Is the F() print macro meant to generate an eth packet per character?

Projects Networking, Protocols, and Devices
1

If I do the following when using the standard Arduino Ethernet2 shield and associated library:-

     if (c == '\n' && currentLineIsBlank) {
          // send a standard http response header
          client.println(F("HTTP/1.1 200 OK"));
          client.println(F("Content-Type: application/octet-stream"));
          client.println(F("Content-Length: 512"));
          client.println(F("Connection: close"));  
          client.println();

every character of the header text seems to be send within it's own tcp packet. Which then need all the associated syn/ack stuff. The F() macro seems to pop out each char individually. It doesn't seem very efficient and produces floods of packets back and forth. So it saves RAM at the expense of packets/transmission time and wears out my wires. Is this correct?

2

Yes, that's correct. It's because of the way the Print class converts the __FlashStringHelper type:

size_t Print::print(const __FlashStringHelper *ifsh)
{
  PGM_P p = reinterpret_cast<PGM_P>(ifsh);
  size_t n = 0;
  while (1) {
    unsigned char c = pgm_read_byte(p++);
    if (c == 0) break;
    if (write(c)) n++;
    else break;
  }
  return n;
}

You can see that write() is called for each character. write is defined in whichever Ethernet library you're using.

3

use StreamLib library BufferedPrinter

4

noiasca:
use something like this as a first step and use as less print/println as possible:

The \n of all but the last line were missing in the above code. :wink:

Added:

An example including R" syntax

void setup() {
  Serial.begin(250000);
  Serial.println(F(" * concat strings"));
  Serial.println(F("HTTP/1.1 200 OK\n"
                   "Content-Type: application/octet-stream\n"
                   "Content-Length: 512\n"
                   "Connection: close\n"));
  Serial.println(F(" * R\""));
  Serial.println(F(R"delimiter(HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 512
Connection: close
)delimiter"));
  Serial.println(F(" * done"));
}
void loop() {}

 * concat strings
HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 512
Connection: close

 * R"
HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 512
Connection: close

 * done
5

noiasca:
and it would be nice if you could leave out the quote of the wrong example because quoting of wrong information is ... somehow ... ugly.

I thought that posting the wrong information is a problem, not quoting or correcting it. :wink:

To post untested code is rarely a good idea, so I try to present a working example if possible.

6

noiasca:
it's not the F Makro but the Println!

use something like this as a first step and use as less print/println as possible:

  client.println(F("HTTP/1.1 200 OK\n"

"Content-Type: application/octet-stream\n"
          "Content-Length: 512\n"
          "Connection: close\n\n"));  // including the additional empty line!

so println is wrog, use println?

7

That's brilliant. Thanks guys.

Can I just ask, how do all the quoted strings work? There's no concatenation between them like a "+". Just returns. Is concatenation inferred by the compiler? Or is it just programmer's magic?

8

In C++, adjacent string literals are automatically concatenated. In fact, there is no concatenation operator such as +.

9

Sorry, is

"Connection: close\n\n"));  // including the additional empty line!

correct? I think that's two additional lines as it's messing up my transfers. Isn't a return automatically appended by println? It works for me with just one slash en.

10

cossoft:
Sorry, is

"Connection: close\n\n"));  // including the additional empty line!

correct? I think that's two additional lines as it's messing up my transfers. Isn't a return automatically appended by println? It works for me with just one slash en.

HTTP request headers should be terminated by an empty line. standard requires them for GET request too. and HTTP requires \r\n not only \n.

use your original code only put BuffredPrint over client. Install StreamLib from Library Manager

if (c == '\n' && currentLineIsBlank) {
     char buff[64];
     BufferedPrint bp(client, buff, sizeof(buff));
     bp.println(F("HTTP/1.1 200 OK"));
     bp.println(F("Content-Type: application/octet-stream"));
     bp.println(F("Content-Length: 512"));
     bp.println(F("Connection: close")); 
     bp.println();
     bp.write(data);
     bp.flush();

you can even use printf with BufferedPrint

    bp.printf(F("Content-Length: %d\r\n"), dataLength);

read about StreamLib purpose
.