/*  Base64是一种基于64个可打印字符来表示二进制数据的表示方法。三个字节有24个比特，对应于4个Base64单元，即3个字节需要用4个可打印字符来表示。  当最后剩余一个八位字节(一个byte)时，最后一个6位的base64字节块有四位是0值，最后附加上两个等号；  如果最后剩余两个八位字节(2个byte)时，最后一个6位的base字节块有两位是0值，最后附加一个等号。  */  #include <stdio.h>  #include "string.h"    #include "stdlib.h"       const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";  static char find_pos(char ch);  char *base64_encode(const char* data, int data_len,int *len);  char *base64_decode(const char* data, int data_len,int *len);         /**   *找到ch在base中的位置   */  static char find_pos(char ch)     {        //the last position (the only) in base[]      char *ptr = (char*)strrchr(base, ch);       return (ptr - base);    }          /**   *BASE64编码   */  char *base64_encode(const char* data, int data_len,int *len)    {           int prepare = 0;        int ret_len;        *len=0;      int temp = 0;        char *ret = NULL;        char *f = NULL;        int tmp = 0;        char changed[4];        int i = 0;        ret_len = data_len / 3;        temp = data_len % 3;        if (temp > 0)        {            ret_len += 1;        }        //最后一位以''结束       ret_len = ret_len*4 + 1;        ret = (char *)malloc(ret_len);                    if ( ret == NULL)        {            printf("No enough memory.n");            exit(0);        }        memset(ret, 0, ret_len);        f = ret;      //tmp记录data中移动位置        while (tmp < data_len)        {            temp = 0;            prepare = 0;            memset(changed, 0, 4);            while (temp < 3)            {                   if (tmp >= data_len)                {                    break;                }               //将data前8*3位移入prepare的低24位               prepare = ((prepare << 8) | (data[tmp] & 0xFF));                tmp++;                temp++;            }            //将有效数据移到以prepare的第24位起始位置          prepare = (prepare<<((3-temp)*8));                         for (i = 0; i < 4 ;i++ )            {                //最后一位或两位              if (temp < i)                {                    changed[i] = 0x40;                }                else              {                    //24位数据                  changed[i] = (prepare>>((3-i)*6)) & 0x3F;                }                *f = base[changed[i]];                        f++;              (*len)++;           }        }        *f = '';                    return ret;                }       /**   *BASE64解码   */  char *base64_decode(const char *data, int data_len,int *len)    {        int ret_len = (data_len / 4) * 3+1;        int equal_count = 0;        char *ret = NULL;        char *f = NULL;        *len=0;      int tmp = 0;        int temp = 0;        char need[3];        int prepare = 0;        int i = 0;        if (*(data + data_len - 1) == '=')        {            equal_count += 1;        }        if (*(data + data_len - 2) == '=')        {            equal_count += 1;        }                 ret = (char *)malloc(ret_len);        if (ret == NULL)        {            printf("No enough memory.n");            exit(0);        }        memset(ret, 0, ret_len);        f = ret;        while (tmp < (data_len - equal_count))        {            temp = 0;            prepare = 0;            memset(need, 0, 4);            while (temp < 4)            {                if (tmp >= (data_len - equal_count))                {                    break;                }                prepare = (prepare << 6) | (find_pos(data[tmp]));                temp++;                tmp++;            }            prepare = prepare << ((4-temp) * 6);            for (i=0; i<3 ;i++ )            {                if (i == temp)                {                    break;                }                *f = (char)((prepare>>((2-i)*8)) & 0xFF);                f++;              (*len)++;            }        }      *f = '';      if(data[data_len-1]=='=')      {          (*len)--;      }      /*      while(*(--f)=='')      {        (*len)--;                 }      */      return ret;    }  int main(){      char *former = "hello";      int len1,len2;      printf("%sn",former);      char *after = base64_encode(former, 5,&len1);      printf("%d %sn",len1,after);      former = base64_decode(after, len1,&len2);      printf("%d %sn",len2,former);  }