$fmonitor

Hi everyone,
I am trying to monitor the signals within my testbench into a file but it always gives me the following warning
** Warning: No matching channel descriptor '2' found.  Ignoring the write in to the invalid channel.
#    Time: 0 ns  Iteration: 2  Instance: /textgenerator

here is an example:

module textgenerator;
    integer fd;
    logic clk;
   
initial begin
        clk
        #2 clk=!clk;
    end
    
initial begin 
  
fd=$fopen("file.txt");
$fmonitor(fd,"clk:%b",clk);
$fclose(fd);
        end
endmodule


could someone please help me on how to save the monitored signals within a file