PSTR如何接收多个不以逗号分隔的字符串？

Question

I'm studying the code from the rbb_server example of ENC28J60's library for Arduino (I would put the link here if I could) and I've noticed this wierd piece of code:

 static word homePage() {
  long t = millis() / 1000;
  word h = t / 3600;
  byte m = (t / 60) % 60;
  byte s = t % 60;
  bfill = ether.tcpOffset();
  bfill.emit_p(PSTR(
    "HTTP/1.0 200 OK\r\n"
    "Content-Type: text/html\r\n"
    "Pragma: no-cache\r\n"
    "\r\n"
    "<meta http-equiv='refresh' content='1'/>"
    "<title>RBBB server</title>" 
    "<h1>$D$D:$D$D:$D$D</h1>"),
      h/10, h%10, m/10, m%10, s/10, s%10);
  return bfill.position();
}

How can that PSTR(........) compile if strings dont even have a comma separating them??

I've looked for that PSTR macro definition and I've found this:

Used to declare a static pointer to a string in program space. */
# define PSTR(s) ((const PROGMEM char *)(s))
#else  /* !DOXYGEN */
/* The real thing. */
# define PSTR(s) (__extension__({static char __c[] PROGMEM = (s); &__c[0];}))
#endif /* DOXYGEN */

Which you can find in the pgmspace.h file somewhere in Arduino's IDE folder.

How can this even compile??

Thanks!

Answer 1

There is a rule in C (C99, 6.4.5p4) that says that two adjacent string literals (be they on different lines):

"HTTP/1.0 200 OK\r\n"
"Content-Type: text/html\r\n"

are concatenated into one string literal and are equivalent to:

"HTTP/1.0 200 OK\r\nContent-Type: text/html\r\n"

This is done by the preprocessor in translation phase 6.

Answer 2

Adjacent string literals are automatically concatenated by the compiler into a single string literal.

printf("H" "e" "l" "l" "o");

is equivalent to

printf("Hello");