vbraun / ld-vulcanize

Self-Contained Library Paths

Self-Contained Library Paths

This program rewrites search paths in executables and shared libraries to be relative instead of absolute, and vice versa. This makes a collection of binaries in a specific path relocatable.

Example (OSX)

Say you have a project tree

• /prefix: Directory
• /prefix/bin/foo: Executable linking to libfoo
• /prefix/lib/libfoo.dylib: Shared library linking to libbar
• /prefix/lib/libbar.dylib: Shared library

Absolute paths means:

• /prefix/bin/foo links to /prefix/lib/libfoo.dylib
• /prefix/lib/libfoo.dylib links to /prefix/lib/libbar.dylib

Relative paths means:

• /prefix/bin/foo links to @executable_path/../lib/libfoo.dylib
• /prefix/lib/libfoo.dylib links to @loader_path/libbar.dylib

This tool converts one to the other. Typically, your build process uses absolute paths everywhere, and if you rename the build tree /prefix then the binaries do not work any more as they cannot find their shared libraries. Solution: Rewrite everything under /prefix to use relative paths:

$ ld_vulcanize --path=/prefix --rewrite=relative

DYLD_LIBRARY_PATH is Evil

Another "solution" is to set DYLD_LIBRARY_PATH=/prefix/lib and have wonky/incorrect paths for the linker paths. Since DYLD_LIBRARY_PATH has precedence, /prefix/bin/foo then still works despites paths pointing to the wrong directories. But it has also severe disadvantages:

• Needs special environment variable. So you can't just execute the binaries any more.

• Name collisions: Other executables that happen to link to their own libfoo.dylib will not work any more, as DYLD_LIBRARY_PATH has precedence. Especially fun if bash is the affected binary because you have an incompatible libreadline.dylib.

• Apple broke DYLD_LIBRARY_PATH: In OSX 10.11, it is not passed through script interpreters.

Help

$ ld-vulcanize --help
usage: ld-vulcanize [-h] [--log LOG] --path PATH [--rewrite REWRITE]

Rewrite Library Paths

optional arguments:
  -h, --help         show this help message and exit
  --log LOG          one of [DEBUG, INFO, ERROR, WARNING, CRITICAL]
  --path PATH        root of the directory tree to operate on
  --rewrite REWRITE  one of [readonly, relative, absolute]. How to rewrite the
                     library search paths. Default: readonly (no changes
                     written to disk)

Caveats

Caveat: Other Relative Path Schemes

There are other strategies for relative paths, e.g. using @rpath or using @executable_path also in intermediate shared libraries. They are ambiguous since multiple binaries might be linking to the very same shared library. Although such an relative path strategy might very well work, ld_vulcanize cannot rewrite it back to absolute paths.

Caveat: DYLD Install Names

This tool does not deal with OSX dyld install names; They are important at link time but not at run time.

Caveat: Hardlinks

Does not work for hardlinked binaries in different paths. For example: Git installs hardlinks of the same binary into both $prefix/bin/git and $prefix/lib/libexec/git-core/git. The correct relative library path is either ../lib/libz.so or ../../libz.so, but since there is only one underlying binary one of them will be wrong. Whichever relative path we pick will break the other hardlink of git.

Solution: Replace hard links with soft links.

Name

Vulcanize

1. to treat (rubber) with sulphur or sulphur compounds under heat and pressure to improve elasticity and strength or to produce a hard substance such as vulcanite

2. to treat (substances other than rubber) by a similar process in order to improve their properties

License

GPLv3