从特定地址初始化多维数组指针的正确方法

Question

I want to create a multidimensional array pointer in C with a particular starting address in memory. I don't fully understand the correct syntax for this. What i want to accomplish to do is something like follows:

unsigned char (*vptr)[240][320] =  (unsigned char*)0x40000000;

where
would write 2 to address ;
would write 3 to address ;
...
would write 4 to address ;


So basically that my would be functionally equivalent to

unsigned char vptr[240][320] = {....};

where .

Could someone please write how this is supposed to be done? Thank you very much for the help.

Answer 1

You could cast the address as (void *)0x40000000 as that can be converted to any object type. Some compilers might issue a warning unless you cast the address as (void *)(uintptr_t)0x40000000 .

With unsigned char (*vptr)[240][320] , you would need to access the individual unsigned char elements using (*vptr)[i][j] . Therefore, to make things easier, it would be better to declare vptr as unsigned char (*vptr)[320] so that vptr is a pointer to the first element of an array of char[320] . Then you can access the individual unsigned char elements using the more natural syntax vptr[i][j] . For example vptr[10][20] would access the unsigned char element at address 0x40000000 + (10 * 320) + 20 ( 0x40000c94 ), or in general, vptr[i][j] would access the unsigned char element at address 0x40000000 + (i * 320) + j .

If you don't want to cast the address to (void *) , you can cast to the correct pointer type instead. Eg unsigned char (*vptr)[240][320] = (unsigned char (*)[240][320])0x40000000; or unsigned char (*vptr)[320] = (unsigned char (*)[320])0x40000000; . (You might still need the intermediate (uintptr_t) cast to avoid a compiler warning.)

You mentioned that &vptr == 0x40000000 (disregarding casting), but that is not the case. vptr is just a pointer variable at whatever address the compiler and linker decide to place it. It is the vptr variable itself (not its address) that has the value 0x40000000 (disregarding casting).