1.1 What is the Dynamic Linking Extension

The Dynamic linking extension that came out of the Aspen group comprises a set of four routines and a header file to provide a portable API for manipulation of an implementation defined class of files, which typically could be shared libraries. These routines are based on those introduced in UNIX System V Release 4.

1.2 The Benefits of Dynamic Linking

Use of dynamic linking allows several benefits for application developers:

• The ability to share commonly used code across many applications, leads to disk and memory savings;
• It allows the implementation of services to be hidden from applications;
• It allows the reimplementation of services, for example to permit bug and performance fixes, or to allow multiple implementations selectable at runtime.

1.3 Functional Overview

dlopen() - gain access to an executable object file

This routine can be used to attach a shared object during a process lifetime, for example

#include <dlfcn.h>
void *mylib;

mylib = dlopen ("mylib.so.1", RTLD_LAZY);

dlsym() - obtain address of a symbol from dlopen() object

This routine can be used to obtain the address of a symbol within an object, for example

int (*myptr) ();

myptr = (int (*) ()) dlsym (mylib, "mysymname");

and then, it is possible to manipulate the data or function by a dereference to the address:

int foobar;

foobar = (*myptr) ();

dlclose() - close a dlopen() object

This function is used to inform the system that the object attached by a previous call to dlopen() is no longer needed by the application, for example:

int foo;

foo = dlcose( mylib );

dlerror() - get diagnostic information

This function is used to find out diagnostic information if any of the other dynamic linking routines returned an error. A call to dlerror() will then return a string describing the error, for example:

char 	*errstr;

errstr = dlerror();

One point to note about this routine, is that it is not thread safe, since the string may reside in a static area which is overwrittem whenever an error occurs.

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