In below code,I think pointer pexam->pf and pf1 will be same, but why pexam->pf++ can't change? What mistake i make? Can someone will help me. thx!
/////////////////////////// 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; }
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.