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Asked: 2026-06-12T03:34:59+00:00

I need the maximum length of a uniform name. ie: given a used program

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I need the maximum length of a uniform name. ie: given a used program with uniforms uniform test and uniform myuniform, glGetProgramiv(program, GL_ACTIVE_UNIFORM_MAX_LENGTH, outParam) should get output of 10 (for “myuniform”).

I have a very simple test shader set up with 1 defined uniform: uniform float time

glGetProgramiv(program, GL_ACTIVE_UNIFORM_MAX_LENGTH, outParam) returns 5, the length of time. If I change “time” to something else, it returns the changed length (ex: change to “timer” it returns 6).

glGetProgramiv with GL_ACTIVE_UNIFORMS tells me that there are 2 uniforms.
The second uniform that it is reporting is gl_ModelViewProjectionMatrix.

I am fine with it including gl_ModelViewProjectionMatrix in the list – I am using it in the shader, but this brings up a problem when combined with the other return value. Why doesn’t glGetProgramiv return the length of “gl_ModelViewProjectionMatrix” if it is including it in the list? I need the full names of attributes and variables for my application, but since I am getting a max length of 5, glGetActiveUniform is returning a uniform name of “gl_M” which is not acceptable.

Either the max name length should include the MVP matrix, or the list of names should not. It does not make sense to include the name in the list but not in the max name length calculation.

Is this happening only for me? I could not find anything else about it using Google. I could abandon a query for max length and always use very big buffers, but I’ve seen some very long variable names before, so the buffers would have to be huge to guarantee no errors. That’s not a real fix anyway.

This test is working correctly for attributes. I use gl_Vertex and have no other attributes. The system reports 1 current attribute with a length of 10 and a name of gl_Vertex correctly. If I remove my time uniform entirely, that leaves the MVP matrix as the only used uniform, the system reports 1 current uniform, max name length of 0, so getting its name with the returned max length gets nothing.

For completeness, I include the code below. The code is in Java and uses JOGL to access OpenGL bindings. To highlight the relevant areas, I have deleted lines not relevant to this issue, which was mostly the GUI updating, including the GUI part that actually shows the values obtained in here. I also deleted the part that gets the attributes since that works fine, as stated above.

FYI for the C people who are wary of the Java-isms: think of a Buffer (IntBuffer, ByteBuffer, FloatBuffer) like a pointer, buffer.get() like buffer[n++], and buffer = IntBuffer.allocate(n) like a malloc(). I also use OpenGL in C and C++, so I can rewrite this in C if the GLSL gurus here prefer that.

Any suggestions?

// add options to panelShaderParameters
public void updateShaderParameters(GLAutoDrawable surface)
{
    GL2 gl = surface.getGL().getGL2();

    IntBuffer outParam = IntBuffer.allocate(1);
    int numParameters = 0,
        maxNameLength = 0;

    IntBuffer size = null,
        type = null;
    ByteBuffer name = null;

    gl.glGetProgramiv(shader.getName(), GL2.GL_ACTIVE_UNIFORMS, outParam);
    numParameters = outParam.get();
    outParam = IntBuffer.allocate(1);
    gl.glGetProgramiv(shader.getName(), GL2.GL_ACTIVE_UNIFORM_MAX_LENGTH, outParam);
    maxNameLength = outParam.get();

    for(int i = 0; i < numParameters; i += 1)
    {
        size = IntBuffer.allocate(1);
        type = IntBuffer.allocate(1);
        name = ByteBuffer.allocate(maxNameLength);
        gl.glGetActiveUniform(shader.getName(), i, maxNameLength, (IntBuffer)null, size, type, name);
        byte[] nameBuffer = new byte[maxNameLength];
        name.position(0);
        name.get(nameBuffer);
    }
}
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1 Answer

  1. Editorial Team

    You have found a driver bug. You can attempt to report it (this forum is a place where it may be seen), but there’s nothing you can do to make it work correctly. So just work around it.

    Rather than having a single max-size, just ask each uniform in turn what it’s length is. In C/C++, you do this by passing NULL for the buffer to glGetActiveUniformName:

    GLsizei length;
glGetActiveUniformName(program, index, 1000 /*large number*/, &length, NULL);
//Use length to allocate a buffer of the appropriate size.

    I don’t know how this would work using JOGL. Perhaps you should switch to LWJGL, which has a much more reasonable Java implementation of this function.

    Also, stop storing the strings as byte arrays. Convert them into proper Java strings (another reason to use LWJGL).

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