指向结构体中的指针的指针

Question

Could someone help explaining why this part of my code isn't working?

typedef struct {
    char *something;
} random;

random *rd;
rd->something = calloc(40, sizeof(char)); // This is the line which crashes
strncpy(rd->something, aChar, 40);

The program works if I write it as such:

random rd;
rd.something = calloc(40, sizeof(char));
strncpy(rd.something, aChar, 40);

But I think this is wrong when handling memory, that's why I want help with the first scenario.

Answer 1

There's no memory allocated to the struct pointed by rd.

Try:

typedef struct {
    char *something;
} random;

random *rd = malloc (sizeof(random));
rd->something = calloc(40, sizeof(char)); // This is the line which crashes
strncpy(rd->something, aChar, 40);

Answer 2

It is because your defined pointer

random *rd;

is not properly initialized and therefore you get a segmentation fault. The second version works, because you actually allocate rd . To make the first version work as well, allocate memory for *rd with

random *rd = (random*)malloc(sizeof(random));

Answer 3

Case 1:

random *rd;

// Create *pointer* to struct of type random . Doesn't point to anything.

rd->something = calloc(40, sizeof(char)); 

// Use it by trying to acquire something which doesnt exist and it crashes

Case 2:

random rd;

// Create a random struct

rd.something = calloc(40, sizeof(char));

// Use it . Works good

===========================

For Case 1, you need to allocate a struct first , make the pointer point to it and then use the -> operator to modify the values

Answer 4

It will work, but first allocate memory to rd. rd = (random *) calloc(1,sizeof(random));