Where to use "static volatile bool"?

Question

Why in the piece of code below we use "static volatile bool"?

static volatile bool wifi_connected = false;

taken from here: https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/examples/WiFiIPv6/WiFiIPv6.ino

• 001: You could easily answer this question yourself with a little research. Maybe a good book, to start: The Definitive C Book Guide and List
• plsp: The use of "static" confused me because I thought that static is a variable that remains in memory while the program is running. Thank you for the list of books!
• 001: There is a lot about C that is confusing, which is why I recommend a good book or article. A quick search found these examples: Static_(keyword) or C Static Variables and Static Functions Explained with Examples

Answer 1

static means that the global will only be accessible inside the current translation unit.

volatile means that while(!wifi_connected){/*...*/} cannot be optimized to if(!wifi_connected)while(true){/*...*/}

These things are orthogonal. So you would use static volatile when you need both properties.

• dandavis: as a C noob, i only see volatile used in interrupt routines.
• zeta-band: @dandavis volatile is a hint to the compiler that the variable may be changed in another thread or an interrupt routine. This is to tell the optimizer not to get rid of loading the variable from memory each time we check the status.
• Brydon Gibson: In your volatile case - would the compiler also include a jump out of the loop if wifi_connected were changed? If not - how would it handle exiting the loop in the (eventual) case that the variable changes?
• ratchet freak: @BrydonGibson variables are only checked when you tell them to be checked so the compiler will not introduce a break into the middle of the while body that you didn't put there. The while acts just like a normal while.
• Brydon Gibson: I understand. In the given example however, the while loop will run forever even if wifi_connected is changed inside. I guess the compiler wouldn't make that optimization in that scenario