We start by calling glCreateProgram to create an empty program object. This function returns a handle to the program object, which we store in a variable named programHandle. If an error occurs with program creation, the function will return 0. We check for that, and if it occurs, we print an error message and exit.
Next, we attach each shader to the program object using glAttachShader. The first argument is the handle to the program object, and the second is the handle to the shader object to be attached.
Then, we link the program by calling glLinkProgram, providing the handle to the program object as the only argument. As with compilation, we check for the success or failure of the link, with the subsequent query.
We check the status of the link by calling glGetProgramiv. Similar to glGetShaderiv, glGetProgramiv allows us to query various attributes of the shader program. In this case, we ask for the status of the link by providing GL_LINK_STATUS as the second argument. The status is returned in the location pointed to by the third argument, in this case named status.
The link status is either GL_TRUE or GL_FALSE, indicating the success or failure of the link. If the value of the status is GL_FALSE, we retrieve and display the program information log, which should contain additional information and error messages. The program log is retrieved by the call to glGetProgramInfoLog. The first argument is the handle to the program object, the second is the size of the buffer to contain the log, the third is a pointer to a GLsizei variable where the number of bytes written to the buffer will be stored (excluding the null terminator), and the fourth is a pointer to the buffer that will store the log. The buffer can be allocated based on the size returned by the call to glGetProgramiv with the GL_INFO_LOG_LENGTH parameter. The string that is provided in log will be properly null terminated.
Finally, if the link is successful, we install the program into the OpenGL pipeline by calling glUseProgram, providing the handle to the program as the argument.
It is a good idea to detach and delete the shader object, regardless of whether the link is successful. However, if the shader objects might be needed to link another program, you should detach it from this program and skip deletion until later.
With the simple fragment shader from this recipe and the vertex shader from the previous recipe compiled, linked, and installed into the OpenGL pipeline, we have a complete OpenGL pipeline and are ready to begin rendering. Drawing a triangle and supplying different values for the Color attribute yields an image of a multi-colored triangle where the vertices are red, green, and blue, and inside the triangle, the three colors are interpolated, causing a blending of colors throughout:
