shift count is too large

STM8S是8 bit單片機
在STM8S中 unsigned long是32位， unsigned short和unsigned int都是16位，unsigned char是8位。

以以下代碼編譯時報了“shift count is too large”的warning， 並且代碼執行時也沒有達到預期效果：

unsigned char temp[4];
unsigned long pd_reg;
...
pd_reg = temp[3] << 24 | temp[2] << 16 | temp[1] << 8 | temp[0];

后面加上了強制類型轉換解決了此問題：

unsigned char temp[4];
unsigned long pd_reg;
...
pd_reg = (unsigned long)temp[3] << 24 | (unsigned long)temp[2] << 16 | (unsigned long)temp[1] << 8 | (unsigned long)temp[0];