Why is install_name_tool and otool necessary for Mach-O libraries in Mac Os X?

Question

I am developing a Cocoa Application using the latest version of Xcode 4, I want to link dynamic libraries to my project (dylibs).

I read somewhere that adding the libraries in my project was not enough as I have to run install_name_tool and otool to make my project use the libraries that were bundled in my project.

I have read the manual pages for install_name_tool, but I do not understand WHY I have to do this.

How do libraries work? Especially interested in the part where the application and the libraries have paths that point to specific places in my machine, like /usr/local/lib/mylibrary.dylib when running otool -L

Answer 1

Apple has several ways of locating shared libraries:

1. @executable_path : relative to the main executable
2. @loader_path : relative to the referring binary
3. @rpath : relative to any of a list of paths.

@rpath is the most recent addition, introduced in OS X 10.5.

If for instance you want to have your executable in Contents/MacOS and libraries in Contents/Libraries you could do the following:

install_name_tool -id @rpath/Libraries/lib_this.dylib   builddir/lib_this.dylib

and in the top-level executable set rpath with:

install_name_tool -add_rpath @loader_path/..  myexecutable

and:

install_name_tool -change builddir/lib_this.dylib @rpath/Libraries/lib_this.dylib myexecutable

Note: that the first path after -change must match exactly what is currently in the binary.

If you get lost otool -l -v myexecutable will tell you what load commands exactly are currently in the executable.

See man dyld and man install_name_tool for more information.