OS X Library basics 2

Here is a second example, of a dynamic library (adapted from a Stack Overflow answer here). The same .c files are used from the first example (here).

step 1, create libadd.dylib

$ gcc -g -Wall -c add*.c $ file add1.o add1.o: Mach-O 64-bit object x86_64 $ gcc -dynamiclib -current_version 1.0 add*.o -o libadd.dylib $ file libadd.dylib libadd.dylib: Mach-O 64-bit dynamically linked shared library x86_64 $ otool -L libadd.dylib libadd.dylib: libadd.dylib (compatibility version 0.0.0, current version 1.0.0) /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.1)

Find out more about otool by doing

man otool | col -b > result.txt

col helps to format the output from man properly. I learned we can do this:

$ otool -tvV "libadd.a(add1.o)" libadd.a(add1.o): (__TEXT,__text) section _f1: 00000000 pushl %ebp 00000001 movl %esp,%ebp 00000003 pushl %ebx 00000004 subl $0x14,%esp 00000007 calll 0x0000000c 0000000c popl %ebx 0000000d movl 0x08(%ebp),%eax 00000010 movl %eax,0x04(%esp) 00000014 leal 0x2fd-0xc(%ebx),%eax 0000001a movl %eax,(%esp) 0000001d calll _printf+0x100000000 00000022 movl 0x08(%ebp),%eax 00000025 incl %eax 00000026 addl $0x14,%esp 00000029 popl %ebx 0000002a leave 0000002b ret

I don't speak assembly, but I can get the general idea.
Step 2, create useadd

$ gcc -c useadd.c $ gcc -v useadd.o ./libadd.dylib -o useadd Using built-in specs. Target: i686-apple-darwin10 Configured with: /var/tmp/gcc/gcc-5664~89/src/configure --disable-checking --enable-werror --prefix=/usr --mandir=/usr/share/man --enable-languages=c,objc,c++,obj-c++ --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin10 --with-gxx-include-dir=/usr/include/c++/4.2.1 --host=i686-apple-darwin10 --target=i686-apple-darwin10 Thread model: posix gcc version 4.2.1 (Apple Inc. build 5664) /usr/libexec/gcc/i686-apple-darwin10/4.2.1/collect2 -dynamic -arch i386 -macosx_version_min 10.6.5 -weak_reference_mismatches non-weak -o useadd -lcrt1.10.6.o -L/usr/lib/i686-apple-darwin10/4.2.1 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1/../../../i686-apple-darwin10/4.2.1 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1/../../.. useadd.o ./libadd.dylib -lSystem -lgcc -lSystem $ nm -gpv useadd 0000202c D _NXArgc 00002030 D _NXArgv 00002038 D ___progname 00001000 A __mh_execute_header 00002034 D _environ 00001f02 T _main 00001ec4 T start U _exit U _f1 U _f2 U _printf U dyld_stub_binder $ ./useadd f1: 1; main 2 f2: 10; main 12

Step 3. Use an environment variable to give output when libraries are loaded:

$ export DYLD_PRINT_LIBRARIES=1 $ ./useadd dyld: loaded: /Users/telliott_admin/Desktop/add/./useadd dyld: loaded: /Users/telliott_admin/Desktop/add/libadd.dylib dyld: loaded: /usr/lib/libSystem.B.dylib dyld: loaded: /usr/lib/system/libmathCommon.A.dylib f1: 1; main 2 f2: 10; main 12 $ mv libadd.dylib /tmp dyld: loaded: /bin/mv dyld: loaded: /usr/lib/libSystem.B.dylib dyld: loaded: /usr/lib/system/libmathCommon.A.dylib $ export DYLD_LIBRARY_PATH=/tmp $ ./useadd dyld: loaded: /Users/telliott_admin/Desktop/add/./useadd dyld: loaded: /tmp/libadd.dylib dyld: loaded: /usr/lib/libSystem.B.dylib dyld: loaded: /usr/lib/system/libmathCommon.A.dylib f1: 1; main 2 f2: 10; main 12 $ unset DYLD_LIBRARY_PATH

Notice in the second run of ./useadd, we loaded /tmp/libadd.dylib.
Finally, let's snoop on Python:

$ python dyld: loaded: /usr/bin/python dyld: loaded: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation dyld: loaded: /usr/lib/libSystem.B.dylib dyld: loaded: /usr/lib/libauto.dylib dyld: loaded: /usr/lib/libicucore.A.dylib dyld: loaded: /usr/lib/libobjc.A.dylib dyld: loaded: /usr/lib/libz.1.dylib dyld: loaded: /usr/lib/libstdc++.6.dylib dyld: loaded: /usr/lib/system/libmathCommon.A.dylib dyld: loaded: /System/Library/Frameworks/Python.framework/Versions/2.6/Resources/Python.app/Contents/MacOS/Python dyld: loaded: /System/Library/Frameworks/Python.framework/Versions/2.6/Python dyld: loaded: /usr/lib/libSystem.B.dylib dyld: loaded: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation dyld: loaded: /usr/lib/libauto.dylib dyld: loaded: /usr/lib/libicucore.A.dylib dyld: loaded: /usr/lib/libobjc.A.dylib dyld: loaded: /usr/lib/libz.1.dylib dyld: loaded: /usr/lib/libstdc++.6.dylib dyld: loaded: /usr/lib/system/libmathCommon.A.dylib Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) [GCC 4.2.1 (Apple Inc. build 5646)] on darwin Type "help", "copyright", "credits" or "license" for more information. dyld: loaded: /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload/readline.so dyld: loaded: /usr/lib/libedit.2.dylib dyld: loaded: /usr/lib/libncurses.5.4.dylib >>> import string dyld: loaded: /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload/strop.so >>> import sys >>> import math dyld: loaded: /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload/math.so >>> [1]+ Stopped python $ unset DYLD_PRINT_LIBRARIES

We can see that some .so libraries can be loaded. However, there are some fundamental differences between the Unix method of shared object .so libraries and the OS X / Mach model of bundles and frameworks.
That's for the future!