扫描数据多于预定义值的数字 [英] Scanning a number having more data than predefined value

问题描述

#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的情况下跟随?

Why is circulation not following while scanning input via scanf(), why is it following in case of char?

推荐答案

来自 this <"%u" 格式的 code>scanf(和系列)参考:

From this scanf (and family) reference for the "%u" format:

数字的格式与 strtoul() 预期的相同，值为 0 为 base 参数(base 由解析的第一个字符决定)

The format of the number is the same as expected by strtoul() with the value ​0​ for the base argument (base is determined by the first characters parsed)

然后我们去strtoul 函数，并读取返回值:

Then we go to the strtoul function, and read about the returned value:

成功时对应 str 内容的整数值.如果转换后的值超出对应返回类型的范围，则发生范围错误，返回ULONG_MAX或ULLONG_MAX.如果无法进行转换，则返回 0 .

Integer value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs and ULONG_MAX or ULLONG_MAX is returned. If no conversion can be performed, ​0​ is returned.

由此我们可以看出，如果您为scanf "%u" 格式输入过大的值，那么结果将是ULONG_MAX 转换为 unsigned int.但是在sizeof(unsigned long) >的系统上，结果会有所不同.大小(无符号整数).有关这方面的信息，请参见下文.

From this we can see that if you enter a too large value for the scanf "%u" format, then the result will be ULONG_MAX converted to unsigned int. However the result will differ on systems where sizeof(unsigned long) > sizeof(unsigned int). See below for information about that.

需要注意的是，在具有 64 位 unsigned long 和 32 位 unsigned int 的平台上，该值在 unsigned 范围内有效long 不会被转换为 egUINT_MAX，而是使用模算术转换 as详情请看这里.

It should be noted that on platforms with 64-bit unsigned long and 32-bit unsigned int, a value that is valid in the range of unsigned long will not be converted to e.g. UINT_MAX, instead it will be converted using modulo arithmetic as detailed here.

让我们取4294967299这样的值.32 位 unsigned int 太大了，但很适合 64 位 unsigned long.因此，对 strtoul 的调用不会返回 ULONG_MAX，而是返回值 4294967299.使用标准转换规则(链接到上面)，这将导致 unsigned int 值为 3.

Lets take the value like 4294967299. It is to big to fit in a 32-bit unsigned int, but fits very well in a 64-bit unsigned long. Therefore the call to strtoul will not return ULONG_MAX, but the value 4294967299. Using the standard conversion rules (linked to above), this will result in an unsigned int value of 3.

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