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?

Good

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. – Wasp

I don't get it... why do you need a pointer ? – Job

See question here In order to declare a pointer you need to allocate memory so your basically asking the same thing. – Dumond

Answer 1

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.

Hernia

+1. Didn't know about this. Where does the memory come from in this case? – Mohler

@Mohler 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 – Hernia

@cnicutar: Lovely answer +1 & chosen . – Good

if defined in a statement block, we do not need to free the struct pointer, right? – Underwriter

Answer 2

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;

Jaques

This one uses dynamic memory allocation hence it is not so elegant. – Orangeman

Answer 3

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;

Good

Answer 4

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.

Mohler

Answer 5

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

struct PipeShm * pMyPipe = &myPipe;

Wireman

Answer 6

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

Disendow

you need to cast the struct before you fill it – Disendow

Consider adding a explanation to your code so it's more helpful to OP and future readers – Devastation

Answer 7

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;

Rabah