Thread: Building program with shared library

  1. #1
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    Jul 2021
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    Building program with shared library

    Hi folks!

    I'm trying to build a (multi-file) sample program with a shared library for demonstration purposes. But unfortunately, I'm getting something wrong. Anybody here to help?

    Code:
    $ make
    gcc -Wall -g -o mylib.o -c mylib.c
    gcc -Wall -g -o libmylib.so -shared mylib.o
    gcc -Wall -g -o prog -L. -lmylib wrapLibraryFunc.c prog.c
    wrapLibraryFunc.c:4:6: warning: ‘wrapLibraryFunc’ defined but not used [-Wunused-function]
        4 | void wrapLibraryFunc(char *message)
          |      ^~~~~~~~~~~~~~~
    In file included from prog.c:1:
    prog.h:1:13: warning: ‘wrapLibraryFunc’ used but never defined
        1 | static void wrapLibraryFunc(char *message);
          |             ^~~~~~~~~~~~~~~
    /usr/bin/ld: /tmp/ccg3TTFt.o: in function `wrapLibraryFunc':
    /home/user/src/cprogramming/wrapLibraryFunc.c:6: undefined reference to `printMessage'
    /usr/bin/ld: /tmp/ccpTloxp.o: in function `main':
    /home/user/src/cprogramming/prog.c:5: undefined reference to `wrapLibraryFunc'
    collect2: error: ld returned 1 exit status
    make: *** [Makefile:11: prog] Error 1
    Attached Files Attached Files

  2. #2
    and the hat of int overfl Salem's Avatar
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    The normal convention is that libraries go at the end of the command line.
    Code:
    gcc -Wall -g -o prog wrapLibraryFunc.c prog.c  -L. -lmylib
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Ok, didn't know that.
    But I'm still getting the same errors.

  4. #4
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    The problem is that you make wrapLibraryFunc static in the header file.
    This causes it to not be seen outside of it's translation unit.
    Get rid of the static.
    Also, your char* parameters should really be const char *.
    The best argument against democracy is a five minute conversation with the average voter. - Churchill

  5. #5
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    Gets me thinking of this.
    Code:
    #include <stdio.h>
    
    int main(int argc, const char **argv) 
    
    {     
         printf(" %s\n", argv[0]);
         return 0;
    }

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