Native function name w/o parens -- expected behavior?

Question

I wrote a sketch that used the millis() function in several places. I got odd behavior because I inadvertently used the plain millis in one line, without the parens. It evaluated to zero when running.

Why didn't the compiler flag this as an undeclared variable? I suspect it's because the name was already in the symbol table, but I still would have thought it'd be caught. Is this a quirk, or expected? If expected, what is the benefit?

C/C++ is not my main language so be gentle. (-:

Edit as requested to show some code. This is an incomplete sample, but shows the usage:

const unsigned long GATE_OPEN_TIMER = 7000;
const unsigned long GATE_CLOSE_TIMER = 12000;
...
unsigned long gateStopTime = 0;
...

  if (isGateCycling) {                 // if open/close cycle is running
    if (isGatePassive) {               // if we're not applying power
      if (millis() >= gateStopTime) {  // if current delay expired
        digitalWrite(GateCloseRelay, ACTIVE);
        gateStopTime = millis() + GATE_CLOSE_TIMER;
        isGatePassive = false;

... followed by similar logic for the reverse direction. If any use of millis() is replaced by millis, the code falls over but the compiler sees nothing wrong.

Answer 1

I've made up a test case:

const unsigned long GATE_OPEN_TIMER = 7000;
const unsigned long GATE_CLOSE_TIMER = 12000;
unsigned long gateStopTime = 0;

void setup()
{
  if (millis >= gateStopTime) {  // if current delay expired
    gateStopTime = millis + GATE_CLOSE_TIMER;
  }
}
void loop()
{
}

Under IDE 1.6.9 I get these errors:

/tmp/arduino_modified_sketch_345879/sketch_oct27a.ino: In function 'void setup()':
sketch_oct27a:7: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   if (millis >= gateStopTime) {  // if current delay expired
                 ^
/tmp/arduino_modified_sketch_345879/sketch_oct27a.ino:8:29: warning: pointer to a function used in arithmetic [-Wpointer-arith]
     gateStopTime = millis + GATE_CLOSE_TIMER;
                             ^
sketch_oct27a:8: error: invalid conversion from 'long unsigned int (*)()' to 'long unsigned int' [-fpermissive]
     gateStopTime = millis + GATE_CLOSE_TIMER;
                           ^
exit status 1
ISO C++ forbids comparison between pointer and integer [-fpermissive]

---

Under IDE 1.6.12 they are downgraded to warnings:

/tmp/untitled109152362.tmp/sketch_oct25b/sketch_oct25b.ino: In function 'void setup()':
/tmp/untitled109152362.tmp/sketch_oct25b/sketch_oct25b.ino:9:21: warning: ISO C++ forbids comparison between pointer and integer [-fpermissive]
       if (millis >= gateStopTime) {  // if current delay expired
                     ^
/tmp/untitled109152362.tmp/sketch_oct25b/sketch_oct25b.ino:10:33: warning: pointer to a function used in arithmetic [-Wpointer-arith]
         gateStopTime = millis + GATE_CLOSE_TIMER;
                                 ^
/tmp/untitled109152362.tmp/sketch_oct25b/sketch_oct25b.ino:10:31: warning: invalid conversion from 'long unsigned int (*)()' to 'long unsigned int' [-fpermissive]
         gateStopTime = millis + GATE_CLOSE_TIMER;
                                   ^

Sketch uses 512 bytes (1%) of program storage space. Maximum is 32,256 bytes.
Global variables use 13 bytes (0%) of dynamic memory, leaving 2,035 bytes for local variables. Maximum is 2,048 bytes.

---

So, the compiler isn't totally letting you get away with it. You should check the warnings. But why are the warnings only warnings?

Checking the generated compile command (using verbose compiling) I see that 1.6.12 has an extra option: -fpermissive.

From the documentation:

Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile.

So you have "nonconformant code" but you are getting away with it.

I don't know why the Arduino devs did that. The whole point of making things like that errors is so you fix them at compile time, rather than scratching your head about why things don't work at run time.

Clearly the symbol millis exists. But it is a function (or perhaps a pointer to a function) and thus shouldn't be just compared to a number or have a number added to it. That's what those errors were about in 1.6.9.

---

Just remember, in C, C++ and the ilk, to call a function you always use parentheses, even if you are passing no arguments.

Answer 2

In short, a function in an Arduino program that gets called without () will be considered as a plain variable. Try writing a small function in your program and call it without (). Nothing will happen.

The reason the compiler didn't flag it as an undeclared variable was because millis is a variable that is declared in a kind of pre-loaded library. Think of it as a library that is always there even if you didn't include it. That is why the compiler only saw it as a regular variable.