【C语言】错题本(2)

题目:

将题目代码粘贴在下面便于分析:

#define MAX_SIZE A+B
struct _Record_Struct
{
  unsigned char Env_Alarm_ID : 4;
  unsigned char Para1 : 2;
  unsigned char state;
  unsigned char avail : 1;

}*Env_Alarm_Record;

struct _Record_Struct *pointer = (struct _Record_Struct*)malloc(sizeof(struct _Record_Struct) * MAX_SIZE);

错误选型:B

正确选择:D

题目分析:

也即最后malloc(     ),括号中的表达式为(3*2+3),等于9个字节.

注意!宏在代码中替换表达式时就是原样替换,不会因为宏定义的整体就替换成带括号的表达式,就像本题,宏替换后的表达式为:

#define MAX_SIZE A+B

malloc(sizeof(struct _Record_Struct) * MAX_SIZE);

//宏替换后:

malloc(sizeof(struct _Record_Struct) * 2 + 3 );

//而不是:

malloc(sizeof(struct _Record_Struct) * (2 + 3) );