Variable of type double not working when read by scanf()

问题

#include <stdio.h>

int main(){
    double d;

    scanf("%f", &d);
    printf("%f\n\n",  d);

    system("pause");
    return 0;
}

This is what I get: error.

This code is meant to read the variable double and display it on the screen but it does only display "0.0...".

If I just change the variable type to float it does exactly what I want but if I make it a double it just reads '0'. Why?

回答1:

In your code,

 scanf("%f", &d);

is wrong. For scanf(), %f expects a pointer to float as argument.

Quoting C11, chapter §7.21.6.2, fscanf()

a,e,f,g

Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the strtod function. The corresponding argument shall be a pointer to floating.

In case you want to scan a double, you have to use

 scanf("%lf", &d);

---

FWIW, passing incompatible type of argument for any format specifier invokes undefined behavior.

Quoting C11,

[...] Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

来源：https://stackoverflow.com/questions/37285056/variable-of-type-double-not-working-when-read-by-scanf