c - 扫描数据多于预定义值的数字

Question

#include<stdio.h>
main()
{

        unsigned int num;
        printf("enter the number:\n");
        scanf("%u",&num);//4294967299 if i'm scanning more than 4G its not scanning
        printf("after scanning num=%u\n",num);// 4294967295 why its giving same 4G

        /*      unsigned char ch;
                printf("enter the character:\n");
                scanf("%d",&ch);// if i/p=257 so its follow circulation
                printf("after scanning ch=%d\n",ch);// 1 its okk why not in int ..
         */
}

为什么通过scanf()扫描输入时循环不跟随，为什么在char的情况下循环？

Answer 1

C11 标准草案 n1570 7.21.6.2 说如下

paragraph 10

[...] the input item [...] is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. 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.

现在，这里的“转换”一词用于字符串=>结果数据类型转换，不能理解为整数转换。由于转换为十进制整数的字符串 "4294967299" 无法在 32 位宽的 unsigned int 类型的对象中表示，因此标准的解读表明行为未定义，即

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

因此，您的问题的答案是 C 标准没有说明这种情况下的行为，您看到的行为是您的编译器和 C 库实现所展示的行为，并且不可移植；在其他平台上，可能的行为可能包括:

ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).