如何动C整数常量带前导零，并且怎么样的atoi？ [英] How does C Handle Integer Literals with Leading Zeros, and What About atoi?

问题描述

在创建带前导零的整数，如何Ç处理呢？它是不同版本的C有何不同？

When you create an integer with leading zeros, how does c handle it? Is it different for different versions of C?

在我的情况下，他们似乎只是被丢弃（但也许这就是printf的呢？）：

In my case, they just seem to be dropped (but maybe that is what printf does?):

#include <stdio.h>

int main() {
    int a = 005;
    printf("%i\n", a);
    return 0;
}

我知道我可以用printf与0垫，但我只是想知道这是如何工作的。

I know I can use printf to pad with 0s, but I am just wondering how this works.

推荐答案

前导零表明该号码在EX pressed 八或基数为8;因此，010 = 8.添加额外的前导零没有影响;正如你所期望的在数学，X + 0 * 8 ^ N = X;有通过其重新presentation不再是该值没有变化。

Leading zeros indicate that the number is expressed in octal, or base 8; thus, 010 = 8. Adding additional leading zeros has no effect; just as you would expect in math, x + 0*8^n = x; there's no change to the value by making its representation longer.

一个地方，你经常会看到这是UNIX文件模式; 0755实际上是指7 * 8 ^ 2 + 5 * 8 + 5 = 493;或使用umasks如0022 = 2 * 8 + 2 = 10。

One place you often see this is in UNIX file modes; 0755 actually means 7*8^2+5*8+5 = 493; or with umasks such as 0022 = 2*8+2 = 10.

的atoi（NPTR）被定义为等同于与strtol（NPTR，（字符**）NULL，10），但它不检测错误 - 这样，的atoi（）总是使用十进制（从而忽略前导零）。 与strtol（NPTR，任何事情，0）执行以下操作：

atoi(nptr) is defined as equivalent to strtol(nptr, (char **) NULL, 10), except that it does not detect errors - as such, atoi() always uses decimal (and thus ignores leading zeros). strtol(nptr, anything, 0) does the following:

的字符串可以被任意的开始
  白色空间的数量（如确定
  通过 isspace为（3）），其次是单
  可选的+或 - 标志。如果基地
  零或16，字符串可能再
  包括0X preFIX，而且数量
  将在基地16读取;否则，一个
  零基础取为10（十进制）
  除非下一个字符是 0 ，在
  这种情况下，被取为8（八进制）。

The string may begin with an arbitrary amount of white space (as determined by isspace(3)) followed by a single optional '+' or '-' sign. If base is zero or 16, the string may then include a "0x" prefix, and the number will be read in base 16; otherwise, a zero base is taken as 10 (decimal) unless the next character is '0', in which case it is taken as 8 (octal).

所以它使用相同的规则与C编译器。

So it uses the same rules as the C compiler.

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