Odtnhj

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);