Cannot pass in uniform color in fragment shader opengl

Question

I am using openGL 3.1 and programming with QGLfunctions. I cannot pass in the uniform vec4 mycolor to the fragment shader I have initialized my program using:

m_program = new QOpenGLShaderProgram(this);
m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
m_program->link();
m_posAttr = m_program->attributeLocation("posAttr");
m_mycolor = m_program->uniformLocation("mycolor");
m_matrixUniform = m_program->uniformLocation("matrix");

and then I try to send color

 GLfloat color[] = {
        0.0f, 1.0f, 0.0f, 1.0f    };
glUniform4fv(m_mycolor, 4, color);

to my fragment shader which is:

 static const char *fragmentShaderSource =
    "uniform vec4 mycolor;\n"
    "void main() {\n"
    "   gl_FragColor = mycolor;\n"
    "}\n";

and I get nothing displayed

Answer 1

glUniform4fv(m_mycolor, 4, color);

From the glUniform documentation about the count parameter:

Specifies the number of elements that are to be modified. This should be 1 if the targeted uniform variable is not an array, and 1 or more if it is an array.

The count parameter in your case should be 1 since your color[] array still only represents one vec4 entry.

you are trying to fill a uniform vec4 color[4] array.

The correct version :

glUniform4fv(m_mycolor, 1, color);

As @Bahbar also pointed out : "GL_INVALID_OPERATION is generated if count is greater than 1 and the indicated uniform variable is not an array variable." So.. check your GL errors!