Shareef Jalloq

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5 years, 4 months ago.

What's wrong with my code? Only works with an extra printf in main().

So I'm a bit of a noob when it comes to C/C++ to go easy on me. However, I've been playing around with some of the examples and am trying to get the following simple NTP example working. I'm using mbed-cli 1.8.3 with a nucleo_f767zi. I'm using the USER_BUTTON on the board to toggle an LED via InterruptIn and also to set a flag that should request and print out the current time.

However, it only works if I add some dummy printf to the main while loop outside of the flag test. What is going on?

#include "mbed.h"
#include "stats_report.h"
#include "ntp-client/NTPClient.h"
#include "EthernetInterface.h"

DigitalOut led1(LED1);
InterruptIn btn(USER_BUTTON);
Serial usb(USBTX, USBRX);
EthernetInterface eth;

int flag = 0;

void buttonIsr() {
    flag = 1;
    led1 = !led1;
}

// main() runs in its own thread in the OS
int main()
{
    eth.connect();
    usb.printf("ip address = %s\n\r", eth.get_ip_address());
    
    btn.rise(&buttonIsr);

    NTPClient ntp(&eth);

    time_t timestamp = ntp.get_timestamp();
    usb.printf("Current time is %s\n\r", ctime(&timestamp));    
    
    while (1) {
        if (flag == 1) {
            usb.printf("button pushed\n\r");

            timestamp = ntp.get_timestamp();

            while(timestamp < 0) {
                wait(1);
                timestamp = ntp.get_timestamp();
            }

            usb.printf("Current time is %s\n\r", ctime(&timestamp));
            flag = 0;
        }
    }
}

1 Answer

Zoltan Hudak
5 years, 4 months ago.

Hello, Shareef

Since the compiler does not know anything about interrupts and the interrupt service routine isn't called directly at any point in your code, it concludes that it's a good candidate for code optimization/reduction. You can prevent such optimization by declaring the flag variable as volatile. 

volatile int flag = 0;

It will mean for the compiler that such variable can change its value any time during the program execution and it won't optimize the related code.
NOTE: When you tried to print the flag variable in the main function the compiler was prevented to optimize the flag related code as well that's why it worked as expected.

Accepted Answer