linker - 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 copy of the library among all executables (I don't mind re-distributing .dylib with them).

Do I need to do some magic to get libpng export symbols?

Does "Link Binary With Libraries" build phase link statically?

Apple's docs mention loading of libraries at run time with dlopen, but how I can make Xcode create executable without complaining about missing symbols?

---

I think I've figured it out:

• libpng wasn't linking properly, because I've built 32/64-bit executables and 32-bit library. Build settings of the library and executables must match.

• libpng's config.h needs to have tons of defines like #define FEATURE_XXX_SUPPORTED

• "Link Binary With Libraries" build phase handles dynamic libraries just fine, and DYLD_FALLBACK_LIBRARY_PATH environmental variable is neccessary for loading .dylibs from application bundle.

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 <stdio.h>
    void mymod( int x )
    {
        printf( "mymod: %d
", 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.)