[英]C - Reassign pointer inside of struct to pointer outside of struct
I have two pointers, each pointing to a different array. 我有两个指针,每个指针指向一个不同的数组。 One of the pointers is inside of a struct, as follows:
指针之一在结构内部,如下所示:
typedef struct
{
int N; /* Number of Elements in array */
double *c; /* Pointer to an array */
/* Other members.... */
} CS;
The struct was initialized by: 该结构通过以下方式初始化:
CS->N = n; /* n is an integer of an initially unknown size */
CS->c = (double *)malloc(n * sizeof(double));
The pointer in the struct, CS->C
, contains data that I no longer care about. 结构中的指针
CS->C
包含我不再关心的数据。
My other pointer was defined as follows: 我的其他指针定义如下:
double *alpha;
alpha = (double *)malloc(CS->N * sizeof(double));
I need to replace the contents of CS->C
with alpha
. 我需要用
alpha
替换CS->C
的内容。 I know I can do something naive like: 我知道我可以做一些天真的事情,例如:
for (i=0;i<CS->N;i++) /* i is an integer */
CS->c[i] = alpha[i];
I could also use memcpy, like this: 我也可以这样使用memcpy:
memcpy(CS->c,alpha,CS->N * sizeof(double));
My understanding is that both of these methods will copy the contents from memory located at alpha
to the memory occupied by CS->C
. 我的理解是,这两种方法都会将内容从位于
alpha
的内存复制到CS->C
占用的内存。 That being a very expensive operation, it would make more sense to simply change the assignment of CS->C
to alpha
. 这是一个非常昂贵的操作,将
CS->C
的分配简单更改为alpha
会更有意义。
How can I do this? 我怎样才能做到这一点?
I've tried to reassign the pointer by doing like CS->C = &alpha
, but this gives me the following warning "assignment from incompatible pointer type". 我试图通过像
CS->C = &alpha
这样来重新分配指针,但这给了我以下警告“从不兼容的指针类型分配”。
Note: This is using ANSI C89 under full compliance, ie the compiler options are: -Wall -pedantic -ansi
注意:这是在完全符合要求的情况下使用ANSI C89,即编译器选项为:
-Wall -pedantic -ansi
Edit 1 编辑1
Freeing CS->c
and assigning it to alpha
by doing: 释放
CS->c
并将其分配给alpha
是:
free(CS->c);
CS->c = alpha;
does not work. 不起作用。 It causes every entry in
CS->c
to become equal to 0.0
and it results in my program seg faulting. 它导致
CS->c
每个条目都等于0.0
,并导致我的程序段错误。
Edit 2 编辑2
I think I realized why the method suggested in my first edit did not work. 我想我知道为什么我第一次编辑中建议的方法不起作用。
alpha
is a temporary pointer, created and initialized inside of a function, so once that function is exited, the memory occupied by alpha
is "freed". alpha
是在函数内部创建和初始化的临时指针,因此一旦退出该函数, alpha
占用的内存将被“释放”。 Since CS->c
points to that memory, it is also freed. 由于
CS->c
指向该内存,因此它也被释放。 Upon this discovery, I think I will rework my code, such that alpha
and CS-c
are initially swapped, such that when they are switched again, the end order will be correct. 基于这一发现,我想我将重新编写代码,以便最初交换
alpha
和CS-c
,以便当再次切换它们时,结束顺序将是正确的。 Thank you all for you valuable input. 谢谢大家的宝贵意见。
Just copy the pointer 只需复制指针
CS->C = alpha;
Alpha is a pointer to double, CS->C too, types matches, you simply change the pointer. Alpha是指向double的指针,CS-> C也是,类型匹配,您只需更改指针即可。
/!\\ Don't forgot to free the old CS->C ... /!\\不要忘记释放旧的CS-> C ...
Free the old pointer and assing the new directly. 释放旧指针,然后直接连接新指针。 (Not the adress of the new pointer)
(不是新指针的地址)
The pointer in the struct,
CS->c
, contains data that I no longer care about.结构体中的指针
CS->c
包含我不再关心的数据。My other pointer was defined as follows:
我的其他指针定义如下:
double *alpha;
alpha = malloc(CS->N * sizeof(double));
... ...
free (CS->c);
CS->c = alpha ; alpha=NULL;
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