Reading value of constant parameter

I’m learning OpenCL and I wrote simple kernel, which copies part (square) of input image and pastes it to different position (in this case (0,0)) of output image. The problem is reading of parameter.

__constant sampler_t sampler = CLK_NORMALIZED_COORDS_FALSE |
                               CLK_ADDRESS_CLAMP_TO_EDGE |

__kernel void image_part_copy( __read_only image2d_t input,
                               __constant int* x_coord,
                               __global float* output,
                               __write_only image2d_t outputImage)
    int width  = get_image_width( input );
    int height = get_image_height( input );

    int2 pixelcoord = (int2) (get_global_id(0), get_global_id(1));
    int xc = x_coord[get_global_id(0)];

    const int top_left = xc; // <--- THE PROBLEM IS HERE
    //const int top_left = 1000;
    const int width_of_cut = 2500;
    const int right_bottom = top_left + width_of_cut;

    int a,b;
    a = (int)(pixelcoord.x + top_left);
    b = (int)(pixelcoord.y + top_left);

    if( a < right_bottom && b < right_bottom )
        float4 first_pixel = read_imagef(input, sampler, (int2)(a,b));
        write_imagef(outputImage, (int2)(pixelcoord.x,pixelcoord.y), first_pixel);
        float4 copy = read_imagef(input, sampler, pixelcoord);
        write_imagef(outputImage, (int2)(pixelcoord.x,pixelcoord.y), copy);

    output[get_global_id(0)] = (float)(top_left); // just for control

If there is const int top_left = 1000; (commented line), it works as I wanted. But if I set parameter __constant int* x_coord in host code and then read it in kernel: int xc = x_coord[get_global_id(0)]; and const int top_left = xc;, it just copies the input image. But the control: output[get_global_id(0)] = (float)(top_left); works. In host code is the output parameter the assigned value (input parameter): __constant int* x_coord.