将数据写入特定的内存地址问题

Question

When I use the following code to write data at specific address at memory i get an error told me that

" type error in argument 1" of function write.

as i understand S_LEN is a pointer to 0x1814 so it should work in correct way but at this case i get the previous error.

when i replace the command write(S_LEN,0); via write(&S_LEN,0); it work correct. although at first case i pass hex number casting to pointer to integer.

can any one explain this matter?

#define RAM_START 0x1800
#define S_LEN  (((int32_t *)(RAM_START))[0xFF])
    int32_t write(int32_t *dest_ptr, int32_t src)
    {                                     
        *dest_ptr = src;
         return 0;
    } 

    main()
    {
     write(S_LEN,0);
    }

Answer 1

Your write function expects int32_t * as first argument, which is a pointer , so you should use something like this instead, if you want S_LEN to be a pointer:

#define S_LEN  (((int32_t *)(RAM_START))+0xFF)

Your expression evaluates to an int32_t since you apply the subscript operator [] on the int32_t * (which is a pointer).

On the other hand &S_LEN is accepted, beause it yields the address of the int32_t you get by applying [] .

Also note that S_LEN relates to the memory location 4*0xFF+0x1800 which is 0x1BFC . (It is 255 int32_t s after 0x1800 ).

Answer 2

In your code, S_LEN does the following:

1. it convert RAM_START to a pointer of type int32_t *
2. subscription of a pointer of type int32_t * should be a int32_t number

But write need first argument to be of type int32_t * , so &S_LEN is right, cause the address of an int32_t returned by S_LEN is of type int32_t *

The output from OSX 10.11:

[oxnz@rmbp:tmp:0]$ cc     a.c   -o a
a.c:14:8: warning: incompatible integer to pointer conversion passing 'int32_t'
      (aka 'int') to parameter of type 'int32_t *' (aka 'int *'); take the
      address with & [-Wint-conversion]
        write(S_LEN,0);
              ^~~~~
              &(   )
a.c:5:16: note: expanded from macro 'S_LEN'
#define S_LEN  (((int32_t *)(RAM_START))[0xFF])
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.c:6:24: note: passing argument to parameter 'dest_ptr' here
int32_t write(int32_t *dest_ptr, int32_t src)
                       ^
1 warning generated.

[oxnz@rmbp:tmp:0]$ cat a.c
#include <stdio.h>
#include <stdlib.h>

#define RAM_START 0x1800
#define S_LEN  (((int32_t *)(RAM_START))[0xFF])
int32_t write(int32_t *dest_ptr, int32_t src)
{
    *dest_ptr = src;
    return 0;
}

int main()
{
    write(S_LEN,0);
}