Книга: Embedded Linux Primer: A Practical, Real-World Approach

13.6.4. ldd

13.6.4. ldd

Although not strictly a binary utility, the ldd script is another useful tool for the embedded developer. It is part of the C library package and exists on virtually every Linux distribution. ldd lists the shared object library dependencies for a given object file or files. We introduced ldd in Chapter 11, "BusyBox." See Listing 11-2 for an example usage. The ldd script is particularly useful during development of ramdisk images. One of the most common failures asked about on the various embedded Linux mailing lists is a kernel panic after mounting root:

VFS: Mounted root (nfs filesystem).
Freeing unused kernel memory: 96k init
Kernel panic - not syncing: No init found. Try passing init=option to kernel.

One of the most common causes is that the root file system image (be it ramdisk, Flash, or NFS root file system) does not have the supporting libraries for the binaries that the kernel is trying to execute. Using ldd, you can determine which libraries each of your binaries requires and make sure that you include them in your ramdisk or other root file system image. In the previous example kernel panic, init was indeed on the file system, but the Linux dynamic loader, ld.so.1, was missing. Using ldd is quite straightforward:

$ xscale_be-ldd init
    libc.so.6 => /opt/mvl/.../lib/libc.so.6 (0xdead1000)
    ld-linux.so.3 => /opt/mvl/.../lib/ld-linux.so.3 (0xdead2000)

This simple example demonstrates that the init binary requires two dynamic library objects: libc and ld-linux. Both must be on your target and must be accessible to your init binary that is, they must be readable and executable.

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