Penguin

The dynamic library loader used in linux (part of glibc) has some neat tricks. One of these is that you can set an environment variable called

LD_DEBUG

to show how symbols (variables and functions, for example) are resolved for a dynamic executable. This can sometimes help resolve obscure bugs where your application isn't doing what you expect (assuming it is caused by symbols being resolved differently to what you were expecting).

This is very useful if you get segmentation violations or aborts for a program - this can sometimes be caused by linking against the wrong version of a library. This is also a really good way to understand what happens when you run any program! It has some self-documentation - for the impatient, you can do

 $ LD_DEBUG=help /path/to/some/dynamic/executable

eg

 $ LD_DEBUG=help ls

prints out:

 Valid options for the LD_DEBUG environment variable are:

  libs       display library search paths
  reloc      display relocation processing
  files      display progress for input file
  symbols    display symbol table processing
  bindings   display information about symbol binding
  versions   display version dependencies
  all        all previous options combined
  statistics display relocation statistics
  help       display this help message and exit

 To direct the debugging output into a file instead of standard output a
 filename can be specified using the LD_DEBUG_OUTPUT environment variable.

As a quick example of what it does:

 $ LD_DEBUG=all ls 2>&1 > /dev/null | less

 13442:
 13442:  file=librt.so.1;  needed by ls
 13442:  find library=librt.so.1; searching
 13442:   search cache=/etc/ld.so.cache
 13442:    trying file=/lib/librt.so.1
 13442:
 13442:  file=librt.so.1;  generating link map
 13442:    dynamic: 0x400263ec  base: 0x40020000   size: 0x00010d14
 13442:      entry: 0x400219c0  phdr: 0x40020034  phnum:          6
 13442:
 13442:
 13442:  file=libc.so.6;  needed by ls
 13442:  find library=libc.so.6; searching
 13442:   search cache=/etc/ld.so.cache
 13442:    trying file=/lib/libc.so.6
 13442:
 13442:  file=libc.so.6;  generating link map
 13442:    dynamic: 0x40146ce4  base: 0x40031000   size: 0x0011ab00
 13442:      entry: 0x4004a184  phdr: 0x40031034  phnum:          6
 13442:
  ...
 13442:  checking for version `GLIBC_2.2' in file /lib/librt.so.1 required by  file ls
 13442:  checking for version `GLIBC_2.1' in file /lib/libc.so.6 required by file  ls
 13442:  checking for version `GLIBC_2.2.3' in file /lib/libc.so.6 required by file ls
  ...
 13442:  relocation processing: /lib/libpthread.so.0 (lazy)
 13442:  symbol=_errno;  lookup in file=ls
 13442:  symbol=_errno;  lookup in file=/lib/librt.so.1
 13442:  symbol=_errno;  lookup in file=/lib/libc.so.6
 13442:  symbol=_errno;  lookup in file=/lib/libpthread.so.0
 13442:  symbol=_errno;  lookup in file=/lib/ld-linux.so.2
 13442:  binding file /lib/libpthread.so.0 to /lib/libc.so.6: normal symbol  `_errno' [GLIBC_2.0]
 13442:  symbol=_h_errno;  lookup in file=ls
 13442:  symbol=_h_errno;  lookup in file=/lib/librt.so.1
 13442:  symbol=_h_errno;  lookup in file=/lib/libc.so.6
 13442:  symbol=_h_errno;  lookup in file=/lib/libpthread.so.0
 13442:  symbol=_h_errno;  lookup in file=/lib/ld-linux.so.2
 13442:  binding file /lib/libpthread.so.0 to /lib/libc.so.6: normal symbol `_h_errno' [GLIBC_2.0]
  ...

In other words, every single function and external variable in the standard library that ls(1) uses must be located each time it is run (kind of obvious, really).

 $ ldd /bin/ls
        librt.so.1 => /lib/librt.so.1 (0x40020000)
        libc.so.6 => /lib/libc.so.6 (0x40031000)
        libpthread.so.0 => /lib/libpthread.so.0 (0x4014c000)
        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)

see ld.so(8) for environment variables.


See also AdvancedUserTips, CommonProgrammingBugs