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Dynamic Loading

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

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.

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.

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

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");
  }

}




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