why pointer doesn't change in struct?

struct tagEXAM
{
 int x;
 void (**pf)(void);
};
void fun1(void);
void fun2(void);
void (*code pf[])(void)=
{
 fun1,fun2
};
struct tagEXAM code exam[] =
{
 {1, pf}
};
int x;
void (**pf1)(void);
struct tagEXAM* pexam;
void main(void)
{
 while (1)
 {
  pexam = exam;
  pf1 = pexam->pf;

  (*pexam->pf)();
  (*pf1)();

  pexam->pf++;
  pf1++;
  (*pexam->pf)();
  (*pf1)();
 }
}
void fun1(void)
{
 x = 1;
}
void fun2(void)
{
 x = 2;
}

While the choice of a '51 target is questionable when using function pointers, there doesn't appear to be anything particularly wrong with your code.

What do you mean by "can't change"?

You are trying to increment a pointer that is located in ROM. That's why it won't increment.

You are trying to increment a pointer that is located in ROM.

Well spotted.

struct tagEXAM code exam[] =
{
 {1, pf}
};

Now I make a little sense, because the struct tagEXAM* pexam is only a pointer for definition of struct tagEXAM, pexam->pf is not a pointer, it's a reference for function pointer fp[], it's located in ROM, so you can't increment it!. Am I right? confuse me, Can someone make a little clear? thx.

According to the declarations, "exam" is a variable that exists in code memory.

struct tagEXAM code exam[] =
    {
    {1, pf}
    };

The 8051 cannot write to code memory. (That's a feature.) Therefore, no bits in the variable "exam" can be changed. The type of exam is irrelevant to this point. "pexam" is a pointer to a "struct tagEXAM". This is not a variable in and of itself. It's a pointer, so it just holds the location of some other actual variable.

struct tagEXAM* pexam;

"pexam" is initialized to point to "exam".

    pexam = exam;

The code then tries to change the "pf" field of the thing to which pexam points:

    pexam->pf++;

The thing to which pexam points is "exam". As we saw at first, "exam" exists in code memory, and so none of it can be changed. pexam->pf is the same memory as exam.pf. It can't be changed. Incrementing would change pf, so you can't increment exam.pf or pexam->pf.

Thanks Davis, it's good comment.