Why does `accept(2)` need the `sockaddr` length as a separate pointer?

Question

I am struggling to understand the prototype of accept(2).

I have a simple server that accepts connections and returns "Hello world!\n" to the clients. The system call accept(2) takes a pointer to struct sockaddr and a pointer to an socklen_t to store the length of the struct. But sockaddr already has a field called sa_len, which seems to be done just for that.

In addition, I have this simple server (compiles under OSX, hopefully Linux too) that prints out my own socklen_t passed to accept and then the value of sa_len: they are the same, 28 in this case, on OSX.

EDIT: After a bit more testing, it seems that sa_len is not necessarily the same as the length stored in the pointer.

Why does accept(2) need the length as a separate pointer?

---

For reference, I'm posting the example server below. You can compile with:

gcc -Wall -Wextra main.c

And then run:

./a.out

In another terminal, connect to it:

telnet 127.0.0.1 3000

---

#include <errno.h>
#include <sys/socket.h>
#include <unistd.h>
#include <netdb.h>
#include <stdio.h>
#include <netinet/in.h>
#include <strings.h>

// default port if none if passed in the program arguments
#define FTP_PORT_DEFAULT ("3000")

// number of connections allow to wait for a call to accept
#define FTP_BACKLOG (5)

int main(int ac, char **av)
{
    struct addrinfo     hints; // hints used to get address information
    struct addrinfo     *sai; // server address information
    int                 ss; // server socket
    char            *port;

    port = (ac >= 2) ? av[1] : FTP_PORT_DEFAULT;
    bzero(&hints, sizeof(hints));

    // using AF_INIT6 instead of PF_INET6
    // http://stackoverflow.com/questions/6729366/
    hints.ai_family = AF_INET6;
    hints.ai_socktype = SOCK_STREAM;

    // intended for use with `bind`
    hints.ai_flags = AI_PASSIVE;

    // passing `NULL` as the hostname tells `getaddrinfo` to use
    // any available local IP address
    if (getaddrinfo(NULL, port, &hints, &sai) != 0)
        return (-1);

    if ((ss =
         socket(sai->ai_family, sai->ai_socktype, sai->ai_protocol))== -1)
        return (-1);

    if (bind(ss, sai->ai_addr, sai->ai_addrlen) == -1 ||
        listen(ss, FTP_BACKLOG) == -1)
    {
        close(ss);
        return (-1);
    }

    while (1)
    {
        int cs;
        struct sockaddr csockaddr;
        unsigned csockaddr_len;

        bzero(&csockaddr, sizeof(csockaddr));
        cs = accept(ss, &csockaddr, &csockaddr_len);
        // handle "fatal" errors that I don't think I can recover from
        // other than by relaunching the server
        if (cs == EBADF || cs == ECONNABORTED || cs == ENOTSOCK ||
                cs == EOPNOTSUPP)
        {
            break ;
        }
        printf("my len %u / internal len %u\n",
                   csockaddr_len, csockaddr.sa_len);

        if (fork() == 0)
        {
            write(cs, "Hello world!\n", 13);
            close(cs);
        }
        break;
    }
    close(ss);

    return 0;
}

Answer 1

sa_len is not portable. It was added to BSD only in 1988, a few years after 4.3BSD, but it's not in POSIX and it's not in Linux.