如何應用結構偏移量？

Question

我有一個結構

typedef struct foo { 
    int lengthOfArray1; 
    int lengthOfArray2; 
    int* array1; 
    int* array2; 
} foo; 

我需要爲整個結構和數組的內容分配足夠的內存。因此，假設每個陣列有長度爲5 ...

foo* bar = (foo*)malloc(sizeof(foo) + (sizeof(int) * 5) + (sizeof(int) * 5)); 

我現在有指向數組1和數組2到正確的位置在分配的緩衝區：

bar->array1 = (int*)(&bar->lengthOfArray2 + sizeof(int)); 
bar->array2 = (int*)(bar->array1 + lengthOfArray2); 

這是正確的嗎？

編輯＃1

只是爲了澄清任何混淆：我試圖保持內存在一個塊，而不是三個。

編輯＃2

我不能使用C99作爲MSVC 2010編譯器不支持它（http://stackoverflow.com/questions/6688895/does-microsoft-visual-studio-2010-supports -c99）。

Answer 1

繼OP的方法，這應該做的工作：

/* 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); 

Answer 2

您必須分配結構的大小。然後您必須分配具有各自大小的整數數組。

foo* bar = malloc(sizeof(foo)); 

/* somewhere in here the array lengths are set then... */ 

bar->array1 = malloc(sizeof(int) * bar->lengthOfArray1); 
bar->array2 = malloc(sizeof(int) * bar->lengthOfArray2); 

Answer 3

隨着一點點額外的內存（僅適用於C99）：

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

typedef struct foo { 
    int lengthOfArray1; 
    int lengthOfArray2; 
    int *array1; 
    int *array2; 
    int array[]; 
} foo; 

int main(void) 
{ 
    foo *bar = malloc(sizeof(foo) + (sizeof(int) * 10)); 

    bar->array1 = &bar->array[0]; 
    bar->array2 = &bar->array[5]; /* 5 or lengthOfArray1 */ 

    return 0; 
}