char *incomming_message;
.....
struct blabla *b;
b = (struct blabla *) incomming_message; /* Так делать нельзя, т.к. компилятор не может ничего узнать будет ли то, куда указывает incomming_message выровнено в памяти! */

b->that = 100500; /* Если incomming_message не было выровнено, то и на доступ внутри структуры тоже будет ошибка выравнивания */

char *incomming_message;
.....
struct blabla b;

memcpy(&b, incomming_message); /* Таким образом мы создаём копию данных, которые гарантированно выровнены в памяти */

b.that = 100500; /* В этом месте компилятор обязан сгенерировать код без невыровненных обращений */

int read_u8(uint8_t **message, size_t *len, uint8_t *retval){
    if (*len >= sizeof(uint8_t) ){
        *retval = **message;
        *message += sizeof(uint8_t);
        *len -= sizeof(uint8_t);
        return 0;  /* Ok */
    }
    return (-1);    /* Fail */
}

int read_u16_LE(uint8_t **message, size_t *len, uint16_t *retval){
    uint8_t msb;
    uint8_t lsb;
    if(read_u8(message, len, &lsb) < 0){
        return (-1);  /* Fail */
    }

    if(read_u8(message, len, &msb) < 0){
        return (-1);  /* Fail */
    }

    *retval = (msb << 8) | lsb;

    return (0);  /* Ok */
}

int read_u16_BE(uint8_t **message, size_t *len, uint16_t *retval){
    uint8_t msb;
    uint8_t lsb;
    if(read_u8(message, len, &msb) < 0){
        return (-1);  /* Fail */
    }

    if(read_u8(message, len, &lsb) < 0){
        return (-1);  /* Fail */
    }

    *retval = (msb << 8) | lsb;

    return (0);  /* Ok */
}

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

typedef struct bpdu_body_y {
unsigned char flags;
unsigned char root_id[8];
unsigned char root_path_cost[4];
unsigned char bridge_id[8];
unsigned char port_id[2];
unsigned char message_age[2];
unsigned char max_age[2];
unsigned char hello_time[2];
unsigned char forward_delay[2];
} BPDU_BODY_Y;

typedef struct bpdu_body_t {
unsigned char flags;
unsigned char root_id[8];
unsigned char root_path_cost[4] __attribute__ ((__aligned__(4)));
unsigned char bridge_id[8];
unsigned char port_id[2];
unsigned char message_age[2];
unsigned char max_age[2];
unsigned char hello_time[2];
unsigned char forward_delay[2];
} __attribute__ ((__aligned__(4))) BPDU_BODY_T;

int main(int argc, char *argv[])
{
int i;
volatile char *memo;
volatile u32 *imemo;
BPDU_BODY_T *bmemo;
BPDU_BODY_Y *ymemo;

printf ("sizeof unaligned struct = %d; sizeof aligned struct = %d\n", sizeof(BPDU_BODY_Y), sizeof(BPDU_BODY_T));

memo = (char *)malloc(sizeof (BPDU_BODY_T));

for (i = 0; i < sizeof (BPDU_BODY_T); i++) {
memo[i] = i;
}

printf ("------------------------------------------------------------------------------------------------------\n\t");
bmemo = memo;
ymemo = memo;
printf ("\tMemory contents: \n\n");
for (i = 0; i < sizeof(BPDU_BODY_T); i++) {
printf("%02X", memo[i]);
if (!((i+1)%4)) {
printf ("|");
}
}
printf ("\n----------------------------------------------------------------\n");
printf ("It should be %02X%02X%02X%02X\n\n",
ymemo->root_path_cost[3], ymemo->root_path_cost[2],
ymemo->root_path_cost[1], ymemo->root_path_cost[0]);
memo = memo + 9;
printf ("Addr of unaligned root_path_cost is %p; Addr of aligned root_path_cost is %p; \n",
&ymemo->root_path_cost, &bmemo->root_path_cost);
printf("\tUnaligned root_path_cost %08X\n\t", *((int *)(ymemo->root_path_cost)));
printf("Aligned root_path_cost %08X\n", *((int *)(bmemo->root_path_cost)));

return 0;
}

root@OpenWrt:/# tst1
sizeof unaligned struct = 31; sizeof aligned struct = 36
------------------------------------------------------------------------------------------------------
Memory contents:

00010203|04050607|08090A0B|0C0D0E0F|10111213|14151617|18191A1B|1C1D1E1F|20212223
|
----------------------------------------------------------------
It should be 0C0B0A09

Addr of unaligned root_path_cost is 0x11011; Addr of aligned root_path_cost is 0x11014;
Unaligned root_path_cost 080B0A09
Aligned root_path_cost 0F0E0D0C
root@OpenWrt:/# echo 2 > /proc/cpu/alignment
root@OpenWrt:/# tst1
sizeof unaligned struct = 31; sizeof aligned struct = 36
------------------------------------------------------------------------------------------------------
Memory contents:

00010203|04050607|08090A0B|0C0D0E0F|10111213|14151617|18191A1B|1C1D1E1F|20212223
|
----------------------------------------------------------------
It should be 0C0B0A09

Addr of unaligned root_path_cost is 0x11011; Addr of aligned root_path_cost is 0x11014;
Unaligned root_path_cost 0C0B0A09
Aligned root_path_cost 0F0E0D0C