以下格式说明符在做什么? [英] What is the following format specifier doing?

问题描述

         else
        {
            printf("\\%.3hho", *data);
        }

我无法在网上或通过阅读 C 编程语言书籍找到有关如何破译它的信息.我在以下代码片段中看到了它.代码试图在 telnet 协议中执行密码嗅探.

I could not find information regarding on how to decipher it on the net or by reading the C programming language book. I saw it in the following code snippet. The code is trying to perform password sniffing in the telnet protocol.

if ((pktlen - sizeof(struct ipheader)) >  dataoffset)
{
    printf("   SrcIP: %s,", inet_ntoa(ip->iph_sourceip));
    printf(" DstIP: %s,", inet_ntoa(ip->iph_destip));
    printf(" Data: ");
    u_char* data = (u_char*)tcp + dataoffset;

    for (unsigned short s = 0; s < (ntohs(ip->iph_len) - (sizeof(struct ipheader) + dataoffset)); s++)
    {
        if (isprint(*data) != 0)
        {
          printf("%c", *data);
                                  }
        else
        {
            printf("\\%.3hho", *data);
        }
        data++;
    }
    printf("\n");
  }

}

推荐答案

根据 CppReference:

转换说明符o

将无符号整数转换成八进制表示oooo,

Precision 指定出现的最小位数.<截断>

Precision specifies the minimum number of digits to appear. <truncated>

所以字符串 \\%.3hho 表示文字反斜杠(它被转义)，加上格式说明符 %.3hho:

So the string \\%.3hho represents a literal backslash (it's escaped), plus the format specifier %.3hho:

• %...o 表示无符号整数的八进制表示.字母 o 也表示此说明符的结尾
• hh 是 C99 规范，表示 unsigned char 而不是 unsigned int，始终为单字节
• .3 表示精度为 3，所以至少 3 个八进制数字 (000 - 377)

• %...o indicates octal representation for unsigned integer. The letter o also indicates the end of this specifier
• hh is a C99 specification, indicates unsigned char instead of unsigned int, always a single byte
• .3 means the precision is 3, so at least 3 octal digits (000 - 377)

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