函数在C中分配了不正确的浮点值

Question

I am trying to return a float value and assign it to a float variable, but the value of the new float is different from the returned one.

float getVoltageReading() {
    return 1.2f;
}


void updateUIReadings(uint8_t menuID) {
    float integerReading = getVoltageReading(); // digital voltage
}

In debugger I see that getVoltageReading return 1.2 , but the integerReading is assigned to be 1.06703091e+009

Why is that?

Answer 1

You are calling the getVoltageReading function without an active prototype in scope, which means it is assuming that it will return an int . It looks like, from the way your question is organised, that it is in scope, but I can assure you it's not.

You can see that with the following two files, testprog1.c :

#include <stdio.h>

//float getVoltageReading(void);

int main(void) {
    float integerReading = getVoltageReading ();
    printf("%e\n", integerReading);
    return 0;
}

and testprog2.c :

float getVoltageReading(void) {
    return 1.2f;
}

When these are compiled and linked together, the output is:

1.067031e+09

because the float value being returned from getVoltageReading() is being interpreted as an int . If you uncomment the prototype in testprog1.c , it works fine because it interprets the float value as a float :

1.200000e+00