least for most versions of make.
  • usmangt
    Thanks Rubberman for your reply, It helped me in understanding a lot :)

    Also i make the makefile which is now working, here i am giving the path of the current directory where all the files are located and this is how i do it

    all: program1

    program1: main.c libelectronic.so libelectronic.h
    gcc -o program1 main.c -L. -lelectronic -Wl,-rpath .

    libelectronic.so: libelectronic.h libelectronic.c
    gcc -fPIC -shared -o libelectronic.so libelectronic.c

    // Only here i am not sure how to do make install, since its in the same directory so i tried this but didn't worked

    install : program1
    mv testelectronic pwd

    any idea about this.

    Thank you

  • usmangt
    i think it's not needed as its already creating the file in the current directory so make all will do the trick.


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