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
– Alok Save, Commented Jul 29, 2012 at 14:19
See question here In order to declare a pointer you need to allocate memory so your basically asking the same thing. — rudolph9
– rudolph9, Commented Jul 29, 2012 at 14:23

cnicutar · Accepted Answer · 2012-07-29 14:33:50Z

62

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, 2012 at 14:33

answered Jul 29, 2012 at 14:22

cnicutar

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4 Comments

Jay Over a year ago

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

cnicutar Over a year ago

@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

JAN Over a year ago

@cnicutar: Lovely answer +1 & chosen .

NewBee Over a year ago

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

Brian Cain · Accepted Answer · 2012-07-29 14:22:00Z

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;

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

JAN · Accepted Answer · 2012-07-29 14:22:09Z

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;

Jay · Accepted Answer · 2012-07-29 14:21:05Z

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.

oseiskar · Accepted Answer · 2012-07-29 14:21:52Z

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

struct PipeShm * pMyPipe = &myPipe;

mousetail · Accepted Answer · 2022-12-19 08:24:31Z

1

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, 2022 at 8:24

mousetail

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answered Dec 15, 2022 at 19:44

user20787710

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2 Comments

user20787710 Over a year ago

you need to cast the struct before you fill it

mousetail Over a year ago

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

Jokester · Accepted Answer · 2012-07-29 14:24:32Z

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;

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