我知道,由于未初始化的填充,memcmp()不能用于比较memset()未归零的结构.但是,在我的程序中,我有一个结构,在开始时有几个不同的类型,然后是几十个相同的类型,直到结构的结尾.我的想法是手动比较前几种类型,然后memcmp()在相同类型成员的剩余连续内存块上使用a .
我的问题是,C标准对结构填充有什么保证？我可以在任何或所有编译器上可靠地实现这一点吗？C标准是否允许在相同类型的成员之间插入struct padding？
我已经实现了我提出的解决方案,它似乎与预期完全一致gcc:
#include
#include
#include
struct foo
{
char a;
void *b;
int c;
int d;
int e;
int f;
};
static void create_struct(struct foo *p)
{
p->a = 'a';
p->b = NULL;
p->c = 1;
p->d = 2;
p->e = 3;
p->f = 4;
}
static int compare(struct foo *p1, struct foo *p2)
{
if (p1->a != p2->a)
return 1;
if (p1->b != p2->b)
return 1;
return
/* Note the typecasts to char * so we don't get a size in ints. */
memcmp(
/* A pointer to the start of the same type members. */
&(p1->c),
&(p2->c),
/* A pointer to the start of the last element to be compared. */
(char *)&(p2->f)
/* Plus its size to compare until the end of the last element. */
+sizeof(p2->f)
/* Minus the first element, so only c..f are compared. */
-(char *)&(p2->c)
) != 0;
}
int main(int argc, char **argv)
{
struct foo *p1, *p2;
int ret;
/* The loop is to ensure there isn't a fluke with uninitialized padding
* being the same.
*/
do
{
p1 = malloc(sizeof(struct foo));
p2 = malloc(sizeof(struct foo));
create_struct(p1);
create_struct(p2);
ret = compare(p1, p2);
free(p1);
free(p2);
if (ret)
puts("no match");
else
puts("match");
}
while (!ret);
return 0;
}