C++ Reference Guide

Dynamic Linking By Example

Last updated Mar 5, 2004.

Last week, I explained the principles of dynamic linking and discussed its advantages. Now it's time to see dynamic linking in action.

My example is based on the POSIX library. This isn't a complete tutorial; rather, my aim is to exemplify the implementation of dynamic linking in general. Therefore, Windows programmers should benefit from this discussion as well.

Building a shared library

The first step is to build a shared library. Use as an example the following simple function hello() that prints a greeting message on the screen:

/*hello.c*/ #include <stdio.h> void hello() { printf("have a nice day.\n"); } 

Next, compile hello.c with the suitable option (e.g., -fPIC in GCC) to build an object file for a shared library, and link it with the appropriate linker options to create a shared library (for the sake of brevity, I will skip these technical details; they are highly dependent on the OS, compiler, and linker). The end result of the linkage process is a file called hello.so that may be installed either in a user's directory or in a system's default directory for shared libraries defined by the LD_LIBRARY_PATH environment variable.

The <dlfcn.h> Interface

const char * dlerror();

void * dlopen(const char * filename, int flags);

void *dlsym(void * handle, const char * symbol);

void * dlclose(void *handle);

A <dlfcn.h> Example

The following program uses the <dlfcn.h> library to open the hello.so library, locate the symbol "hello", and call this function. Notice how the program checks for errors after each step.

#include <dlfcn.h> #include <stdio.h> /*for printf() */ #include <stdlib.h> /* for exit() */ typedef void (*pf)(); int main() { void *lib; pf greet; const char * err; lib=dlopen("hello.so", RTLD_NOW); if (!lib) { printf("failed to open hello.so: %s \n", dlerror()); exit(1); } dlerror(); /*first clear any previous error; redundant in this case but a useful habit*/ greet= (pf) dlsym(lib, "hello");/*locate hello() */ err=dlerror();/*check for errors and copy error message*/ if (err) { printf("failed to locate hello(): %s \n", err); exit(1); } greet(); /*call hello() */ dlclose(lib); return 0; }

Summary

Although the <dlfcn.h> interface is confined to the POSIX world, dynamic linking in Windows and other modern operating systems is implemented more or less in the same manner, albeit with a different set of APIs. Hopefully, a standard dynamic linkage library will materialize in the next revision round of the C++ standard.

Related Books

"Linux Programming by Example," by Arnold Robbins (to appear in April 2004) describes the Linux/C interface very well. Although it misses two important topics, i.e., dynamic linking and multithreading, it's an excellent guide for programmers who are making their first steps in the Linux world.

"Linux Application Development" by Michael K. Johnson and Eric W. Troan has been my favorite guide to Linux system programming. The title is somewhat out of focus — it doesn't really cover full-blown application development issues such as GUI and multimedia — but it's exactly the book every system programmer should have at his or her disposal: succinct, accurate, authoritative, and totally fluff-free.