Error while executing CGI script "premature end of script headers"

Question

Apache HTTP Server

I ask for your help, habravchane!
I installed Apache on the machine and wrote a script in C for remote control of a home robot.

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <string.h>
#include "PortDriver.h"

int main(void)
{
    HANDLE h;
    char *query;
    query=(char*)malloc(sizeof(char)*256);
    h=InitPort("\\\\.\\COM2",57600);
    query=getenv("QUERY_STRING");

    if (h==INVALID_HANDLE_VALUE)
    {
        printf("Error\n");
        return 0;
    }

    if (strstr(query,"COMM=W")!=0)
    {
        SendData(h,'W');

    }

    if (strstr(query,"COMM=A")!=0)
    {
        SendData(h,'A');

    }

    if (strstr(query,"COMM=S")!=0)
    {
        SendData(h,'S');

    }

    if (strstr(query,"COMM=D")!=0)
    {
        SendData(h,'D');

    }

    if (strstr(query,"COMM=Q")!=0)
    {
        SendData(h,'Q');

    }
    free(query);
    printf("Status: 204 No Change\n");
    return 0;
}

A page is generated in the browser where I display video from the camera and process the keyboard. I send commands to the server with GET, the script itself processes them, but an error page arrives in response, the Apache log gives out: “premature end of script headers”.
What should be done so that after pressing a button on the keyboard and after executing the script, the page in the browser is not updated and nothing else appears?

Answer 1

Headings must end with \r\n\r\n.
For example:

printf("Content-type:text/plain\r\n\r\n");