C printf％d前导零是否正确？ [英] C printf %d incorrect value with leading zeros?

问题描述

C函数printf似乎根据是否存在前导零而打印不同的值。

The C function printf seems to print different values depending on whether or not leading zeros are present.

我试图确定其中的mode参数的数值


printf（ mode：％d\n，S_IRWXU）;

printf（ mode：％d\n，00700）;

I was trying to determine the numerical values for the mode argument in the Linux 'open' system call.

printf("mode:%d\n",S_IRWXU);
printf("mode:%d\n",00700);

都给了我448 ，而

both gave me 448, while

printf（ mode：％d\n，700）; 给了我700，正如我期望的那样。

printf("mode:%d\n",700); gives me 700, as I would expect from both.

这是怎么回事？

我正在使用gcc（Ubuntu 5.4.0-6ubuntu1〜16.04.5）5.4.0 20160609

I am using gcc (Ubuntu 5.4.0-6ubuntu1~16.04.5) 5.4.0 20160609

推荐答案

一个数值常量带有一个或多个前导零的是一个八进制常数，而没有一个前导零的一个是一个十进制常数，而一个带有前导的 0x 是十六进制常量。在任何情况下，无论将值传递给 printf 还是任何其他函数，都适用。

A numerical constant with one or more leading zeros is an octal constant, while one without a leading zero is a decimal constant and one with a leading 0x is a hexadecimal constant. This holds in any context, whether the value is passed to printf or any other function.

printf ，您正在使用％d 格式说明符，该说明符以十进制打印值。如果您输入八进制常量，则会看到该八进制数字的十进制值。在此示例中，0700b8 == 7 * 8 ^ 2 + 0 * 8 ^ 1 + 0 * 8 ^ 0 == 7 * 64 == 448b10

In the case of printf, you're using the %d format specifier which prints the value in decimal. If you pass in an octal constant, you'll see the decimal value of that octal number. In this example, 0700b8 == 7 * 8^2 + 0 * 8^1 + 0 * 8^0 == 7 * 64 == 448b10

在处理文件权限时，这些值通常表示为八进制，因此您应始终在这些值前使用0开头。

If you're dealing with file permissions, those values are typically denoted as octal, so you should always use a leading 0 with those.

这篇关于C printf％d前导零是否正确？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！