install_name_tool difference between -change and -

Question

I have been struggling with this concept for a while and I cannot really understand what the difference is between -change and -id.The man page states

 -id name
              Changes  the  shared  library identification name of a dynamic shared library to name.  If the Mach-O binary is not a dynamic
              shared library and the -id option is specified it is ignored.

-change old new
              Changes  the dependent shared library install name old to new in the specified Mach-O binary.  More than one of these options
              can be specified.  If the Mach-O binary does not contain the old install name in a specified -change  option  the  option  is
              ignored.

So far I have experimented with -change. Suppose I have the following structure

Test.App
|_Contents
    |_MacOS
    |   |_test -----> item A
    |_Library
        |_test_library.dylib     --->item B
        |_another_library.dylib  --->item C

Now suppose I ran the following on itemB

$ otool -L test_library.dylib
   test_library.dylib
   /some/path/another_library.dylib  -->item D

The above result indicates that test_library.dylib depends on another_library.dylib now if I needed to change the location of another_library.dylib I would do this

install_name_tool -change /some/path/another_library.dylib some/new/path/another_library.dylib  test_library.dylib 

this would change the location of item D. My question is what does install-name_tool -id do and when do I use that ?

Answer 1

id is used at link time and install name is used at runtime. They are all information provided for the linker to locate the dylib. I followed this tutorial.

Let me show an example,

$ cat a.cc
#include <iostream>
void a() { std::cout << "a()" << std::endl; }
$ clang++ -c a.cc

$ clang++ -o liba.dylib -dynamiclib a.o
$ otool -L liba.dylib
liba.dylib:
        liba.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)

As you can see, the first line is the id. Let's link with libb.dylib,

$ cat b.cc
#include <iostream>
void a();
void b() { std::cout << "b()" << std::endl; a(); }
$ clang++ -c b.cc
$ clang++ -o libb.dylib -dynamiclib b.o -L. -la
$ otool -L libb.dylib
libb.dylib:
        libb.dylib (compatibility version 0.0.0, current version 0.0.0)
        liba.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)

Just notice the second line, the id for liba.dylib is used here. Let's change the id to foo/liba.dylib and link again,

$ install_name_tool -id foo/liba.dylib liba.dylib
$ otool -D liba.dylib
liba.dylib:
foo/liba.dylib
liba.dylib:
        foo/liba.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)

So you see the -D and -L all outputs the current id as foo/liba.dylib.

Let's link again with liba.dylib again,

$ clang++ -o libb.dylib -dynamiclib b.o -L. -la
$ otool -L libb.dylib
libb.dylib:
        libb.dylib (compatibility version 0.0.0, current version 0.0.0)
        foo/liba.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)

See the difference? The run time location to find liba.dylib is changed to foo/liba.dylib at the second line.

Basically, it tells libb.dylib to find liba.dylib from current_dir/foo

Answer 2

install-name_tool -id is used for change the install name of dylib, you can use the otool -D see a dylib install name in the terminal, it will show the default value for you, the /some/path/another_library.dylib is the default install name of another _library.dylib, of course, you can change it use install-name_tool -id in the terminal, just use like this in terminal

install-name_tool -id /some/path/another_library_newname.dylib /some/path/another_library.dylib

now,you use the otool -D /some/path/another_library.dylib, you will find the install name is /some/path/another_library_newname.dylib

here is my example in picture

Answer 3

My understanding is:

-id: This sets the "install name" that will be used when linking against the dynamic library. It would be run on the target dynamic library file.

-change: This changes the "install name" after linking and would be run on the executable or dynamic library that links against the target dynamic library.