C++ Dynamic Shared Library on Linux
This is a follow-up to Dynamic Shared Library compilation with g++. I’m trying to create a shared class library in C++ on Linux. I’m able to get the library to compile, and I can call some of the (non-class) functions using the tutorials that I found here and here. My problems start when I try to use the classes that are defined in the library. The second tutorial that I linked to shows how to load the symbols for creating objects of the classes defined in the library, but stops short of using those objects to get any work done. Does anyone know of a more complete tutorial for creating shared C++ class libraries that also shows how to use those classes in a separate executable? A very simple tutorial that shows object creation, use (simple getters and setters would be fine), and deletion would be fantastic. A link or a reference to some open source code that illustrates the use of a shared class library would be equally good. Although the answers from codelogic and nimrodm do work, I just wanted to add that I picked up a copy of Beginning Linux Programming since asking this question, and its first chapter has example C code and good explanations for creating and using both static and shared libraries. These examples are available through Google Book Search in an older edition of that book.
I’m not sure I understand what you mean by «using» it, once a pointer to the object is returned, you could use it like you use any other pointer to an object.
The article I linked to shows how to create a function pointer to an object factory function using dlsym. It doesn’t show the syntax for creating and using objects from the library.
You will need the header file describing the class. Why do you think you have to use «dlsym» instead of just letting the OS find and link the library at load time? Let me know if you need a simple example.
@nimrodm: What’s the alternative to using «dlsym»? I’m (supposed to be) writing 3 C++ programs that will all use the classes defined in the shared library. I also have 1 Perl script that will use it, but that’s a whole other problem for next week.
Understanding Shared Libraries in Linux
In programming, a library is an assortment of pre-compiled pieces of code that can be reused in a program. Libraries simplify life for programmers, in that they provide reusable functions, routines, classes, data structures, and so on (written by another programmer), which they can use in their programs.
For instance, if you are building an application that needs to perform math operations, you don’t have to create a new math function for that, you can simply use existing functions in libraries for that programming language.
Examples of libraries in Linux include libc (the standard C library) or Glibc (GNU version of the standard C library), libcurl (multiprotocol file transfer library), libcrypt (library used for encryption, hashing, and encoding in C), and many more.
Linux supports two classes of libraries, namely:
- Static libraries – are bound to a program statically at compile time.
- Dynamic or shared libraries – are loaded when a program is launched and loaded into memory and binding occurs at run time.
Dynamic or shared libraries can further be categorized into:
- Dynamically linked libraries – here a program is linked with the shared library and the kernel loads the library (in case it’s not in memory) upon execution.
- Dynamically loaded libraries – the program takes full control by calling functions with the library.
Shared Library Naming Conventions
Shared libraries are named in two ways: the library name (a.k.a soname) and a “filename” (absolute path to file which stores library code).
For example, the soname for libc is libc.so.6: where lib is the prefix, c is a descriptive name, so means shared object, and 6 is the version. And its filename is: /lib64/libc.so.6. Note that the soname is actually a symbolic link to the filename.
Locating Shared Libraries in Linux
Shared libraries are loaded by ld.so (or ld.so.x) and ld-linux.so (or ld-linux.so.x) programs, where x is the version. In Linux, /lib/ld-linux.so.x searches and loads all shared libraries used by a program.
A program can call a library using its library name or filename, and a library path stores directories where libraries can be found in the filesystem. By default, libraries are located in /usr/local/lib, /usr/local/lib64, /usr/lib and /usr/lib64; system startup libraries are in /lib and /lib64. Programmers can, however, install libraries in custom locations.
The library path can be defined in /etc/ld.so.conf file which you can edit with a command-line editor.
The line(s) in this file instruct the kernel to load file in /etc/ld.so.conf.d. This way, package maintainers or programmers can add their custom library directories to the search list.
If you look into the /etc/ld.so.conf.d directory, you’ll see .conf files for some common packages (kernel, mysql, and postgresql in this case):
# ls /etc/ld.so.conf.d kernel-2.6.32-358.18.1.el6.x86_64.conf kernel-2.6.32-696.1.1.el6.x86_64.conf mariadb-x86_64.conf kernel-2.6.32-642.6.2.el6.x86_64.conf kernel-2.6.32-696.6.3.el6.x86_64.conf postgresql-pgdg-libs.conf
If you take a look at the mariadb-x86_64.conf, you will see an absolute path to package libraries.
# cat mariadb-x86_64.conf /usr/lib64/mysql
The method above sets the library path permanently. To set it temporarily, use the LD_LIBRARY_PATH environment variable on the command line. If you want to keep the changes permanent, then add this line in the shell initialization file /etc/profile (global) or ~/.profile (user-specific).
# export LD_LIBRARY_PATH=/path/to/library/file
Managing Shared Libraries in Linux
Let us now look at how to deal with shared libraries. To get a list of all shared library dependencies for a binary file, you can use the ldd utility. The output of ldd is in the form:
library name => filename (some hexadecimal value) OR filename (some hexadecimal value) #this is shown when library name can’t be read
This command shows all shared library dependencies for the ls command.
# ldd /usr/bin/ls OR # ldd /bin/ls
Sample Output
linux-vdso.so.1 => (0x00007ffebf9c2000) libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003b71e00000) librt.so.1 => /lib64/librt.so.1 (0x0000003b71600000) libcap.so.2 => /lib64/libcap.so.2 (0x0000003b76a00000) libacl.so.1 => /lib64/libacl.so.1 (0x0000003b75e00000) libc.so.6 => /lib64/libc.so.6 (0x0000003b70600000) libdl.so.2 => /lib64/libdl.so.2 (0x0000003b70a00000) /lib64/ld-linux-x86-64.so.2 (0x0000561abfc09000) libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003b70e00000) libattr.so.1 => /lib64/libattr.so.1 (0x0000003b75600000)
Because shared libraries can exist in many different directories, searching through all of these directories when a program is launched would be greatly inefficient: which is one of the likely disadvantages of dynamic libraries. Therefore a mechanism of caching is employed, performed by the program ldconfig.
By default, ldconfig reads the content of /etc/ld.so.conf, creates the appropriate symbolic links in the dynamic link directories, and then writes a cache to /etc/ld.so.cache which is then easily used by other programs.
This is very important especially when you have just installed new shared libraries or created your own, or created new library directories. You need to run the ldconfig command to effect the changes.
# ldconfig OR # ldconfig -v #shows files and directories it works with
After creating your shared library, you need to install it. You can either move it into any of the standard directories mentioned above and run the ldconfig command.
Alternatively, run the following command to create symbolic links from the soname to the filename:
# ldconfig -n /path/to/your/shared/libraries
To get started with creating your own libraries, check out this guide from The Linux Documentation Project(TLDP).
That’s all for now! In this article, we gave you an introduction to libraries and explained shared libraries, and how to manage them in Linux. If you have any queries or additional ideas to share, use the comment form below.