c - avr内联汇编错误: impossible constraint

Question

#include <avr/io.h>

int main(void){

    asm volatile("ldi r16, %0\n\t"
                 "out %1, r16\n\t"
                 "ldi r16, %0\n\t"
                 "out %2, r16\n\t"::"M" (0xff),"I" (_SFR_IO_ADDR(DDRB)),"I" (_SFR_IO_ADDR(PORTB)));

    while(1) {
        asm volatile("cbi %0, %1\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "nop\n\t"
                     "sbi %0, %1\n\t"::"I" (SFR_IO_ADDR(PORTB)), "M" (0xff));
    }
}

编译时:错误:不可能的约束 指针设置到asm语句位置:11-9 (asm volatile("cbi %0, %1\n\t")

但是为什么呢？

Answer 1

您缺少 SFR_IO_ADDR(PORTB) 上的前导下划线，因此它被编译为对返回整数的外部函数的调用。如果您启用了警告，您会看到以下内容:警告:函数“SFR_IO_ADDR”的隐式声明