casting pointers

RE: casting pointers

Stanislav Silnitskiy — Wed, 16 Dec 2015 12:11:38 GMT

Awesome. although gcc doesn't want this pragma, it just works!

RE: casting pointers

monpetit — Wed, 16 Dec 2015 06:04:54 GMT

...

void test_something(void)
{
    p_addr = &addr1;
    p_mddr = (my_gap_addr_t*)&addr1;

    show(p_addr);
    show(p_mddr);
    printf("0x%04x\r\n", p_mddr->host);
    puts("---");
    
    addr1.addr[0] = 0xab;
    addr1.addr[1] = 0xcd;
    show(p_addr);
    show(p_mddr);
    printf("0x%04x\r\n", p_mddr->host);
    puts("---");
    
    const uint8_t buffer[] = { 0xfe, 0xdc, 0xba, 0x98, 0x76, 0x54 };
    memcpy((void*)addr1.addr, (const void*)buffer, sizeof(buffer));
    show(p_addr);
    show(p_mddr);
    printf("0x%04x\r\n", p_mddr->host);
    puts("---");
}

RE: casting pointers

monpetit — Wed, 16 Dec 2015 06:03:58 GMT

I have tested in another way.

#pragma anon_unions
typedef struct {
    uint8_t addr_type;                    /**< See @ref BLE_GAP_ADDR_TYPES. */
    union {
        uint8_t addr[BLE_GAP_ADDR_LEN];   /**< 48-bit address, LSB format. */
        __packed struct {
            uint16_t host;
            uint32_t net;
        };
    };
} my_gap_addr_t;


static ble_gap_addr_t addr1 = { 0x00, { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 } };
static ble_gap_addr_t *p_addr;
static my_gap_addr_t  *p_mddr;

#define show(P) \
    printf("(0x%02x, 0x%02x, 0x%02x, 0x%02x, 0x%02x, 0x%02x)\r\n", \
        P->addr[0], P->addr[1], P->addr[2], P->addr[3], P->addr[4], P->addr[5])

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 19:34:23 GMT

.. even editing SDK's internals will not help though.. SoftDevice expects the intact ble_gap_addr_t struct.

So, the only option is to copy the address from/to SDK's structs into specially aligned space:

typedef struct {
   uint16_t extra_aligner; // required to access addr[2] by uint32_t pointer
   uint8_t addr[BLE_GAP_ADDR_LEN];
} aligned_addr __attribute__((aligned(32)));

static aligned_addr addr;    
static uint32_t *const p_net = (void*)&aligned_addr.addr[2];
static uint16_t *const p_host = (void*)&aligned_addr.addr[0];

and now it is possible to assign some (see BL specs. as per PUBLIC address!) address in the form NE:TW:OR:K_:HO:ST

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 17:01:33 GMT

I tried to avoid editing SDK, but the only plausible solution I see, is to switch places of struct. members and add alignment (not sure, either it is really required):

ble_gap.h:

/**@brief Bluetooth Low Energy address. */
typedef struct
{
  uint8_t addr[BLE_GAP_ADDR_LEN];       /**< 48-bit address, LSB format. */
  uint8_t addr_type;                    /**< See @ref BLE_GAP_ADDR_TYPES. */
} ble_gap_addr_t __attribute__((aligned(16)));

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 16:28:07 GMT

it will surely work, but slightly estrange me from the goal. initial purpose of using 16 bit pointer is simplification of code. What I need, is to distinguish a subset of addresses, which may consist of more than 255 ones (a resolution, provided by one octet). So I decided to use 48 bit address, expressed by uint8_t array, in manner of IP: a "network mask" part, as higher 32 bits (conveniently accessed by uint32_t pointer) and "host" part - remaining 16 bits for . So not to bother myself with traveling across 8-bit array, proposed solution had been chosen. And it is true, that the issue is solely alignment related, so, respecting the alignment restrictions of ARM arch., it will work.

RE: casting pointers

monpetit — Mon, 14 Dec 2015 15:17:45 GMT

Oh I see. Then you can assign the pointer of uint8_t like this:

static uint8_t *p_host1;
void init_beacon(void) {
    p_host1 = &addr1.addr[0];
}

then use it like array, p_host1[0], p_host1[1]...

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 14:56:09 GMT

p_host1 holds the address of the value, I know, so when I need the value, I use *p_host1 - dereference it. I just tried to avoid copying of memory as much as possible...

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 14:51:53 GMT

definitely it is:

**Cortex-M0 Devices Generic User Guide

Home > The Cortex-M0 Instruction Set > About the instruction descriptions > Address alignment

3.3.4. Address alignment

An aligned access is an operation where a word-aligned address is used for a word, or multiple word access, or where a halfword-aligned address is used for a halfword access. Byte accesses are always aligned. There is no support for unaligned accesses on the Cortex-M0 processor. Any attempt to perform an unaligned memory access operation results in a HardFault exception.**

RE: casting pointers

monpetit — Mon, 14 Dec 2015 14:51:41 GMT

Stan, it seems to me that you have misunderstood the C pointer. Your way of assigning and casting of array elements doesn't fulfill your intension. The p_host1 of your code holds not values of addr but the address of addr. To hold values of array elements you should use memcpy.

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 14:33:22 GMT

Hi monpetit, Thank you for advice! Does it have better performance, comparing with assignment of pointer and reading the pointed value? I will need to compare the addr value pretty intensively, so I thought the pointer might optimize memory access a little bit - initialize the pointer once and then just dereference its value when needed...

RE: casting pointers

monpetit — Mon, 14 Dec 2015 14:25:42 GMT

Stan,

If you want copy 2 bytes of array elements, use memcpy(void * destination, const void * source, size_t num) function.

like this:

static uint8_t host1[2];
void init_beacon(void)
{
    memcpy((void*)host1, (void*)addr1.addr, 2);
}

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 14:21:20 GMT

It must be memory alignment issue.

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 14:20:43 GMT

I think, its rather a memory alignment issue... when my code runs, addr1.addr has address 0x200029b5 So when I try to initialize a pointer to 32 bit value by ..29b5 an exception seems to happen. I tied to init a pointer by address of addr.add[3] (..29b8) and it works fine. I saw some warnings about such or similar cases in ARM docs, but it was pretty long time ago...

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 13:51:16 GMT

I know, its 48 bit. I'm trying to cast first two bytes of the array element of struct(.addr) to be unit16_t, so I provide the address of the first one &addr.add[0].

RE: casting pointers

Anders Strand — Mon, 14 Dec 2015 13:46:04 GMT

I see. Why is the p_host1 16 bit? The address will be 6 byte = 6*8 = 48 bit. This seems to be a pure C language question. I would advice you to search stack overflow or similar sites.

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 13:38:42 GMT

as for the address, I just intent to use 6byte address in manner of IP to easily group/manage big net of beacons (project related). Under hungs I mean freezes... I don't have an access to SWD debugger, so my only option to debug is UART line... and I just see, that code doesn't run after the assignment of the pointer in beacon.c

RE: casting pointers

Anders Strand — Mon, 14 Dec 2015 13:34:26 GMT

Can you explain why you are doing this? Also explain what you mean by "hungs in assignment"

RE: casting pointers

Stanislav Silnitskiy — Mon, 14 Dec 2015 13:32:52 GMT

Hi Andres! its not from SDK. Its my own. Looks like I miss something important...

RE: casting pointers

Anders Strand — Mon, 14 Dec 2015 13:30:23 GMT

Have you found this code in the SDK? Which example are you using?