函數調用值後的C返回結構指針值丟失

Question

我想知道爲什麼我的結構值在函數返回後丟失。

struct A { 
    int val1; 
    int val2; 
} 

main() { 
    struct A *a; 
    a->val1 = 1; 
    a->val2 = 2; 
    calc((void*)a); 
    // calc returns and a's values are 1 and 2 respectively 
} 

void calc(void* v) { 
    struct A *a = (struct A*) v; 
    ... 
    a->val1 = 2; 
    a->val2 = 3; 
} 

Answer 1

當你想處理指針時，你必須引入動態內存分配。創建一個指針意味着你告訴編譯器'我將存儲變量的地址'。因此，在這種情況下，您可以爲其創建對象分配地址或動態地爲其分配內存。

情況1：

main() { 
    struct A *a; 
    struct A obj;//assign address of other object manually 
    a = &obj; 
    a->val1 = 1; 
    a->val2 = 2; 
    calc((void*)a); 
    // calc returns and a's values are 1 and 2 respectively 
} 

情況2：

main() { 
    struct A *a; 
    a = (A *) malloc(sizeof(A));//dynamic memory allocation 
    a->val1 = 1; 
    a->val2 = 2; 
    calc((void*)a); 
    // calc returns and a's values are 1 and 2 respectively 
}