Nrthugu

i am trying to use flexible array for int variable.

Below is my code:

struct S2 {

    int foo;

    int bar;

    int stud_roll;

}s2g;



void test(){

        s2g.stud_roll = {0};

}



int main(){

        test();

        return 0;

}

But its not working.

How can fix the problem? what is my error?

In order to use your struct with a Flexible Array Member (FAM) you need a pointer to struct, not type struct. The FAM provides the convenience of allowing allocation for the struct and the FAM in a single allocation, rather than needing to allocate for the struct and then for stud_roll separately. For example:

struct S2 {

    int foo;

    int bar;

    int stud_roll;

} *s2g;   /* note the declaration as a pointer */



void test (void)

{

    s2g = malloc (sizeof *s2g + ELEMENTS * sizeof *s2g->stud_roll);

    if (!s2g) {

        perror ("malloc-s2g");

        exit (EXIT_FAILURE);

    }

    ...

There you allocate storage for both the struct sizeof *s2g plus the element of stud_roll, e.g. ELEMENTS * sizeof *s2g->stud_roll. This provides the single allocation/single free.

A short example would be:

#include <stdio.h>

#include <stdlib.h>



#define ELEMENTS 10



struct S2 {

    int foo;

    int bar;

    int stud_roll;

} *s2g;



void test (void)

{

    s2g = malloc (sizeof *s2g + ELEMENTS * sizeof *s2g->stud_roll);

    if (!s2g) {

        perror ("malloc-s2g");

        exit (EXIT_FAILURE);

    }



    s2g->foo = 1;

    s2g->bar = 2;



    for (int i = 0; i < ELEMENTS; i++)

        s2g->stud_roll[i] = i + 1;

}



int main (void) {



    test();



    printf ("s2g->foo: %dns2g->bar: %dn", s2g->foo, s2g->bar);

    for (int i = 0; i < ELEMENTS; i++)

        printf (" %d", s2g->stud_roll[i]);

    putchar ('n');

    free (s2g);



    return 0;

}

(note: since the flexible array member is static, you cannot have an array of struct containing a flexible array member -- but you can have an array of pointers with separate allocation for each)

Example Use/Output

$ ./bin/famtst

s2g->foo: 1

s2g->bar: 2

 1 2 3 4 5 6 7 8 9 10

You need to allocate memory for (struct S2).stud_roll. Without any memory you will write out of bounds. Assimung there is no padding sizeof(struct S2) == sizeof(int) + sizeof(int) - there is no memory allocated for stud_roll cause the member takes no memory "by itself".

s2g.stud_roll = {0};

You can't assign arrays in that way in C.

You can use compound literal to allocate some memory on stack:

#define STUD_ROLL_SIZE  4



struct S2 * const s2g = (void*)((char[sizeof(struct S2) + STUD_ROLL_SIZE * sizeof(int)]){ 0 });



void test(void) {

    s2g->stud_roll[0] = 1;

    // or

    memcpy(s2g->stud_roll, (int){ 1, 2, 3, 4 }, 4 * sizeof(int));

}

or use malloc to dynamically allocate the memory, you need to allocate more memory than the sizeof(struct S2):

struct S2 *s2g = NULL;



void test(void) {

    s2g = malloc(sizeof(struct S2) + STUD_ROLL_SIZE * sizeof(int));

    if (s2g == NULL) {

        fprintf(stderr, "Abort ship! Abort ship!n");

        exit(-1);

    }

    s2g->stud_roll[0] = 1;

    free(s2g);

}