strsep和strtok_r替代strtok

char *strtok(char *str, const char *delim)

 会修改数据源。外部加锁才线程安全（strtok执行结束再解锁执行另一个strtok循环知道工作完成）

主要是以互斥访问strtok实现文件中的static外部变量char*old。源码如下。

#include <string.h>


static char *olds;

#undef strtok

/* Parse S into tokens separated by characters in DELIM.
   If S is NULL, the last string strtok() was called with is
   used.  For example:
        char s[] = "-abc-=-def";
        x = strtok(s, "-");             // x = "abc"
        x = strtok(NULL, "-=");         // x = "def"
        x = strtok(NULL, "=");          // x = NULL
                // s = "abc=-def"
*/
char *
strtok (s, delim)
     char *s;
     const char *delim;
{
  char *token;

  if (s == NULL)
    s = olds;

  /* Scan leading delimiters.  */
  s += strspn (s, delim);
  if (*s == '')
    {
      olds = s;
      return NULL;
    }

  /* Find the end of the token.  */
  token = s;
  s = strpbrk (token, delim);
  if (s == NULL)
    /* This token finishes the string.  */
    olds = __rawmemchr (token, '');
  else
    {
      /* Terminate the token and make OLDS point past it.  */
      *s = '';
      olds = s + 1;
    }
  return token;
}

char *strsep(char **stringp, const char *delim)

会修改数据源。可重入的，注意这里虽然改动stringp的内容，主要是不在使用static静态变量了。

#include <string.h>

#undef __strsep
#undef strsep

char *
__strsep (char **stringp, const char *delim)
{
  char *begin, *end;

  begin = *stringp;
  if (begin == NULL)
    return NULL;

  /* A frequent case is when the delimiter string contains only one
     character.  Here we don't need to call the expensive `strpbrk'
     function and instead work using `strchr'.  */
  if (delim[0] == '' || delim[1] == '')
  {
      char ch = delim[0];

        if (ch == '')
            end = NULL;
         else
        {
            if (*begin == ch)
                end = begin;
            else if (*begin == '')
                end = NULL;
            else
                end = strchr (begin + 1, ch);
        }
  }
  else
    /* Find the end of the token.  */
    end = strpbrk (begin, delim);

  if (end)
  {
      /* Terminate the token and set *STRINGP past NUL character.  */
      *end++ = '';
      *stringp = end;
  }
  else
    /* No more delimiters; this is the last token.  */
    *stringp = NULL;

  return begin;
}

注意事项，会多次出现空字符串的问题，这是因为strsep在处理多于一个的delimit字符是会返回空字符串代替NULL（见源码便知），http://blog.csdn.net/striver1205/article/details/25601885

对比strtok，man手册提到：

The strsep() function was introduced as a replacement for strtok(3), since the latter cannot handle empty fields.
However, strtok(3) conforms to C89/C99 and hence is more portable.

stringp必须是二级指针，不能是指向字符数组的指针！

正确用法：

char a[]=......;char *p=a;strsep(&p,delim)

错误用法：

char a[]=...;strsep(&a,delim);会包参数类型错误

错误用法：

char *a=”xxxxx”;strsep(&a,delim)//错误，注意原串会被修改内容，字符常量会seg fault

char *strtok_r(char *str, const char *delim, char **saveptr) 

会修改数据源。可重入，理由和strsep类似。