How to read this expression?

(long)&((FLASH_CONF_STRUCT*)0)->vlan

FLASH_CONF_STRUCT is a struct type, and vlan is a member of this struct.

How to spell this expression?

(FLASH_CONF_STRUCT*) is a type cast. It's casting 0 to a pointer that points to a FLASH_CONF_STRUCT. Let's call this ptr.

ptr->vlan is equivalent to (*ptr).vlan. It accesses the vlan field of the structure pointed by ptr.

& gets the address of what follows, so the offset of vlan added to ptr.

(long) casts the address to a long.

Overall, this is meant to get the offset of vlan within the structure. But I suspect it invokes Undefined Behaviour (because it dereferences a NULL pointer, at the very least). And it does so needlessly.

Replace

long ofs = (long)&((FLASH_CONF_STRUCT*)0)->vlan;

with the far more readable

#include <stddef.h>

size_t ofs = offsetof(FLASH_CONF_STRUCT, vlan);

For example,

#include <stddef.h>
#include <stdio.h>

typedef struct {
   int a;
   int b;
   int vlan;
} FLASH_CONF_STRUCT;

int main(void) {
   printf("%ld\n", (long)&((FLASH_CONF_STRUCT*)0)->vlan);
   printf("%zu\n", offsetof(FLASH_CONF_STRUCT, vlan));
   return 0;
}

Output:

$ gcc -Wall -Wextra -pedantic-errors -std=c99 a.c -o a && a
8
8