Assign value to field in array of nested structs - c

Question

I have a function test that accepts a pointer to a structure. How can I set the value of a field in a structure in the nested structure array? The structures are created and deleted elsewhere, I just want to set the value.
typedef struct {
int a;
} inner_struct_t;
typedef struct {
int b;
inner_struct_t innerStructsArr[];
} outer_struct_t;
void test(outer_struct_t *outerStruct);
void test(outer_struct_t *outerStruct) {
outerStruct->b = 123; // set a value in the outer struct
(outerStruct->innerStructsArr)[0].a = 456; // but this DOESN'T work
}

Answer 1

Here's a complete example that should work for you:
#include <stdio.h>
#include <malloc.h>
typedef struct {
int a;
} inner_struct_t;
typedef struct {
int b;
inner_struct_t *innerStructsArr;
} outer_struct_t;
void test(outer_struct_t *outerStruct, int n) {
outerStruct->b = 123; // set a value in the outer struct
outerStruct->innerStructsArr = malloc(sizeof(inner_struct_t) * n); // Allocate space
outerStruct->innerStructsArr[0].a = 456; // Assign a value
}
int main (int argc, char *argv[]) {
outer_struct_t s;
test(&s, 1);
printf("b=%d, inner[0]=%d\n", s.b, s.innerStructsArr[0].a);
free(s.innerStructsArr);
return 0;
}

Answer 2

When you have a structure with a flexible array member (FAM) in it, you need to allocate the space for the array at the same time that you allocate the space for the structure.
ISO/IEC 9899:2011 §6.7.2.1 Structure and union specifiers
¶18 As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member. In most situations,
the flexible array member is ignored. In particular, the size of the structure is as if the
flexible array member were omitted except that it may have more trailing padding than
the omission would imply. However, when a . (or ->) operator has a left operand that is
(a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object being accessed; the
offset of the array shall remain that of the flexible array member, even if this would differ
from that of the replacement array. If this array would have no elements, it behaves as if
it had one element but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.
Given:
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
int a;
} inner_struct_t;
typedef struct
{
int b;
inner_struct_t innerStructsArr[];
} outer_struct_t;
static void test(outer_struct_t *outerStruct)
{
outerStruct->b = 123;
outerStruct->innerStructsArr[0].a = 456;
}
int main(void)
{
outer_struct_t *os = malloc(sizeof(*os) + 4 * sizeof(os->innerStructsArr[0]));
for (int i = 0; i < 4; i++)
os->innerStructsArr[i].a = 10 * i;
test(os);
printf(" b = %d\n", os->b);
for (int i = 0; i < 4; i++)
printf("[%d].a = %d\n", i, os->innerStructsArr[i].a);
free(os);
return 0;
}
Sample output:
b = 123
[0].a = 456
[1].a = 10
[2].a = 20
[3].a = 30