how to access a double pointer to a structure from another structure in c

Question

the error is : request for member 'a' in something not a structure or union.

#include<stdio.h>

typedef struct mia {
    int a;
}hola;

typedef struct m {
    hola **r;
}bic;

int main() {
    bic y;
    scanf("%d", &(y.r->a));
    printf("%d", (y.r->a));
    return 0;
}

Before you address this question, you may want to ensure r is holding a valid pointer. As it is now, it has not one, but two levels of indeterminate indirection. — WhozCraig
– WhozCraig, Commented Feb 12, 2013 at 1:28
@gia, your other question that you deleted, you never set nr_query to a value but used it as a control to a loop. — Hogan
– Hogan, Commented Feb 13, 2013 at 20:27

WhozCraig · Accepted Answer · 2013-02-12 01:34:43Z

2

You will need a more complex syntax for this should do it:

int main()
{
    hola x;
    hola *ptr = &x;
    bic y = { &ptr };

    scanf("%d", &((*y.r)->a));
    printf("%d\n", (*y.r)->a);
    printf("%d\n", ptr->a);
    printf("%d\n", x.a);

    return 0;
}

Input

Output

100
100
100

Note you can fetch out the hola * pointer:

hola *ptr = *y.r;
printf("%d", ptr->a);

edited Feb 12, 2013 at 1:34

WhozCraig

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answered Feb 12, 2013 at 1:28

Mats Petersson

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1 Comment

gia Over a year ago

and what can i do if i have bic y[10] ? how do i access 'a'? or if 'a' is a pointer how do i do ?

Sergey Kalinichenko · Accepted Answer · 2013-02-12 01:32:44Z

2

Before you can start accessing data off a pointer, you need to allocate some memory first; otherwise, it's undefined behavior. Your current structure needs two levels of allocation - one for the pointer to hola, and another for the hola itself. You also need to add another level of dereference, because -> works on pointers to structs, not on pointer to pointers to structs:

bic y;
y.r = malloc(sizeof(hola*));
*y.r = malloc(sizeof(hola));
// You need an extra level of dereference (i.e. an asterisk)
scanf("%d", &((*y.r)->a));
printf("%d", ((*y.r)->a));
// Don't forget to free the memory you allocated,
// in reverse order:
free(*y.r);
free(y.r);

Here is a demo on ideone.

answered Feb 12, 2013 at 1:32

Sergey Kalinichenko

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