Can't set uniform value in OpenGL shader

Question

I am trying to get simple shaders to work. To be more specific, I am trying to set a uniform value of the shader. The shaders compile successfully and work correctly if I eliminate the uniform value. Here is the (vertex) shader code:

#version 440 core

layout(location = 0) in vec3 vertex;

uniform mat4 mvp;

void main()
{
    gl_Position = mvp * vec4(vertex, 1);
}

And the code I use to set the value (glGetUniformLocation returns 0 as expected):

mvp = glm::mat4(
    glm::vec4(3.0f, 0.0f, 0.0f, 0.0f),
    glm::vec4(0.0f, 1.0f, 0.0f, 0.0f),
    glm::vec4(0.0f, 0.0f, 1.0f, 0.0f),
    glm::vec4(0.0f, 0.0f, 0.0f, 2.0f)
);

GLuint matrix = glGetUniformLocation(program, "mvp");
glUniformMatrix4fv(matrix, 1, GL_FALSE, glm::value_ptr(mvp));

The problem is that it doesn't seem to set the value at all. If I query the uniform's value using glGetUniformfv, it returns the default value (not the one I just set), and the geometry doesn't show up at all (but that is just a consequence, that's not the problem here).

If I hardcode the uniform's value in the shader, the application displays the geometry correctly, and when asked for the value (again, using glGetUniformfv), it returns the correct value.

#version 440 core

layout(location = 0) in vec3 vertex;

uniform mat4 mvp = mat4(
    vec4(1.0f, 0.0f, 0.0f, 0.0f),
    vec4(0.0f, 1.0f, 0.0f, 0.0f),
    vec4(0.0f, 0.0f, 1.0f, 0.0f),
    vec4(0.0f, 0.0f, 0.0f, 2.0f)
);

void main()
{
    gl_Position = mvp * vec4(vertex, 1);
}

Any idea why does this happen? I am using OpenGL 4.4, but I have tried many different versions with the same result.

Answer 1

Are you calling glUseProgram before setting the uniforms? A full working listing can be found in many examples online, including what I had written sometime back (Github: sgxperf/sgxperf_gles20_vg.cpp) that shows the sequence to be:

glCreateProgram()
glAttachShader()
glLinkProgram()
glUseProgram()
glGetUniformLocation()
glUniformMatrix4fv()