Help understanding SD printing Code (2 replies)

Hello,

I am trying to learn how Marlin reads from the SD and prints the GCODE files. I'm am a beginner with c++, I have gotten this far,

Selecting the file from the menu in menu_media.cpp

inline void sdcard_start_selected_file() {
  card.openAndPrintFile(card.filename);
  ui.return_to_status();
  ui.reset_status();
}

This goes to the openAndPrintFile() in cardreader.cpp

void CardReader:: openAndPrintFile(const char *name) {
  char cmd[4 + strlen(name) + 1]; // Room for "M23 ", filename, and null
  extern const char M23_STR[];
  sprintf_P(cmd, M23_STR, name);
  for (char *c = &cmd[4]; *c; c++) *c = tolower(*c);
  queue.enqueue_one_now(cmd);
  queue.enqueue_now_P(M24_STR);
}

This sends the M23 command with the file name to open the file, followed by the M24 command to start printing from the file.
From M24_25.cpp

if (card.isFileOpen()) {
    card.startFileprint();            // SD card will now be read for commands
    startOrResumeJob();               // Start (or resume) the print job timer
    TERN_(POWER_LOSS_RECOVERY, recovery.prepare());
  }

The startFileprint() in cardreader.cpp

void CardReader::startFileprint() {
  if (isMounted()) {
    flag.sdprinting = true;
    TERN_(SD_RESORT, flush_presort());
  }
}

this seems to set the sdprinting flag to true but that seems to be it, i could not find this flag getting picked up anywhere.

Any help would be appreciated.