一个C语言中的 位运算有关问题 （求高手解答）

一个C语言中的 位运算问题 （求高手解答）
近期在写一个简单程序的时候，遇到了“奇怪”的现象：
VC++6.0上结果与预期相符，而GCC下看到的结果很奇怪。

示例程序如下：


#include <stdio.h>
#include <stdlib.h>

/*
How to store val to assgined bit in a data ?
1. clear the assigned position in the original value
2. get the value to fill in :
  just set the bit (from the start bit to end bit )
  of the value to store , and clear other bits 
  beyond range "from start bit to end bit "
3. "OR" operation of the result in 1,2

  For clear operation, we need a mask , other bits are 1,
  start bit to end bit are all 0.
*/ 

unsigned short store_bit_field(unsigned short original_val,
  unsigned short val_to_store,
  unsigned int start_bit,
  unsigned int end_bit)
{
  unsigned short mask = ~0;
  unsigned short clear_orig = 0;
  unsigned short val_to_fill = 0;
  // int orig_bit_num = (sizeof(original_val) * 8 );
   
  // unsigned short u_mask = 0;
  if(start_bit > orig_bit_num - 1)
  {
printf("invalid start bit pos , return 0 \n");
return 0;
}

if(end_bit > orig_bit_num - 1)
{
end_bit = orig_bit_num - 1 ;
}
   
  printf("ori_bit_num = %d ,start_bit = %d , end_bit = %d \n",
  orig_bit_num,start_bit,end_bit);
   
// printf("original_val = %x\n",original_val);  
// method 1:  
  // mask = (mask << start_bit) & (~(mask << (end_bit+1)));
   
  // if end_bit + 1 == 16, (1 << (end_bit + 1)) will be 0 (unsigned)  
  // method 2 :
  // mask = (1 << (end_bit + 1)) - ((1 << start_bit) - 1) - 1; 
   
  // method 3 :
  mask = (mask << (start_bit)) - (mask << (end_bit + 1));
  printf("mask is %x \n",mask);
   
  //u_mask = ~mask;
  // printf(" u_mask = %x \n", u_mask);
  clear_orig = (original_val & (~mask));
   
  printf(" orignal value after bits are cleared is %x \n",
  clear_orig);
   
  val_to_fill = (val_to_store << start_bit) & mask;
   
  printf(" value to store after other bits are cleared is %x \n",
  val_to_fill);
   
  return (val_to_fill | clear_orig);

}

int main()
{
unsigned short orig_val;
unsigned short val_to_set;
unsigned int start_bit;
unsigned int end_bit;

printf(" please input original value : ");
scanf("%x",&orig_val);

printf(" please input value to set : ");
scanf("%x",&val_to_set);

printf("please input start bit & end bit pos : ");
scanf("%d%d",&start_bit,&end_bit);

  // printf("orig_val = %x , val_to_set = %x\n",orig_val,val_to_set);
printf("value is set , now original changed to %x \n",
store_bit_field(orig_val,val_to_set,start_bit,end_bit)); 

  system("PAUSE");
return 0;
} 


GCC 运行结果如下：

 please input original value : ffff
 please input value to set : 123
please input start bit & end bit pos : 4 15
ori_bit_num = 16 ,start_bit = 4 , end_bit = 15
mask is fff0
 orignal value after bits are cleared is 0