An OpenGL Loading Library is a library that loads pointers to OpenGL functions at runtime, core as well as extensions. This is required to access functions from OpenGL versions above 1.1 on most platforms. Extension loading libraries also abstracts away the difference between the loading mechanisms on different platforms.

Most extension loading libraries override the need to include gl.h at all. Instead, they provide their own header that must be used. Most extension loading libraries use code generation to construct the code that loads the function pointers and the included headers. Information is available if you wish to perform this manually, but you are encouraged to use one of these libraries yourself.

GLEW (OpenGL Extension Wrangler)

The OpenGL Extension Wrangler library provides access to all GL entrypoints. It supports Windows, MacOS X, Linux, and FreeBSD.

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As with most other loaders, you should not include gl.h, glext.h, or any other gl related header file before glew.h, otherwise you'll get an error message that you have included gl.h before glew.h. In fact, you shouldn't be including gl.h at all; glew.h replaces it.

GLEW also provides wglew.h which provides Windows specific GL functions (wgl functions). If you include wglext.h before wglew.h, GLEW will complain. GLEW also provides glxew.h for X windows systems. If you include glxext.h before glxew.h, GLEW will complain.

The latest release as of February 2019 is version 2.1.0.

Initialization of GLEW 1.13.0 and earlier

GLEW up to version 1.13.0 has a problem with core contexts. It calls glGetString(GL_EXTENSIONS), which causes GL_INVALID_ENUM on GL 3.2+ core context as soon as glewInit() is called. It also doesn't fetch the function pointers. GLEW version 2.0.0+ uses glGetStringi instead. The only fix for earlier versions is to use glewExperimental:

glewExperimental is a variable that is already defined by GLEW. You must set it to GL_TRUE before calling glewInit().

You might still get GL_INVALID_ENUM (depending on the version of GLEW you use), but at least GLEW ignores glGetString(GL_EXTENSIONS) and gets all function pointers.

If you are creating a GL context the old way or if you are creating a backward compatible context for GL 3.2+, then you don't need glewExperimental.

GL3W

The GL3W library focuses on the core profile of OpenGL 3 and 4. It only loads the core entrypoints for these OpenGL versions. It supports Windows, Mac OS X, Linux, and FreeBSD.

Note: GL3W loads core OpenGL only by default. All OpenGL extensions will be loaded if the --ext flag is specified to gl3w_gen.py.

GL3W relies on a Python script for its code generation. Unlike other extension loaders, GL3W actually does the code generation on your machine. This is based on downloading and parsing the glcorearb.h file from the OpenGL Registry website.

On the one hand, this means that it is always up-to-date. On the other hand, this also makes it beholden to the format of glcorearb.h (which has no true format), as well as requiring that the user of GL3W have a Python installation.

GL3W is used like this:

glLoadGen (OpenGL Loader Generator)

The OpenGL Loader Generator This tool is similar to GL3W in several respects. It generates GL loaders, rather than being a library you download and compile. The difference is that it is less specialized; GL3W will only generate core OpenGL functions/enumerators, so no extensions. And it only does so for the most recent version of OpenGL (currently 4.6).

The GL Loader Generator is much more generalized. You can generate a header/source pair for any version of OpenGL, from 1.1 to 4.6. You can include whichever OpenGL extensions you so desire extensions or none at all. And it has the ability to generate source code in a variety of styles, from the standard C-style that most users are familiar with, to C++ functions/enums wrapped in namespaces, and so forth. The system is also user-extensible.

Also, unlike GL3W, it works for WGL and GLX extensions too. So you can generate headers for the platform-specific APIs.

Like GL3W, the loader generator is built in a scripting language. Unlike GL3W, this tool is written in Lua, which is downloadable for a variety of platforms (and has a much smaller install package than Python, if you care about that sort of thing).

The tool is fairly simple to use, and its use is specified in some detail on the website, with several examples.

Galogen

Galogen is a GL loader generator in the spirit of glLoadGen, but with no additional dependencies (like Lua). Given an API version and a list of extensions, Galogen will produce corresponding headers and code that load the exact OpenGL entry points you need. The produced code can then be used directly by your C or C++ application, without having to link against any additional libraries. Galogen uses the official Khronos OpenGL Registry as its source of truth, thus ensuring that the produced code always corresponds to the latest specifications. An in-browser version of Galogen is also available.

glad (Multi-Language GL/GLES/EGL/GLX/WGL Loader-Generator)

Glad is pretty similiar to glLoadGen, it generates a loader for your exact needs based on the official specifications from the Khronos SVN. This means they are always up to date! It was written in a way that you can easily extend it to other languages (e.g. at the date of writing this there are 4 different code generating backends).

You can use the glad website to generate a loader for your needs, download it and use it in your project. Another method of using glad is cloning/downloading the repository and generating your own loader. The tool itself is pretty easy to use and works with any Python version above 2.6, you can also include the source directly into your CMake project.

Glad gives you the option to also generate a very basic loader (similiar to gl3w or glxw), but it is recommended to use the loading function provided by your context creation framework, like glfwGetProcAddress. Here is how it looks:

Glad is able to generate a debugging header, which allows you to hook into your OpenGL calls really easily using glad_set_pre_callback and glad_set_post_callback, you can find a more detailed guide on the github repository.

Glatter

Glatter is a generated library, with support for GL, GLES, GLU, EGL, GLX, WGL.

Its usage requires the inclusion of one header:

In C++ code, it may as well be used as header-only.

It requires no explicit loading or initialization calls, and its startup cost is minimal.

Besides loading, it offers debugging and tracing functionality.

More details can be found on its github page.

glsdk (Unofficial OpenGL SDK)

The Unofficial OpenGL SDK includes a component for loading OpenGL functions. This component, called GL Load, has a C and C++ interface for loading OpenGL functions. It also provides different headers for different OpenGL versions. It even has headers that remove compatibility enums and functions for versions of OpenGL 3.1 or greater.

Here is a code example:

GL Load even offers special headers for C++ code, that moves as much of OpenGL as possible into a namespace. The equivalent code in the C++ interface is as follows:

glbinding (C++)

glbinding is a new, generated, cross-platform C++ binding for OpenGL which is solely based on the new xml-based OpenGL API specification (gl.xml). It leverages modern C++11 features like enum classes, lambdas, and variadic templates, instead of relying on macros (all OpenGL symbols are real functions and variables). It provides type-safe parameters, per feature API header, lazy function resolution, multi-context and multi-thread support, global function callbacks, meta information about the generated OpenGL binding and the OpenGL runtime, as well as multiple examples for quick-starting your projects.

Current gl code, written with a typical C binding for OpenGL is fully compatible for the use with glbinding:

glbinding also supports OpenGL feature (version) specific symbols for functions, enums, and bitfields. For example:

  • functions32.h provides all OpenGL commands available up to 3.2 in namespace gl32.
  • functions32core.h provides all non-deprecated OpenGL commands available up to 3.2 in namespace gl32core.
  • functions32ext.h provides all OpenGL commands specified either in 3.3 and above, or by extension in gl32ext.

More details and examples can be found on the github project page and examples wiki respectively.

libepoxy

libepoxy requires no initialization code. The only thing you have to do is:

GLee

The project seems more or less defunct.

While there is activity in the Git repository on Sourceforge, there has not been a new official version and distribution in years. The recent activity could represent a project coming back, but currently you would be better advised to look elsewhere for an OpenGL loader.

Retrieved from 'http://www.khronos.org/opengl/wiki_opengl/index.php?title=OpenGL_Loading_Library&oldid=14497'

When you create or update a dynamic library, you should carefully consider how it may be used by developers in their products. It’s also important to give these developers flexibility by allowing their products to work with earlier or later versions of the library without them having to update their products.

This article demonstrates how to write a dynamic library so that it’s easy to use by developers who want to take advantage of it in their own development. This article also describes how to update existing libraries and manage their version information to maximize client compatibility.

Creating Libraries

When creating a dynamic library, you should perform these tasks:

  • Define the library’s purpose: This information provides the focus required to define the library’s public interface.

  • Define the library’s interface (header files): This is the interface through which the library’s clients access its functionality.

  • Implement the library (implementation files): This is where you define the public functions that the library’s clients use. You may also need to define private variables and functions needed to implement the interface but not required by clients.

  • Set the library’s version information: The library’s version information is divided in three parts: major, minor, and compatibility. You specify all the parts of the library’s version information when you create the .dylib file. See Managing Client Compatibility With Dependent Libraries for details.

  • Test the library: At the very least, you should define a test for each of the public functions your library exposes, to ensure that they perform the correct action given particular test inputs or after performing a specific set of operations.

The following sections provide an example of the process taken to develop a simple dynamic library, called Ratings. The files mentioned in the following sections are included in Ratings/1.0 in this document’s companion-file package.

Defining the Library’s Purpose

The purpose of the Ratings library is to provide a ratings analyzer to its clients. Ratings are strings made up of asterisks (*) that represent levels of satisfaction for a particular item. For example, in Apple’s iTunes app, you can specify whether you like a particular song a lot with a five-star rating (*****) or not at all with a one-star rating (*).

The initial release of the library provides a way for clients to add ratings, get a count of the ratings they have added, get the mean rating, and clear the rating set.

Defining the Library’s Interface

Before you implement the library, you must define the interface the library’s clients use to interact with it. You should carefully specify each function’s semantics. A clear definition benefits you because it makes clear the purpose of each public function. It also benefits developers who use your library because it tells them exactly what to expect when they call each function in their code.

This is an example of a list of interface functions with each function’s semantic meaning:

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  • void addRating(char *rating): Adds a rating value to the set kept by the library. The rating argument is a string; it must not be NULL. Each character in the string represents one rating point. For example, an empty string (') is equivalent to a rating of 0, '*' means 1, '**' means 2, and so on. The actual characters used in the string are immaterial. Therefore, ' ' means 1 and '3456' means 4. Each call to this function increments the value returned by the ratings function.

  • int ratings(void): Returns the number of rating values in the set. This function has no side effects.

  • char *meanRatings(void): Returns the mean rating in the rating set as a string, using one asterisk per rating point. This function has no side effects.

  • clearRatings(void): Clears the rating set. After calling this function (with no subsequent calls to addRating), the ratings function returns 0.

Listing 1 shows the header file that the Ratings library’s clients include to access its interface.

Listing 1 Interface to Ratings 1.0

Implementing the Library

The interface declared in Defining the Library’s Interface is implemented in Ratings.c , shown in Listing 2.

Listing 2 Implementation of Ratings 1.0

The following list describes the tagged lines.

  1. /laserbase-mf3220-driver-download.html. This comment is only to remind the library’s developers to compile this file with the compiler -fvisibility=hidden option, so that only the symbols with the visibility('default') attribute are exported.

  2. This initializer is defined only to show at which point of a client’s execution the library’s initializers are called by the dynamic loader.

  3. This finalizer is defined only to show at which point of client’s execution the library’s finalizers are called by the dynamic loader.

  4. The _add function is an example of an internal function. Clients don’t need to know about it and, therefore, it’s not exported. Also, because internal calls are trusted, no validation is performed. However, there’s no requirement that internal-use functions lack validation.

  5. The addRating function is an example of an exported function. To ensure that only correct ratings are added, the function’s input parameter is validated.

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Setting the Library’s Version Information

When you compile the library’s source files into a .dylib file, you set version information that specifies whether clients can use versions of the library earlier or later than the version they were linked with. When the client is loaded into a process, the dynamic loader looks for the .dylib file in the library search paths and, if it finds it, compares the version information of the .dylib file with the version information recorded in the client image. If the client is not compatible with the .dylib file, the dynamic loader doesn’t load the client into the process. In effect, the client’s load process is aborted because the dynamic loader was unable to locate a compatible dependent library.

Listing 3 shows the command used to generate version 1.0 of the Ratings library.

Listing 3 Generating version 1.0 of the Ratings dynamic library

This list indicates where the major version, minor version, and compatibility version are specified:

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  • The major version number is specified in the library’s filename as “A” in -o libRatings.A.dylib.

  • The minor version number is specified in -current_version 1.0.

  • The compatibility version number is specified in -compatibility_version 1.0.

Note: You can use libtool to create a dynamic library from a set of object files.

Testing the Library

Before publishing a dynamic library, you should test its public interface to ensure that it performs as you specified in the interface’s documentation (see Defining the Library’s Interface. To provide maximum flexibility to clients, you should make sure that your library can be used as a dependent library (clients link with it, and the library is loaded when the client is loaded) or as a runtime-loaded library (clients don’t link with it and use dlopen(3) OS X Developer Tools Manual Page to load it).

Listing 4 shows an example of a test client that uses the Ratings library as a dependent library.

Listing 4 Testing Ratings 1.0 as a dependent library

The following command generates the Dependent client program. Note that libRatings.A.dylib is included in the compile line.

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The Dependent program produces output that shows whether calling each of the library’s exported functions produces the expected results. In addition, the initializer and finalizer functions defined in the library produce output lines that indicate when they are invoked in relation to the program’s normal process. This output is shown in Listing 5.

Listing 5 Test results for Ratings 1.0 as a dependent library

Notice that initializer is called before the main function. The finalizer function, on the other hand, is called after exiting the main function.

Testing the Ratings library as a runtime-loaded library requires another test program, Runtime. Listing 6 shows its source file.

Listing 6 Testing Ratings 1.0 as a runtime-loaded library

The Runtime program is very similar to the Dependent program. However, Runtime must load libRuntime.A.dylib using the dlopen(3) OS X Developer Tools Manual Page function. After that, it must get the address of each function exported by the library using dlsym(3) OS X Developer Tools Manual Page before using it.

The following command generates the Runtime client program.

Listing 7 shows the output produced by Runtime.

Listing 7 Test results for Ratings 1.0 as a runtime-loaded library

Note that the Ratings library’s initializer function is called within the main function execution before the call to dlopen returns, which differs from its execution point in the Dependent program Listing 5. The finalizer function, however, is called after main has exited, the same point at which it’s called in Dependent’s execution. You should consider this when writing dynamic library initializers and finalizers.

Updating Libraries

Making revisions to a previously published dynamic library is a delicate task. If you want existing clients to be able to use the library (that is, load the new version of the .dylib file without recompilation), you must ensure that the API those clients know about is unchanged, including its semantic meaning.

When you can guarantee that the API of the new version of the library is compatible with the API that clients linked with earlier versions know about, you can deem the new version a minor revision. When you want to publish a minor revision of a dynamic library to users of the library’s clients (for example, the users of a program that uses your library) the only version information item you need to change is the library’s current version. The major version (filename) and compatibility version of the library must remain the same. When end users replace the early version of the library with the new version in their computers, the library’s clients use the new version without any problems. Managing Client Compatibility With Dependent Libraries.

Making Compatible Changes

From the point of view of a client image, there are two sides to compatibility with the dynamic libraries it uses: compatibility with versions of a library later than the one the client knows about, known as forward compatibility, and compatibility with versions of a library earlier than the one the client is familiar with, known as backward compatibility. You maintain forward compatibility by ensuring that the library’s API remains compatible across revisions. You facilitate backward compatibility by exporting new functions as weak references. In turn, your library’s clients must ensure that weakly imported functions actually exist before using them.

The Ratings library, introduced in Creating Libraries, has a second version, 1.1. Listing 8 shows the updated header for the Ratings library.

Listing 8 Interface to Ratings 1.1

The tagged lines declare the new functions. Notice that both declarations include the weak_import attribute, informing client developers that the function is weakly linked. Therefore, clients must make sure the function exists before calling it.

Listing 9 shows the library’s new implementation file.

Listing 9 Implementation of Ratings 1.1

Listing 10 shows the updated source code for the Runtime program that tests the Ratings 1.1 library.

Listing 10 Testing Ratings 1.1 as a runtime-loaded library

Updating the Library’s Version Information

Listing 11 shows the command used to generate version 1.1 of the Ratings library.

Listing 11 Generating version 1.1 of the Ratings dynamic library

Notice that this time the library’s current version is set to 1.1 but its compatibility version remains 1.0. When a client is linked against version 1.1 of this library, the compatibility version is encoded in the client image. Therefore, the dynamic loader loads the client image whether it finds version 1.1 or 1.0 of libRatings.A.dylib. That is, the client is backwards compatible with version 1.0 of the library. For details on why /usr/local/lib/libAverages.dylib was added to the compile line, see Using Dependent Libraries.

Note: Weak references are a feature of OS X v10.2 and later. Therefore, when using weak references, your library can be used only by programs running in OS X v10.2 or later.

Testing the New Version of the Library

Similar to creating initial versions of a library, updating libraries require thorough testing. You should at least add tests for each function you added to your test programs. If you used weak references, your test programs should ensure the existence of the new functions before calling them.

The Ratings/1.1 directory in this document’s companion-file package contains the source files for the Ratings 1.1 library. Listing 12 shows an updated version of the Dependent program. The tagged lines show how to check for the presence of a function before using it.

Listing 12 Testing Ratings 1.1 as a dependent library

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Ratings 1.1 depends on Averages 1.1. Therefore, to build the library as well as the test programs, libAverages.A.dylib must be installed in ~/lib. To accomplish this, run this command after opening this document’s companion-file package:

These are commands needed to compile the library and the test programs from the Ratings/1.1 directory:

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The output the program produces is shown in Listing 13.

Listing 13 Test results for Ratings 1.1 used as a dependent library

Programs that use dlopen to load your library should not fail if they are unable to obtain an address for your weakly exported functions with dlsym. Therefore, the program that tests your library as a runtime-loaded library, should allow for the absence of weakly imported functions.

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Listing 14 shows an updated version of the Runtime program. Notice that it uses dlsym to try get the addresses of the new functions but doesn’t exit with an error if they are unavailable. However, just before the program uses the new functions, it determines whether they actually exists. If they don’t exist, it doesn’t perform the test.

Listing 14 Testing Ratings 1.1 as a runtime-loaded library

Listing 15 shows the output produced by Runtime.

Listing 15 Test results for Ratings 1.1 used as a runtime-loaded library

To ensure that clients linked with version 1.0 of libRatings.A.dylib can use version 1.1 of the library, you can copy Ratings/1.1/libRatings.A.dylib to Ratings/1.0. When you run the first Dependent program, its output is the identical to the output it produced when it used version 1.0 of the library. The first Dependent program knows nothing about the functions introduced in version 1.1 of the Ratings library; therefore, it doesn’t call them.

A more interesting test, however, is making sure that clients linked with version 1.1 of the Ratings library can run when their copy of the library is replaced with version 1.0. To test this, execute the following commands in Terminal:

Listing 16 shows the output produced by the second version of Dependent, linked with Ratings 1.1, when it loads Ratings 1.0 instead of Ratings 1.1. The highlighted lines show where the client program did not find a particular function at runtime because the function does not exist in the version of the Ratings library that it’s using.

Listing 16 Test results for Ratings 1.0 used as a dependent library by a client linked against Ratings 1.1

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The second version of the Runtime test program produces similar output when using Ratings 1.0, as shown in Listing 17.

Listing 17 Test results for Ratings 1.0 used as a runtime-loaded library by a client liked with Ratings 1.1 when using Ratings 1.0


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