strtok的多线程实现
下面这段是vc库里strtok的源码，应该是实现了多线程的吧，但其中有些代码不明白，我在代码后做了标记
/***
*strtok.c - tokenize a string with given delimiters
*
* Copyright (c) 1989-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
* defines strtok() - breaks string into series of token
* via repeated calls.
*
*******************************************************************************/

#include <cruntime.h>
#include <string.h>
#ifdef _MT
#include <mtdll.h>
#endif /* _MT */

/***
*char *strtok(string, control) - tokenize string with delimiter in control
*
*Purpose:
* strtok considers the string to consist of a sequence of zero or more
* text tokens separated by spans of one or more control chars. the first
* call, with string specified, returns a pointer to the first char of the
* first token, and will write a null char into string immediately
* following the returned token. subsequent calls with zero for the first
* argument (string) will work thru the string until no tokens remain. the
* control string may be different from call to call. when no tokens remain
* in string a NULL pointer is returned. remember the control chars with a
* bit map, one bit per ascii char. the null char is always a control char.
*
*Entry:
* char *string - string to tokenize, or NULL to get next token
* char *control - string of characters to use as delimiters
*
*Exit:
* returns pointer to first token in string, or if string
* was NULL, to next token
* returns NULL when no more tokens remain.
*
*Uses:
*
*Exceptions:
*
*******************************************************************************/

char * __cdecl strtok (
  char * string,
  const char * control
  )
{
  unsigned char *str;
  const unsigned char *ctrl = control;

  unsigned char map[32];
  int count;

#ifdef _MT
  _ptiddata ptd = _getptd();*****************************作用？？？？？****************************
#else /* _MT */
  static char *nextoken;
#endif /* _MT */

  /* Clear control map */
  for (count = 0; count < 32; count++)
  map[count] = 0;

  /* Set bits in delimiter table */
  do {
  map[*ctrl >> 3] |= (1 << (*ctrl & 7));
  } while (*ctrl++);

  /* Initialize str. If string is NULL, set str to the saved
  * pointer (i.e., continue breaking tokens out of the string
  * from the last strtok call) */
  if (string)
  str = string;
  else
#ifdef _MT****************************************************************************************
  str = ptd->_token;******************************这里是怎样实现多线程的？
#else /* _MT */**********************************************************************************
  str = nextoken;
#endif /* _MT */

  /* Find beginning of token (skip over leading delimiters). Note that
  * there is no token iff this loop sets str to point to the terminal
  * null (*str == '\0') */
  while ( (map[*str >> 3] & (1 << (*str & 7))) && *str )
  str++;

  string = str;

  /* Find the end of the token. If it is not the end of the string,