UU编码简介

1、简介

      uu 编码 将3字节BIN数据转换成4字节可打印ASCII字符。

       uuencode 编码方式用于将任意的二进制文件转换为文本文件,比如email.转换后的文件中仅包含可打印字符.
      uuencode 运算法则将连续的 3字节编码转换成 4字节(8-bit 到 6-bit)的可打印字符.
 

2、示例
      从二进制文件中读取 3字节的数据, 表示如下(a7 表示 a字节的第 7位):
              a7a6a5a4a3a2a1a0   b7b6b5b4b3b2b1b0   c7c6c5c4c3c2c1c0
      转换它们到4字节里为如下所示:
              0 0 a7a6a5a4a3a2   0 0 a1a0b7b6b5b4   0 0 b3b2b1b0c7c6   0 0 c5c4c3c2c1c0
      然后, 每个字节再加 0x20转换为可打印的字符.
      注意: 如果是一个 0字节那它应该被转换为0x60而不是0x20, 因为(前引用'`')优于 0x20(空格' ').
 
      例如: 从文件中读取的 3字节如下:
              14                   0F                    A8
        00010100      00001111       10101000
      转换为 6-bit:
        000101      000000       111110       101000
      每字节高两位补 0后为:
        00000101    00000000     00111110      00101000
      最后每字节再加 0x20,则 4字节输出应该为:
        25   60   5E   48
      注意: 00字节被转换为 0x60而不是 0x20.
 
      因此, 在一个 uuencoded文件中仅包含字符 0x21 '!'到 0x60 '`',它们都是可打印和可被 email传送的.
      这个转换过程也意味着 uuencoded 文件要比原文件大 33%的.
 
3、 编码与解码

3.1编码
      outbuf  [4] 输出 uu编码数据.
      inbytep [3] 输入二进制数据.
      #define   ENCODE_BYTE(b)   (((b) == 0) ? 0x60 : ((b) + 0x20))
          outbuf [0] = ENCODE_BYTE  ((inbytep [0] & 0xFC) >> 2);
          outbuf [1] = ENCODE_BYTE (((inbytep [0] & 0x03) << 4) + ((inbytep [1] & 0xF0) >> 4));
          outbuf [2] = ENCODE_BYTE (((inbytep [1] & 0x0F) << 2) + ((inbytep [2] & 0xC0) >> 6));
          outbuf [3] = ENCODE_BYTE   (inbytep [2] & 0x3F);
 
3.2 解码 
      linep [4]   输入 uu编码数据.
      outbyte [3] 输出二进制数据.
      #define    DECODE_BYTE(b)    ((b == 0x60) ? 0 : b - 0x20)
            outbyte [0] = DECODE_BYTE (linep [0]);
            outbyte [1] = DECODE_BYTE (linep [1]);
            outbyte [0] <<= 2;
            outbyte [0] |= (outbyte [1] >> 4) & 0x03;
            outbyte [1] <<= 4;
            outbyte [2] = DECODE_BYTE (linep [2]);
            outbyte [1] |= (outbyte [2] >> 2) & 0x0F;
            outbyte [2] <<= 6;
            outbyte [2] |= DECODE_BYTE (linep [3]) & 0x3F;

4、源码

4.1编码

#include "stdio.h"
#include "stdlib.h"

#define MAX_LINELEN 45
#define ENCODE_BYTE(b) (((b) == 0) ? 0x60 : ((b) + 0x20))
typedef unsigned char BYTE;

int main (int argc, char *argv [])
{
  FILE *infile = NULL;
  FILE *outfile = NULL;
  int linelen;
  int linecnt;
  BYTE inbuf [MAX_LINELEN];
  BYTE *inbytep;
  char outbuf [5];

  if (argc != 3)
  {
    fprintf (stderr, "Syntax: uuencode <infile> <outfile>\n");
    exit (1);
  }

  /* Try and open the input file (binary) */
  infile = fopen (argv[1], "rb");
  if (infile == NULL)
  {
    fprintf (stderr, "uuencode: Couldn't open input file %s\n", argv[1]);
    exit (1);
  }

  /* Try and open the output file (text) */
  outfile = fopen (argv[2], "wt");
  if (outfile == NULL)
  {
    fprintf (stderr, "uuencode: Couldn't open output file %s\n", argv[2]);
    exit (1);
  }

  /* Write the 'begin' line, giving it a mode of 0600 */
  fprintf (outfile, "begin 600 %s\n", argv[1]);

  do
  {
    /* Read a line from input file */
    linelen = fread (inbuf, 1, MAX_LINELEN, infile);
    /* Write the line length byte */
    fputc (ENCODE_BYTE (linelen), outfile);

    /* Encode the line */
    for (linecnt = linelen, inbytep = inbuf;
         linecnt > 0;
         linecnt -= 3, inbytep += 3)
    {

      /* Encode 3 bytes from the input buffer */
      outbuf [0] = ENCODE_BYTE ((inbytep [0] & 0xFC) >> 2);
      outbuf [1] = ENCODE_BYTE (((inbytep [0] & 0x03) << 4) + ((inbytep [1] & 0xF0) >> 4));
      outbuf [2] = ENCODE_BYTE (((inbytep [1] & 0x0F) << 2) + ((inbytep [2] & 0xC0) >> 6));
      outbuf [3] = ENCODE_BYTE (inbytep [2] & 0x3F);
      outbuf [4] = '\0';

      /* Write the 4 encoded bytes to the file */
      fprintf (outfile, "%s", outbuf);
    }
    fprintf (outfile, "\n");
  } while (linelen != 0);

  /* Write the 'end' marker */
  fprintf (outfile, "end\n");

  /* Tidy up and return */
  fclose (infile);
  fclose (outfile);

  return 0;
}

4.2 解码

/*
* uudecode.c -
*  Simple uudecode utility
*  Jim Cameron, 1997
*/
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* We all hate magic numbers! */
#define LINE_BUF_SIZE 256

/* Decode a byte */
#define DECODE_BYTE(b) ((b == 0x60) ? 0 : b - 0x20)
typedef unsigned char BYTE;

int main (int argc, char *argv[])
{
    FILE *infile = NULL;
    FILE *outfile = NULL;
    char linebuf [LINE_BUF_SIZE];
    char *linep = NULL;
    char *tempcp = NULL;                           
    int   linelen = 0;
    int   linecnt = 0;
    char outfname [LINE_BUF_SIZE];
    BYTE outbyte [3];

    /* Check that we have the right number of arguments */
    if (argc != 2)
    {
        fprintf (stderr, "Syntax: uudecode <filename>\n");
        exit (1);
    }

    /* Open the input file */
    infile = fopen (argv[1], "rt");
    if (infile == NULL)
    {
        fprintf (stderr, "uudecode: Couldn't open file %s\n", argv[1]);
        exit (1);
    }

    /* uu-encoded files always have a 'begin' marker, so go and look for this */
    for (;;)
    {
        /* Read a line */
        if (fgets (linebuf, LINE_BUF_SIZE, infile) == NULL)
        {
            fprintf (stderr, "uudecode: Not a valid uu-encoded file\n");
            exit (1);
        }

        /* See if it is the 'begin' marker */
        if ((strncmp (linebuf, "begin", 5) == 0) && isspace (linebuf[5]))
        {
            break;
        }
    }

    /* If we have reached this point, we have found a begin marker */
    linep = linebuf + 5;

    /* Next comes the mode, which we ignore */
    while (isspace (*linep))
    {
        linep++;
    }
    if (*linep == '\0')
    {
        fprintf (stderr, "uudecode: Not a valid uu-encoded file\n");
        exit (1);
    }

    while (isdigit (*linep))
    {
        linep++;
    }
    while (isspace (*linep))
    {
        linep++;
    }
    if (*linep == '\0')
    {
        fprintf (stderr, "uudecode: Not a valid uu-encoded file/n");
        exit (1);
    }

    /* The rest of the begin line is the output file name */
    strcpy (outfname, linep);
    tempcp = outfname;
    while (!isspace (*tempcp) && (*tempcp != '\0'))
    {
        tempcp++;
    }
    *tempcp = '\0';


    /* Now open the output file */
    outfile = fopen (outfname, "wb");
    if (outfile == NULL)
    {
        fprintf (stderr, "uudecode: Couldn't open output file %s\n", outfname);
        exit (1);
    }

    /* Now for the uu-decode proper */
    do
    {
        if (fgets (linebuf, LINE_BUF_SIZE, infile) == NULL)
        {
            fprintf (stderr, "uudecode: Read error\n");
            exit (1);
        }


        /* The first byte of the line represents the length of the DECODED line */
        linelen = DECODE_BYTE (linebuf[0]);
        linep = linebuf + 1;
        for (linecnt = linelen; linecnt > 0; linecnt -= 3, linep += 4)
        {
            /* Check for premature end-of-line */
            if ((linep[0] == '\0') || (linep[1] == '\0') ||
                (linep[2] == '\0') || (linep[3] == '\0'))
            {
                fprintf (stderr, "uudecode: Error in encoded block\n");
                exit (1);
            }


            /* Decode the 4-byte block */
            outbyte[0] = DECODE_BYTE (linep[0]);
            outbyte[1] = DECODE_BYTE (linep[1]);
            outbyte[0] <<= 2;
            outbyte[0] |= (outbyte[1] >> 4) & 0x03;
            outbyte[1] <<= 4;
            outbyte[2] = DECODE_BYTE (linep[2]);
            outbyte[1] |= (outbyte[2] >> 2) & 0x0F;
            outbyte[2] <<= 6;
            outbyte[2] |= DECODE_BYTE (linep[3]) & 0x3F;

            /* Write the decoded bytes to the output file */
            if (linecnt > 3)
            {
                if (fwrite (outbyte, 1, 3, outfile) != 3)
                {
                    fprintf (stderr, "uudecode: Error writing to output file\n");
                    exit (1);
                }
            }
            else
            {
                if (fwrite (outbyte, 1, linecnt, outfile) != linecnt)
                {
                    fprintf (stderr, "uudecode: Error writing to output file\n");
                    exit (1);
                }
                linecnt = 3;
            }
        }

    } while (linelen != 0);

    /* All is ok, tidy up and exit */
    fclose (infile);
    fclose (outfile);

    return 0;
}