Embedded: pointer pointers, linked lists, UCOS osmemcreate.

At first glance, UCOS's osmemcreate code, feels a bit strange, for example, to force the pointer to the address of the pointer to the pointer to the address? What is the pointer to the post-conversion pointer?

void  osmemcreate (os_mem       *p_mem,                   cpu_char     *p_name,                   void         *p_addr,                   os_mem_qty    n_ Blks,                   os_mem_size   blk_size,                   os_err       *p_err) {#if os_cfg_arg_chk_en > 0u    cpu_data       Align_msk; #endif    os_mem_qty     i;    Os_mem_qty     loops;    cpu_int08u    *p_blk;    void         **p_link;

...

    P_link = (void * *) p_addr;                               /* Create linked list of free memory               blocks    *  /p_blk = (cpu_int08u *) p_addr;    Loops  = n_blks-1u;    for (i = 0u; i < loops; i++) {        p_blk + =  blk_size;       *p_link = (void  *) p_blk;                            /* Save Pointer to NEXT block in current block            *        /P_link = (void * *) (void *) p_blk;                    /* Position to     NEXT                             block    *   /} *p_link             = (void *) 0;                         /* Last memory block points to NULL                       */

(1), P_link = (void * *) p_addr; The P_ADDR value (that is, the address) is assigned to P_link, but the type is different, so cast.

(2), p_blk = (cpu_int08u *) p_addr; Casts, because P_ADDR is a void* parameter, any type of pointer.

(3), loops = n_blks-1u; Number of Cycles

(4), for (i = 0u; i < loops; i++) {//Loop

(5), p_blk + = blk_size; Add an address to a block space size

(6), *p_link = (void *) p_blk; The increment of the address, assigned to *p_link, that is written to the * (P_addr + blk_size) array, because, in (1) the P_ADDR address to the P_link.

(7), P_link = (void * *) (void *) p_blk; P_BLK, is the cpu_int08u * type pointer, so first cast to (void *) any type, and then, with (void * *) cast, with 1;

(8),}//Cycle end
(9), *p_link = (void *) 0; Null is assigned at the end of the list.

Here, **p_link meaningless, because *p_link equals * (p_addr + loops * blk_size), that is, the value of the array, what is the **p_link (* value)? If this value is exactly 32-bit and the requested space, such as 0x12345678, then **p_link, maybe it will make sense?

Also, since the **p_link, have not been used to, whether the use of one-level hands on the line? No need to use level two pointers?

So I did some tests with Xcode, as follows:

1. Traditional textbook usage, C-B->a

2, imitate the method of Ucos, b[0]= (address of a), b[1]= (address +3 of a).

2-1, found that there is a problem, adjusted to find, any type of pointer void *, self-increment, only increase by 1?

2-2, try the next address, add a pointer size, you can. Here, sizeof (void *) = sizeof (long int *) = 8. Tested with Xcode from the Mac.

3, the same imitation ucos, different from 2, pointer types are used long int *, without void *.

3-1, and 2 instead, the self-increment with the pointer is right.

3-2, and 2 instead, adding a pointer size is wrong.

4, later, think, since Ucos, with pointers, but all useless to **p_link, only use the first level of pointers, then I can p_link all parameters are added *, into the use of **p_link, and *p_link?

The result was wrong at first. Compile no problem, syntax is not wrong, however, P_p_b does not point to any address, cannot assign a value to *p_p_b.

5, the modification of 4, and ucos the same as the use of pointers, **p_p_b equivalent to **p_link. Use **p_p_b to modify the outer array, use P_p_b to point to the address of the continuously increasing outer array, and do not need to repeat p_p_b = &p_b (UCOS writing needs, see (7), because, UCOS, equivalent to use a first-level pointer, address change, need to re-assign address, and, Here the level two pointer points to a first-level pointer, a level change, and a level two track.

6, then fine think, since ucos only use the effect of a first-level pointer. Then whether I can use a level 1 pointer directly. (Here is wrong, the original effect should be p_b_1++, similar to ucos, need for constant p_b_2 = p_b_1)

At this point, the end.

Embedded: pointer pointers, linked lists, UCOS osmemcreate.