How do I create a dynamic library (dylib) with Xcode?

Question

I\'m building few command-line utilities in Xcode (plain C, no Cocoa). I want all of them to use my customized version of libpng, and I want to save space by sharing one cop

Answer 1

Dynamic linking on Mac OS X, a tiny example

Steps:

1. create a library libmylib.dylib containing mymod.o
2. compile and link a "callmymod" which calls it
3. call mymod from callmymod, using DYLD_LIBRARY_PATH and DYLD_PRINT_LIBRARIES

Problem: you "just" want to create a library for other modules to use. However there's a daunting pile of programs -- gcc, ld, macosx libtool, dyld -- with zillions of options, some well-rotted compost, and differences between MacOSX and Linux. There are tons of man pages (I count 7679 + 1358 + 228 + 226 lines in 10.4.11 ppc) but not much in the way of examples, or programs with a "tell me what you're doing" mode.

(The most important thing in understanding is to make a simplified OVERVIEW for yourself: draw some pictures, run some small examples, explain it to someone else).

Background: apple OverviewOfDynamicLibraries, Wikipedia Dynamic_library

---

Step 1, create libmylib.dylib --

mymod.c:
    #include 
    void mymod( int x )
    {
        printf( "mymod: %d\n", x );
    }
gcc -c mymod.c  # -> mymod.o
gcc -dynamiclib -current_version 1.0  mymod.o  -o libmylib.dylib
    # calls libtool with many options -- see man libtool
    # -compatibility_version is used by dyld, see also cmpdylib

file libmylib.dylib  # Mach-O dynamically linked shared library ppc
otool -L libmylib.dylib  # versions, refs /usr/lib/libgcc_s.1.dylib

---

Step 2, compile and link callmymod --

callmymod.c:
    extern void mymod( int x );
    int main( int argc, char** argv )
    {
        mymod( 42 );
    }
gcc -c callmymod.c
gcc -v callmymod.o ./libmylib.dylib -o callmymod
    # == gcc callmymod.o -dynamic -L. -lmylib
otool -L callmymod  # refs libmylib.dylib
nm -gpv callmymod  # U undef _mymod: just a reference, not mymod itself

---

Step 3, run callmymod linking to libmylib.dylib --

export DYLD_PRINT_LIBRARIES=1  # see what dyld does, for ALL programs
./callmymod
    dyld: loaded: libmylib.dylib ...
    mymod: 42

mv libmylib.dylib /tmp
export DYLD_LIBRARY_PATH=/tmp  # dir:dir:...
./callmymod
    dyld: loaded: /tmp/libmylib.dylib ...
    mymod: 42

unset DYLD_PRINT_LIBRARIES
unset DYLD_LIBRARY_PATH

That ends one tiny example; hope it helps understand the steps.
(If you do this a lot, see GNU Libtool which is glibtool on macs, and SCons.)
cheers
-- denis