我该如何申请的结构失调? [英] How do I apply a structure offset?
问题描述
我有一个结构
typedef struct foo {
int lengthOfArray1;
int lengthOfArray2;
int* array1;
int* array2;
} foo;
我要整个结构和数组的内容分配足够的内存。因此,假设每个阵列有长度为5 ...
I need to allocate enough memory for the entire structure and its array's contents. So assuming each array had a length of 5...
foo* bar = (foo*)malloc(sizeof(foo) + (sizeof(int) * 5) + (sizeof(int) * 5));
我现在有指向数组1和数组2在该分配的缓冲区中的正确位置:
I now have to point array1 and array2 to the correct location in that allocated buffer:
bar->array1 = (int*)(&bar->lengthOfArray2 + sizeof(int));
bar->array2 = (int*)(bar->array1 + lengthOfArray2);
这是正确的?
编辑#1
只是为了澄清任何混乱。我试图保持在内存中一个块,而不是三
Just to clear up any confusion: I am trying to keep the memory in one block, and not three.
编辑#2
我不能使用C99作为MSVC 2010编译器不支持它(http://stackoverflow.com/questions/6688895/does-microsoft-visual-studio-2010-supports-c99)。
I cannot use C99 as the MSVC 2010 compiler does not support it (http://stackoverflow.com/questions/6688895/does-microsoft-visual-studio-2010-supports-c99).
推荐答案
继OP的做法本应做的工作:
Following the OP's approach this should do the job:
/* Defining these types allows to change the types without having the need to modify the code. */
typedef int Foo_ArrayElement1_t;
typedef int Foo_ArrayElement2_t;
typedef struct Foo_s {
size_t lengthOfArray1; /* 'size_t' is the type of choice for array/memory dimensions. */
size_t lengthOfArray2;
Foo_ArrayElement1_t * array1;
Foo_ArrayElement2_t * array2;
} Foo_t;
/*
* Allocates memory to hold a structure of type Foo_t including size for
* 's1' elements referenced by 'array1' and 's2' elements by 'array2'.
* The arrays' elements are set to 0.
*
* Returns a pointer to the freshly allocated memory or NULL if the memory could not
* be allocated.
*/
Foo_t * Foo_CreateAndInit(size_t s1, size_t s2)
{
/* At once allocate all 'Foo_t' (including the memory Foo_t's array pointers shall point to). */
Foo_t * pfoo = calloc(1,
sizeof(*pfoo) +
s1 * sizeof(*(pfoo->array1) +
s2 * sizeof(*(pfoo->array2)));
if (pfoo)
{
pfoo->lengthOfArray1 = s1;
pfoo->lengthOfArray2 = s2;
/* The first array starts right after foo. */
pfoo->array1 = (Foo_ArrayElement1_t *) (pfoo + 1);
/* The second array starts right after s1 elements of where the first array starts. */
pfoo->array2 = (Foo_ArrayElement2_t *) (pfoo->array1 + s1); /* That casting here is not
necessaryas long as 'Foo_t.array1' and 'Foo_t.array2' point to the same type but makes
the code work even if those types were changed to be different. */
}
return pfoo;
}
...
Foo_t * foo = Foo_CreateAndInit(5, 5);
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