Explanation of piece of code used in header files for socket programming

Can anybody explain me this piece of code?

/* Pad to size of `struct sockaddr'.  */
unsigned char sin_zero[sizeof (struct sockaddr) -
                       __SOCKADDR_CO开发者_Go百科MMON_SIZE -
                       sizeof (in_port_t) -
                       sizeof (struct in_addr)];

here sin_zero is a char array but what is remaining part? It should be some integer. what this sign "-" means? Can anybody explain this to me?

Well, the - is called a "minus" :-) Seriously, everything between the square brackets is meant to calculate the size of sin_zero, which is a so-called padding. It's a member inside struct sockaddr_in, and it's just here to make sure that struct sockaddr_in is exactly of a certain size, most likely 16 bytes. The idea is to ensure that most(*) struct sockaddr variants are of the same size to avoid malloc problems.

Quoting a document I found on the subject:

The POSIX specification requires only three members in the structure: sin_family, sin_addr, and sin_port. It is acceptable for a POSIX-compliant implementation to define additional structure members, and this is normal for an Internet socket address structure. Almost all implementations add the sin_zero member so that all socket address structures are at least 16 bytes in size.

(*) In an earlier version, I wrote all. @MaximYegorushkin correctly noted that this is not true, for example struct sockaddr_un is bigger. If you want the maximum size that fits all types of addresses, use sizeof(struct sockaddr_storage). This struct exists only for this purpose (AFAIK).

sin_zero is a structure member that is used to pad out the structure to a certain minimum size. In this case, the amount of padding is calculated by starting with the desired size (sizeof (struct sockaddr)) and subtracting the space taken up by the other struct members from it.

So, to answer the question: The sign "-" here just means "subtraction".

I run into this question while learning C, too. For other beginners (like me) it could be interesting to know that it seems to be kind of important clearing *sin_zero* in your programs (more on this topic can be found HERE).

On some systems/architectures it can cause problems and on others it can be fine not explicitly clearing *sin_zero*.

Example for clearing sin_zero:

struct sockaddr_in serversin;
memset(&serversin, 0, sizeof(serversin));
serversin.sin_family = ...;
serversin.sin_addr = ...;
serversin.sin_port = ...;

To prevent some future bugs I think it's best practice to explicitly clear it.

Sockaddr contains some things like in_addr and in_port_t, and it has room for some more. The sin_zero is the remaining size in sockaddr. It is the size of sockaddr which is not filled with something else.

Supposedly the sin_zero variable would be initialised to all null bytes, and set as the last field or at the end of the sockaddr struct. The purpose of this is to set the remaining bytes to null.

sin_zeor is a char array with the size calculated with the formula in the [] brackets The '-' sign is actually a mathematical minus sign :-)

hth

Mario

It calculates the size of the array based on the size of some structs and a constant. The sign "-" means minus.