MQX 4.1 RTCS web_hvac response.content_length question

pbanta · ‎10-10-2014

It's not clear to me how to set response.content_length in my CGI functions.

The MQX_RTCS User Guide says in section 8.2.14:

content_length

Length of the response entity from CGI script.

If you look at the web_hvac example there are conflicting examples of how to set the content_length field.

Example 1 - content_length is set to data_length

In cgi_index.c there is this code:

/* Calculate content length while saving it to buffer */
length = snprintf(str, BUFF_SIZE, "%ld\n%ld\n%ld\n", hour, min, sec);
response.data = str;
response.data_length = length;
response.content_length = response.data_length;
/* Send response */
HTTPSRV_cgi_write(&response);
return (response.content_length);

This seems to match what it says in the MQX RTCS User Guide.

Example 2 - content_length is set to zero

In cgi_hvac.c there is this code:

response.ses_handle = param->ses_handle;
response.content_type = HTTPSRV_CONTENT_TYPE_HTML;
response.status_code = 200;
response.data = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 Transitional//EN\">"
    "<html><head><title>HVAC Settings response</title>"
    "<meta http-equiv=\"REFRESH\" content=\"0;url=hvac.shtml\"></head>\n<body>\n";
response.data_length = strlen(response.data);
response.content_length = 0;
HTTPSRV_cgi_write(&response);

if (!bParams)
{
    response.data = "No parameters received.<br>\n";
    response.data_length = strlen(response.data);
    HTTPSRV_cgi_write(&response);
}
response.data = "<br><br>\n</body></html>";
response.data_length = strlen(response.data);
HTTPSRV_cgi_write(&response);
return (response.content_length);

In this case there are multiple calls to HTTPSRV_cgi_write() and content_length is set to zero on line 8. It is never set to non-zero.

My question:

1. When should content_length be set to data_length and when should it be set to zero?

Thanks,

Paul

karelm_ · ‎10-14-2014

Hi Paul,

If content_length is set to zero in first call of HTTPSRV_cgi_write(), The server will signalize end of response entity by closing the connection and thus ignoring keep-alive flag of session. If you set content_length to non-zero value it is send to client to inform him about length of response entity and thus it is not required to close the connection and keep-alive can be used. Variable data_length is used for storing length of data to be written in one specific call of HTTPSRV_cgi_write().

So in short:

You MUST set content_length non-zero if you have keep-alive enabled. Otherwise client will not know that it has all the data and the connection will stall. But you CAN have it set to zero with keep-alive disabled.
Set data_length to length of data pointed to by data variable for every call of HTTPSRV_cgi_write().

I hope this answered your question.

P.S: In MQX 4.2 there will be support for chunked transmission encoding so you will not have to worry about content length.

Best regards,

Karel

MQX 4.1 RTCS web_hvac response.content_length question

MQX 4.1 RTCS web_hvac response.content_length question

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