How to initialize a pointer to a struct in C?

Question

Given this struct:

struct PipeShm
{
    int init;
    int flag;
    sem_t *mutex;
    char * ptr1;
    char * ptr2;
    int status1;
    int status2;
    int semaphoreFlag;

};

That works fine:

static struct PipeShm myPipe = { .init = 0 , .flag = FALSE , .mutex = NULL , 
        .ptr1 = NULL , .ptr2 = NULL , .status1 = -10 , .status2 = -10 , 
        .semaphoreFlag = FALSE };

But when I declare static struct PipeShm * myPipe , that doesn't work , I'm assuming that I'd need to initialize with the operator ->, but how?

static struct PipeShm * myPipe = {.init = 0 , .flag = FALSE , .mutex = NULL , 
        .ptr1 = NULL , .ptr2 = NULL , .status1 = -10 , .status2 = -10 , 
        .semaphoreFlag = FALSE };

Is it possible to declare a pointer to a struct and use initialization with it?

You need to allocate memory to the pointer so that you can do anything meaningful with it. A pointer by itself just points to an random address.You need to make sure that the address pointed by the pointer is big enough to hold the structure contents. — Alok Save, Jul 29 '12 at 14:19
See question [here](http://stackoverflow.com/questions/2177391/allocating-memory-for-a-structure-in-c) In order to declare a pointer you need to allocate memory so your basically asking the same thing. — rudolph9, Jul 29 '12 at 14:23

cnicutar · Accepted Answer · 2012-07-29T14:33:50.550

58

You can do it like so:

static struct PipeShm * myPipe = &(struct PipeShm) {
    .init = 0,
    /* ... */
};

This feature is called a "compound literal" and it should work for you since you're already using C99 designated initializers.

Regarding the storage of compound literals:

6.5.2.5-5

If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.

edited Jul 29 '12 at 14:33

answered Jul 29 '12 at 14:22

cnicutar

178,505
25
365
392

+1. Didn't know about this. Where does the memory come from in this case? – Jay Jul 29 '12 at 14:29
2

@Jay *The storage for this object is either static (if the compound literal occurs at file scope) or automatic (if the compound literal occurs at block scope), and the storage duration is associated with its immediate enclosing block* – cnicutar Jul 29 '12 at 14:31
@cnicutar: Lovely answer +1 & chosen . – JAN Jul 29 '12 at 14:34
if defined in a statement block, we do not need to free the struct pointer, right? – NewBee May 12 '19 at 02:05

score 5 · Answer 2 · answered Jul 29 '12 at 14:22

5

Is it possible to declare a pointer to a struct and use initialization with it ?

Yes.

const static struct PipeShm PIPE_DEFAULT = {.init = 0 , .flag = FALSE , .mutex = NULL , .ptr1 = NULL , .ptr2 = NULL ,
        .status1 = -10 , .status2 = -10 , .semaphoreFlag = FALSE };

static struct PipeShm * const myPipe = malloc(sizeof(struct PipeShm));
*myPipe = PIPE_DEFAULT;

answered Jul 29 '12 at 14:22

Brian Cain

14,403
3
50
88

This one uses dynamic memory allocation hence it is not so elegant. – Matthias Dieter Wallnöfer Feb 13 '19 at 14:50

score 4 · Answer 3 · answered Jul 29 '12 at 14:22

4

Okay I got it :

static struct PipeShm  myPipeSt = {.init = 0 , .flag = FALSE , .mutex = NULL , .ptr1 = NULL , .ptr2 = NULL ,
        .status1 = -10 , .status2 = -10 , .semaphoreFlag = FALSE };

static struct PipeShm  * myPipe = &myPipeSt;

answered Jul 29 '12 at 14:22

JAN

21,236
66
181
318

score 2 · Answer 4 · answered Jul 29 '12 at 14:21

First you need to allocate memory for the pointer as below:

myPipe = malloc(sizeof(struct PipeShm));

Then, you should assign values one by one as below:

myPipe->init = 0;
myPipe->flag = FALSE;
....

Please note that for each individual pointer inside the structure, you need allocate memory seperately.

score 1 · Answer 5 · answered Jul 29 '12 at 14:21

1

First initialize the struct (static struct PipeShm myPipe = {...). Then take the address

struct PipeShm * pMyPipe = &myPipe;

answered Jul 29 '12 at 14:21

oseiskar

3,282
2
22
22

score 0 · Answer 6 · answered Jul 29 '12 at 14:24

0

you have to build that struct by hand, and then make a pointer pointing to that.

either

static struct PipeShm myPipe ={};
static struct PipeShm *pmyPipe = &myPipe;

or

static struct PipeShm *myPipe = malloc();
myPipe->field = value;

answered Jul 29 '12 at 14:24

Jokester

5,501
3
31
39

score 0 · Answer 7 · edited Dec 19 '22 at 08:24

0

static struct PipeShm * myPipe = &(struct PipeShm) {.init = 0 , .flag = FALSE , .mutex = NULL , 
        .ptr1 = NULL , .ptr2 = NULL , .status1 = -10 , .status2 = -10 , 
        .semaphoreFlag = FALSE };

edited Dec 19 '22 at 08:24

mousetail

7,009
4
25
45

answered Dec 15 '22 at 19:44

user20787710

1

you need to cast the struct before you fill it – user20787710 Dec 15 '22 at 19:45
2

Consider adding a explanation to your code so it's more helpful to OP and future readers – mousetail Dec 19 '22 at 08:24

How to initialize a pointer to a struct in C?

7 Answers7

Linked

Related