How to count the size of the structure?

Question

In the man unix(7) I found the following:

pathname: a UNIX domain socket can be bound to a null-terminated filesystem pathname using bind(2). When the address of a pathname socket is returned (by one of the system calls noted above), its length is
offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1

The struct sockaddr_un is defined as

struct sockaddr_un {
    sa_family_t sun_family;               /* AF_UNIX */
    char        sun_path[108];            /* pathname */
};

So I would guess that we can simply do strlen(sun_path) + sizeof(sun_family) I don't understand the +1 they added. Can you please give an explanation? I understand the offsetof uses for portabiliy as described

On Linux, the above offsetof() expression equates to the same value as sizeof(sa_family_t), but some other implementations include other fields before sun_path, so the offsetof() expression more portably describes the size of the address structure.

But this +1 is not clear to me.

The `+1` accounts for the terminating nullchar of the string data held in `sun_path`. The space occupied by the terminator is *not* accounted for in the result of `strlen` itself. — WhozCraig, Dec 07 '18 at 06:17
The size of the structure is `sizeof(struct sockaddr_un)`. `offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1` is closer to the sizeof the data and +1 needed assuming a _null character_ exist. — chux - Reinstate Monica, Dec 07 '18 at 06:26

chux - Reinstate Monica · Accepted Answer · 2018-12-07T07:09:50.900

How to count the size of the structure?

The size of the structure is sizeof(struct sockaddr_un).

I don't understand the +1 they added. Can you please give an explanation?

Here the code is trying to determine the length of the data used (or to be used) in the structure.

offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1

offsetof(struct sockaddr_un, sun_path) This is the offset up to, but not including, the .sun_path member. It is a worse cast calculation of the data needs of prior members and prior padding.
strlen(sun_path) is the length of the string without the null character in .sun_path[]. Should sun_path not include a null character, the result is undefined behavior (UB).
+1 is for the assumed null character in sun_path.

Such a calculation is useful to send the data in struct sockaddr_un someplace as the data needs may be significantly less than the structure needs.

I would guess that we can simply do strlen(sun_path) + sizeof(sun_family)

This is a problem (depending on usage) because

1) It does not include the null character.

2) It does not account for potential padding between members .sun_family and .sun_path. Although in this case I would be surprised to see any.

How to count the size of the structure?

1 Answers1