Created attachment 34438 [details]
vertex shader used with above code snippet

In the following code snippet we are using RGBA color buffer which means,

glGetIntegerv(GL_RED_BITS, &red_bits);
glGetIntegerv(GL_GREEN_BITS, &green_bits);
glGetIntegerv(GL_BLUE_BITS, &blue_bits);
glGetIntegerv(GL_ALPHA_BITS, &alpha_bits);

All of the above return 8. 

The shaders are attached. I am unable to give complete code at the moment.

<Code Snippet starts>
projMat_loc = glGetUniformLocation(uiProgram, "mvpMatrix_uni");
glUniformMatrix4fv(projMat_loc, 1, 0, matProjection);

vertices_loc = glGetAttribLocation(uiProgram, "vertex_att");
colors_loc = glGetAttribLocation(uiProgram, "color_att");
txtCoord_loc = glGetAttribLocation(uiProgram, "multiTexCoord_att");
txtSampler = glGetUniformLocation(uiProgram, "texture0");

glEnableVertexAttribArray(vertices_loc);
glEnableVertexAttribArray(colors_loc);
glEnableVertexAttribArray(txtCoord_loc);

glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

texCol[0] = 0.56;
texCol[1] = 0.56;
texCol[2] = 0.56;
texCol[3] = 0.56;

glClear(GL_COLOR_BUFFER_BIT);

glVertexAttribPointer(txtCoord_loc, 2, GL_FLOAT, GL_FALSE, 0, texCoords1);

vertex[0] = 4.5;
vertex[1] = 4.5;

glVertexAttribPointer(vertices_loc, 2, GL_FLOAT, GL_FALSE, 0, &vertex[0]);
glVertexAttribPointer(colors_loc, 4, GL_FLOAT, GL_FALSE, 0, texCol);

glDrawArrays(GL_POINTS, 0, 1);

glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 4, 4, 1, 1, 0); 

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, (GLfloat)GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, (GLfloat)GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, (GLfloat)GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, (GLfloat)GL_CLAMP_TO_EDGE);

glActiveTexture(GL_TEXTURE0);

glUniform1i(txtSampler, 0); 
glVertexAttribPointer(txtCoord_loc, 2, GL_FLOAT, GL_FALSE, 0, texCoords0);

vertex[0] = 12.5;
vertex[1] = 12.5;
 
col[0] = col[1] = col[2] = col[3] = 1; 

glVertexAttribPointer(vertices_loc, 2, GL_FLOAT, GL_FALSE, 0, &vertex[0]);
glVertexAttribPointer(colors_loc, 4, GL_FLOAT, GL_FALSE, 0, col);

glDrawArrays(GL_POINTS, 0, 1);

tmpBuf = (GLubyte *)malloc(4*sizeof(GLubyte));
glReadPixels(12, 12, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, tmpBuf);  

/*##############################################################
##############################################################
##############################################################
tmpBuf[0]=tmpBuf[1]=tmpBuf[2] = 143 //Expected
tmpBuf[3] = 143 //Not expected - This should be 255 as we are not using this component. I have checked this with other vendors NVIDIA and ATI and they indeed report correct behavior (tmpBuf[3] = 255)
##############################################################
##############################################################
##############################################################*/
</Code Snippet ends>