Question

How can I “pack” and “write” a struct to a file using C so that:

struct a {
    uint64_t a;
    char* b;
    uint16_t c;
} a;
a b;
b.a = 3;
b.b = "Hello";
b.c = 4;

gets written to the file as

00 00 00 00 00 00 00 03 48 65 6c 6c 6f 00 00 04

Answer 1

In C, you’ll have to code a function to do this for you. You can’t just blat the structure out to disk because b is a pointer that makes no sense without the backing string. And, unless you know (and can control) how your compiler packs its structures, you’re better off with a utility function anyway, even without pointers.

And, as if that wasn’t enough, you should output the length of the string as well so you know how many bytes to read back.

You’ll be looking for something like:

int better_than_blat (FILE *f, struct a *x) {
    size_t len = strlen (x->b);
    if (fwrite (&(x->a), sizeof(long), 1, f) != 1) return -1;
    if (fwrite (&len, sizeof(size_t), 1, f) != 1) return -1;
    if (fwrite (x->b, len, 1, f) != 1) return -1;
    if (fwrite (&(x->c), sizeof(short), 1, f) != 1) return -1;
    return 0;
}

int better_than_unblat (FILE *f, struct a *x) {
    size_t len;
    if (fread (&(x->a), sizeof(long), 1, f) != 1) return -1;
    if (fread (&len, sizeof(size_t), 1, f) != 1) return -1;
    x->b = malloc (len + 1);
    if (x->b == NULL) return -1;
    memset (x->b, 0, len + 1);
    if (fread (x->b, len, 1, f) != 1) return -1;
    if (fread (&(x->c), sizeof(short), 1, f) != 1) return -1;
    return 0;
}