c语言实现php的trim标签

/*
 * 清除字符串开始结尾处的空白
 * 修改原字符串
 * */
static char* trim(char*str){
  char* p1 = str,*p2 = NULL,*p3 = NULL;
  int flag = 0;
  while(*(p1) != '\0'){
    if(isspace(*p1)){
      if(p3 == NULL) p3 = p2 == NULL?p1:p2;//标记结尾空白
      if(0 == flag) {//标记开头为空白
        flag = 1;
        p2 = p1;
      }else if(2 == flag){//中间空白
        *(p2++) = *p1;
      }
    }else {
      if(1 == flag || flag == 2){//开头空白结束标志
        *(p2++) = *p1;
        flag = 2;
      }else flag = -1;//开头无空白
      p3 = NULL;//清除结尾空白
    }
    p1++;
  }
  //标记结尾
  if(p3 != NULL) *p3 = '\0';
  else if(p2 != NULL) *p2 = '\0';
  return str;
}