[嵌入式]字节序及内存对齐

1. 字节序

低                       高
|————————————————————————|
        内存左低右高

0x12345678
大端序： 0x12345678 高位存低地址，低位存高地址 也是肉眼序、网络序列 只要少数cpu是大端序
小端序： 0x78563412 高位存高地址，低位存低地址 x86 arm等主流cpu是小端序

intel x86_64上程序

uint64_t hton64(uint64_t host)
{
    uint64_t   ret = 0;
    uint64_t   high,low;
    low   =   host & 0xFFFFFFFF;
    high   =  (host >> 32) & 0xFFFFFFFF;
    low   =   htonl(low);
    high   =   htonl(high);
    ret   =   low;
    ret   <<= 32;
    ret   |=   high;
    return   ret;
}

int main(int argc, char ** argv)
{
	int i;
    uint64_t ulValue = 0x12345678aabbccdd;
	unsigned char *puc = (unsigned char *)(&ulValue);
	for(i = 0; i < 8; i++)
        printf("%X  ", *(puc + i));
	
	printf("\n");
	
	ulValue = hton64(ulValue);
	for(i = 0; i < 8; i++)
        printf("%X  ", *(puc + i));
	
	printf("\n");
	return;
}

[root@localhost test]# gcc main_test.c
[root@localhost test]# ./a.out
DD  CC  BB  AA  78  56  34  12
12  34  56  78  AA  BB  CC  DD
[root@localhost test]#

2.字节对齐

struct memtest{
    unsigned int uiV1;
    unsigned short usV2;
    unsigned int uiV3;
	unsigned char ucV4;
};

void outMembuf(void * buf, unsigned int uiLen)
{
    int i, count = 0;
	printf("buf len:%u\n",uiLen);
	for(i=0; i < uiLen; i++)
	{
		if (0 ==(count%16))
			printf("0x");
		printf("%02X", *(unsigned char *)(buf+i));
		count++;

        if (0 ==(count%8))
			printf("  ");	
		
		if (0 ==(count%16))
			printf("\n");	
	}
}

int main(int argc, char ** argv)
{
	struct memtest stmem = {0};
	stmem.uiV1 = 0x11223344;
	stmem.usV2 = 0x5566;
	stmem.uiV3 = 0x778899aa;
	stmem.ucV4 = 0xbb;
	outMembuf(&stmem,sizeof(struct memtest));
}

[root@localhost test]# ./a.out
buf len:16
0x4433221166550000  AA998877BB000000