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 .dylib
s from application bundle.
At the time this question was asked, Dynamic libraries were not supported by iOS and will result in your app getting rejected. Only static libraries are allowed.
Dynamic linking on Mac OS X, a tiny example
Steps:
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\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.)
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