7. Dynamic Loading

This section is a tad short right now; it will be expanded over time as I gut the ELF howto

7.1 Concepts

Linux has shared libraries, as you will by now be sick of hearing if you read the whole of the last section at a sitting. Some of the matching-names-to-places work which was traditionally done at link time must be deferred to load time.

7.2 Error messages

Send me your link errors! I won't do anything about them, but I might write them up ...

can't load library: /lib/libxxx.so, Incompatible version

(a.out only) This means that you don't have the correct major version of the xxx library. No, you can't just make a symlink to another version that you do have; if you are lucky this will cause your program to segfault. Get the new version. A similar situation with ELF will result in a message like

ftp: can't load library 'libreadline.so.2'

warning using incompatible library version xxx

(a.out only) You have an older minor version of the library than the person who compiled the program used. The program will still run. Probably. An upgrade wouldn't hurt, though.

7.3 Controlling the operation of the dynamic loader

There are a range of environment variables that the dynamic loader will respond to. Most of these are more use to ldd than they are to the average user, and can most conveniently be set by running ldd with various switches. They include

• LD_BIND_NOW --- normally, functions are not `looked up' in libraries until they are called. Setting this flag causes all the lookups to happen when the library is loaded, giving a slower startup time. It's useful when you want to test a program to make sure that everything is linked.
• LD_PRELOAD can be set to a file containing `overriding' function definitions. For example, if you were testing memory allocation strategies, and wanted to replace `malloc', you could write your replacement routine, compile it into malloc.o and then

  $ LD_PRELOAD=malloc.o; export LD_PRELOAD
  $ some_test_program
  

  LD_ELF_PRELOAD and LD_AOUT_PRELOAD are similar, but only apply to the appropriate type of binary. If LD_something_PRELOAD and LD_PRELOAD are set, the more specific one is used.
• LD_LIBRARY_PATH is a colon-separated list of directories in which to look for shared libraries. It does not affect ld; it only has effect at runtime. Also, it is disabled for programs that run setuid or setgid. Again, LD_ELF_LIBRARY_PATH and LD_AOUT_LIBRARY_PATH can also be used to direct the search differently for different flavours of binary. LD_LIBRARY_PATH shouldn't be necessary in normal operation; add the directories to /etc/ld.so.conf/ and rerun ldconfig instead.
• LD_NOWARN applies to a.out only. When set (e.g. with LD_NOWARN=true; export LD_NOWARN) it stops the loader from issuing non-fatal warnings (such as minor version incompatibility messages).
• LD_WARN applies to ELF only. When set, it turns the usually fatal ``Can't find library'' messages into warnings. It's not much use in normal operation, but important for ldd.
• LD_TRACE_LOADED_OBJECTS applies to ELF only, and causes programs to think they're being run under ldd:

  $ LD_TRACE_LOADED_OBJECTS=true /usr/bin/lynx
          libncurses.so.1 => /usr/lib/libncurses.so.1.9.6
          libc.so.5 => /lib/libc.so.5.2.18
  

7.4 Writing programs with dynamic loading

This is very close to the way that Solaris 2.x dynamic loading support works, if you're familiar with that. It is covered extensively in H J Lu's ELF programming document, and the dlopen(3) manual page, which can be found in the ld.so package. Here's a nice simple example though: link it with -ldl

#include <dlfcn.h>
#include <stdio.h>

main()
{
  void *libc;
  void (*printf_call)();

  if(libc=dlopen("/lib/libc.so.5",RTLD_LAZY))
  {
    printf_call=dlsym(libc,"printf");
    (*printf_call)("hello, world\n");
  }

}