Setting a number inside a struct causes timer to misbehave

Question

The following code:

struct l{
 uint8_t pin;
 uint8_t sensePin;
 float brightnessTarget;
 uint8_t PWMvalue;
 uint8_t inc;
};
typedef struct l led;
led* redLed;
led* whiteLed;
unsigned long secondsOfDay = 79140;
unsigned long minutesOfDay = 0;
unsigned int hoursOfDay = 0;
uint8_t interrupted=0;
uint8_t runPID;
int pin13=0;
void setup() {
 cli();
 TCCR1A = 0;// set entire TCCR1A register to 0
 TCCR1B = 0;// same for TCCR1B
 TCNT1 = 0;//initialize counter value to 0
 // set compare match register for 1hz increments
 OCR1A = OCR1A_val;// = (16*10^6) / (1*1024) - 1 (must be <65536)
 // turn on CTC mode
 TCCR1B |= (1 << WGM12);
 // Set CS10 and CS12 bits for 1024 prescaler
 TCCR1B |= (1 << CS12) | (1 << CS10); 
 // enable timer compare interrupt
 TIMSK1 |= (1 << OCIE1A);
 pinMode(13,OUTPUT);
 pinMode(3, OUTPUT);
 pinMode(4, OUTPUT);
 sei();
}
void loop() {
 if (runPID){
 PIControl(redLed);
 PIControl(whiteLed);
 pin13 = (~pin13 & 1);
 digitalWrite(13, pin13);
 runPID=0;
 }
}
SIGNAL(TIMER1_COMPA_vect){ //1 second timer fire
 runPID = 1;
}

Toggles a pin. However, if I add:

redLed->pin = 3;
whiteLed->pin = 4; 
redLed->sensePin = 0;
whiteLed->sensePin = 1;

In the setup anywhere (before cli(), after sei(), or within the block), the interrupt no longer toggles.

I'm absolutely baffled, and I have no idea what's going on.

Edit: everything but whiteLed->sensePin is fine. That line is what is causing the issue (and I still don't understand)

Answer 1

This:

led* redLed;

defines a global variable implicitly initialized to zero (i.e. a NULL pointer). This is an invalid pointer you are not allowed to use until you properly initialize it.

Here:

redLed->pin = 3;

you are writing to the memory pointed to by this invalid pointer. This invokes what the C and C++ standards call undefined behavior. What that means is that, according to the definition of the C++ language, the behavior of your program is not defined. In other words, anything can happen. Typically you get a program crash, but even that behavior is not guaranteed.

Do not ever access memory through an invalid pointer.

• Brydon Gibson: Ugh. I knew it was a dumb oversight. Thank you!