第四步：调用base64_init_ex函数，初始化g_base64_context变量

main
|-----log_init，getExeAbsoluteFilename
|-----tracker_load_from_conf_file
|-----tracker_load_status_from_file
|-----base64_init_ex
|-----tracker_mem_init
|-----.....

在main函数中调用base64_init_ex的代码：

base64_init_ex(&g_base64_context, 0, '-', '_', '.')

这个函数中初始化了全局变量g_base64_context,该变量的结构如下：

struct base64_context
{
    char line_separator[16]; //行分隔符
    int line_sep_len; //行分隔符长度

    /**
     * max chars per line, excluding line_separator. A multiple of 4.
     */
    int line_length; //每行最大的字节数，不包括行分隔符

    /**
     * letter of the alphabet used to encode binary values 0..63
     */
    unsigned char valueToChar[64]; //存储A-Z,a-z,0-9的ascii值，如valueToChar[0]的值为‘A’，就是65

    /**
     * binary value encoded by a given letter of the alphabet 0..63
     */
    int charToValue[256]; //存储ascii值对应的序号，如: charToValue[65]就是charToValue[A]的值为0
    int pad_ch;
}

//初始化context变量
void base64_init_ex(struct base64_context *context, const int nLineLength, \
        const unsigned char chPlus, const unsigned char chSplash, \   //chplus='-'，chsplash='_'
        const unsigned char chPad)   //chpad='.'
{
      int i;

      memset(context, 0, sizeof(struct base64_context));  //变量context置0

      context->line_length = nLineLength;
      context->line_separator[0] = '\n';
      context->line_separator[1] = '\0';
      context->line_sep_len = 1;

      // build translate valueToChar table only once.
      // 0..25 -> 'A'..'Z'
      for (i=0; i<=25; i++)//构建valuetochar表，将数字换成对应的char
      {
         context->valueToChar[i] = (char)('A'+i);  //这里valueToChar[0]='A'
      }
      // 26..51 -> 'a'..'z'
      for (i=0; i<=25; i++ )
      {
         context->valueToChar[i+26] = (char)('a'+i);//这里valueToChar[26]='a'
      }
      // 52..61 -> '0'..'9'
      for (i=0; i<=9; i++ )
      {
         context->valueToChar[i+52] = (char)('0'+i);//这里valueToChar[52]='0'
      }
      context->valueToChar[62] = chPlus;   //valueToChar[62]=‘-’
      context->valueToChar[63] = chSplash;  //valueToChar[63]=‘_’

      memset(context->charToValue, BASE64_IGNORE, sizeof(context->charToValue));//charToValue置为-1
      for (i=0; i<64; i++ )
      {
         context->charToValue[context->valueToChar[i]] = i;//这里把charToValue['A']=0;记录字符对应的序号，也就是charToValue[65]=0
      }

      context->pad_ch = chPad;
      context->charToValue[chPad] = BASE64_PAD;  //这里charToValue['.']=-1,也就是charToValue[46]=-1
}