C：对“ clock（）”的未定义引用

Question

这个问题已经在这里有了答案：

• 什么是未定义的引用/未解决的外部符号错误，如何解决？ 32个答案

我不太熟悉C，所以请多多包涵...

我正在用AtmelStudio开发的Atmel Tiny 85上为看门狗定时器编写一些简单的代码。 我想访问time.h库的clock（）函数：

#include <time.h>

.....

void main() {
  clock_t start = clock();
  ....
}

不幸的是，编译器抱怨时钟是未定义的引用。 我在网上查找的每个代码示例似乎都在执行我正在执行的操作。 我缺少一些基本的东西吗？ 谢谢！

这是完整的代码：

#define F_CPU 1000000UL // 1 MHz
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#include <time.h>
#include <stdio.h>
#include <avr/wdt.h>
#include <stdint.h>
#include <util/atomic.h>

#define LED_PORT PB1
#define RST0_PORT PINB3
#define RST1_PORT PINB4
#define HEARTBEAT PINB0

volatile int HEARTBEAT_RECEIVED;
#define CLOCKS_PER_SECOND 1000000;
const int TIMEOUT_PERIOD = 120; //reset raspberry pi after this many seconds

void set_output();
void set_input();
void set_interrupts();
void run_timer(clock_t start);
void restart_raspberry_pi();

int main(void)
{
    clock_t start;

    HEARTBEAT_RECEIVED = false;

    //Configure IO pins
    set_output();
    set_input();
    PORTB &= ~(1 << RST0_PORT); // Set pin low (common ground pin for pi reset)
    PORTB &= ~(1 << RST1_PORT); // Set pin low (initialize for no pi reset)

    //Configure and enable interrupts
    set_interrupts();

    while (1) {
        sei();
        start = clock();
        run_timer(start);
    }

    return (0);
}


void run_timer(clock_t start) {

    double time_elapsed;

    do {
        _delay_ms(1000);

        if (HEARTBEAT_RECEIVED) { //If heartbeat detected, reset timer.
            HEARTBEAT_RECEIVED = false;
            return;
        }

        time_elapsed = ( clock() - start ) / CLOCKS_PER_SECOND;
    } while (time_elapsed < TIMEOUT_PERIOD);

    restart_raspberry_pi(); //Timeout period has elapsed, reset the pi
}

ISR(PCINT0_vect) {
    //Indicate that a heartbeat has been received
    HEARTBEAT_RECEIVED = true;
}

void restart_raspberry_pi() {
    cli();

    PORTB |= (1 << RST1_PORT); // Set pin high (sets RUN high to reset)
    _delay_ms(500);
    PORTB &= ~(1 << RST1_PORT); // Set pin low (release reset control)
}

void set_output()
{
    //The DDxn bit in the DDRx Register selects the direction of this pin.
    //If DDxn is written logic one, Pxn is configured as an output pin.
    //If DDxn is written logic zero, Pxn is configured as an input pin.

    //PORTB = (0<<PB0) | (1<<PB3);
    DDRB = (1<<DDB5) | (1<<DDB4) | (1<<DDB3) | (1<<DDB2) | (1<<DDB1); // Set pins as output.
}

void set_input()
{
    DDRB |= (0<<DDB0); // set pin 0 as input. Added |=.
}

void set_interrupts()
{
    GIMSK |= (1 << PCIE);   // pin change interrupt enable
    PCMSK |= (1 << PCINT0); // pin change interrupt enabled for PCINT0
}

Answer 1

@肖恩说的没错...。我深入到了time.h库代码，显然我可以使用：

7.23.2.1节Clock（）类型clock_t，宏CLOCKS_PER_SEC和函数clock（）未实现。 我们认为这些项目属于操作系统代码，或者当不存在操作系统时属于应用程序代码。

所以我想这对我是一个教训。 谢谢您的帮助。