How do I write an array to a file?

Your code is fine (almost - see below). For structures, basically the same. Use sizeof() always, since the compiler might not generate the structure size you might think.

typedef struct xyz {
int a[5];
char b[7];
double c[2];
struct xyz *next;
char z;
} sxyz;

sxyz arr[8];

...

file.write(arr, sizeof(arr));  // right
file.write(&arr[0], 8 * sizeof(sxyz));  // wrong

Note I've done two things in two different ways:
1. arr in this context (as a pointer) means the same as &arr[0] - the address of the first element of the array. Both versions are correct in this regard.
2. (No of structs) * (size of struct) is not necessarily equal to size of array of structs

As an exercise, try and work out in your head the size of sxyz above. Compare that with what your compiler says.
[optional advanced exercise]Find the compiler option that changes the size of sxyz without breaking your program. What does that do to the size of arr?

Cheers,
Peter