数组的内存对齐方式

Question

我在Cell处理器上调整DMA传输的内存时遇到问题.我需要地址的最后4位为0.

我有4个数组,unsigned int其中每个元素必须在内存中对齐,以便其(十六进制)地址以零结尾.

例如

int main()
{
    size_t i;

    static unsigned int a[2] __attribute__ ((aligned (16)));
    static unsigned int b[2] __attribute__ ((aligned (16)));
    static unsigned int c[2] __attribute__ ((aligned (16)));
    static unsigned int d[2] __attribute__ ((aligned (16)));

    for (i = 0; i < 2; ++i) {
        printf("a[%u] = %p\n", &a[i]);
        printf("b[%u] = %p\n", &b[i]);
        printf("c[%u] = %p\n", &c[i]);
        printf("d[%u] = %p\n", &d[i]);
    }

    return 0;
}

输出:

a[0] = 0x10010b60
b[0] = 0x10010b50
c[0] = 0x10010b40
d[0] = 0x10010b30
a[1] = 0x10010b64
b[1] = 0x10010b54
c[1] = 0x10010b44
d[1] = 0x10010b34

这里的问题是每个数组的第二个元素似乎不是16位对齐的(它们的地址'以4结尾).

我需要地址看起来像这样:

a[0] = 0xXXXXXXX0
b[0] = 0xXXXXXXX0
c[0] = 0xXXXXXXX0
d[0] = 0xXXXXXXX0
a[1] = 0xXXXXXXX0
b[1] = 0xXXXXXXX0
c[1] = 0xXXXXXXX0
d[1] = 0xXXXXXXX0

Answer 1

alignment属性指定变量或结构字段的对齐方式,而不是单个数组元素.有关详细信息,请参阅指定变量的属性.

如果您始终希望将两个整数对齐,则可以定义结构

struct dma_transfer {
    unsigned int e0 __attribute__ ((aligned (16)));
    unsigned int e1 __attribute__ ((aligned (16)));
};

这将对齐16字节边界上的元素.

int main(int argc, char **argv)
{
    static struct dma_transfer a;
    static unsigned int b[2];

    printf("a.e0 = %p\n", &a.e0);
    printf("a.e1 = %p\n", &a.e1);
    printf("b[0] = %p\n", &b[0]);
    printf("b[1] = %p\n", &b[1]);

    return 0;
}

给出,例如

a.e0 = 0x601060
a.e1 = 0x601070
b[0] = 0x601080
b[1] = 0x601084

但这也意味着你在两个整数值之间有漏洞.在32位系统上,您将拥有

| int 4字节| 洞12字节|
| int 4字节| 洞12字节|

Answer 2

如果arr是一个32位元素的数组,并且地址arr[0]是0xXXXXXXX0,那么地址arr[1]必然是0xXXXXXXX4.

出于您的目的,您需要使用16字节元素的数组:

typedef struct
{
    unsigned int x;
    unsigned char reserved[16-sizeof(unsigned int)];
}
element_t;

static element_t a[2] __attribute__ ((aligned (16)));
static element_t b[2] __attribute__ ((aligned (16)));
static element_t c[2] __attribute__ ((aligned (16)));
static element_t d[2] __attribute__ ((aligned (16)));

或者,您可以完全避免使用数组.

相反,使用纯变量,并告诉编译器将它们与16个字节对齐:

static unsigned int a0 __attribute__ ((aligned (16)));
static unsigned int a1 __attribute__ ((aligned (16)));
static unsigned int b0 __attribute__ ((aligned (16)));
static unsigned int b1 __attribute__ ((aligned (16)));
static unsigned int c0 __attribute__ ((aligned (16)));
static unsigned int c1 __attribute__ ((aligned (16)));
static unsigned int d0 __attribute__ ((aligned (16)));
static unsigned int d1 __attribute__ ((aligned (16)));