Socket层实现系列 — send()类发送函数的实现

主要内容:socket发送函数的系统调用、Socket层实现。

内核版本:3.15.2 

我的博客:http://blog.csdn.net/zhangskd

 

发送流程图

 

以下是send()、sendto()、sendmsg()和sendmmsg()的发送流程图,这四个函数除了在系统调用层面

上有些差别,在Socket层和TCP层的实现都是相同的。

 

 

应用层

 

应用层可以使用以下Socket函数来发送数据:

ssize_t write(int fd, const void *buf, size_t count);

ssize_t send(int s, const void *buf, size_t len, int flags);

ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);

ssize_t sendmsg(int s, const struct msghdr *msg, int flags);

int sendmmsg(int s, struct mmsghdr *msgvec,  unsigned int vlen, unsigned int flags);

 

这些发送函数有什么区别呢?

当flags为0时,send()和write()功能相同。

send(s, buf, len, flags)和sendto(s, buf, len, flags, NULL, 0)功能相同。

write()和send()在套接字处于连接状态时可以使用,而sendto()、sendmsg()和sendmmsg()在任何时候都可用。

 

用户层的数据最终都是以消息头来描述的。

struct msghdr {
    void *msg_name; /* optional address,目的地址 */
    socklen_t msg_namelen; /* size of address,目的地址的长度 */
    struct iovec *msg_iov; /* scatter/gather array,分散的数据块数组 */
    size_t msg_iovlen; /* #elements in msg_iov,分散的数据块个数 */
    void *msg_control; /* ancillary data, 控制数据 */
    socklen_t msg_controllen; /* ancillary data buffer len,控制数据的长度 */
    int msg_flags; /* flags on received message */
};
/* Structure for scatter/gather I/O. */
struct iovec {
    void *iov_base; /* Pointer to data. */
    size_t iov_len; /* Length of data. */
};

 

发送默认为阻塞发送,也可以设置为非阻塞发送。

非阻塞标志:O_NONBLOCK、MSG_DONTWAIT

When the message does not fit into the send buffer of the socket, send() normally blocks, unless the

socket has been placed in non-blocking I/O mode.

Enables non-blocking operations; if the operation would block, EAGAIN is returned (this can also be enabled

using the O_NON-BLOCK with the F_SETEL fcntl(2)).

 

系统调用

 

发送函数是由glibc提供的,声明位于include/sys/socket.h中,实现位于sysdeps/mach/hurd/connect.c中,

主要是用来从用户空间进入名为sys_socketcall的系统调用,并传递参数。sys_socketcall()实际上是所有

socket函数进入内核空间的共同入口。

SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
{
    ...
    switch(call) {
    ...
    case SYS_SEND:
        err = sys_send(a0, (void __user *)a1, a[2], a[3]);
        break;

    case SYS_SENDTO:
        err = sys_sendto(a0, (void __user *)a1 a[2], a[3], (struct sockaddr __user *)a[4], a[5]);
        break;

    ...
    case SYS_SENDMSG:
        err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]);
        break;

    case SYS_SENDMMSG:
        err = sys_sendmmsg(a0, (struct msghdr __user *)a1, a[2], a[3]);
        break;
    ...
    }
}

send()其实是sendto()的一种特殊情况。

SYSCALL_DEFINE4(send, int, fd, void __user *, buff, size_t, len, unsigned, flags)
{
    return sys_sendto(fd, buff, len, flags, NULL, 0);
}

sendto()初始化了消息头,接着就调用sock_sendmsg()来处理。

/* Send a datagram to a given address. We move the address into kernel space
 * and check the user space data area is readable before invoking the protocol.
 */

SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len, unsigned, flags,
    struct sockaddr __user *, addr, int, addr_len)
{
    struct socket *sock;
    struct sockaddr_storage address;
    int err;
    struct msghdr msg;
    struct iovec iov;
    int fput_needed;

    if (len > INT_MAX)
       len = INT_MAX;

    /* 通过文件描述符fd,找到对应的socket实例。
     * 以fd为索引从当前进程的文件描述符表files_struct实例中找到对应的file实例,
     * 然后从file实例的private_data成员中获取socket实例。
     */
    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (! sock)
        goto out;

    /* 初始化消息头 */
    iov.iov_base = buff;
    iov.iov_len = len;
    msg.msg_name = NULL;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1; /* 只有一个数据块 */
    msg.msg_control = NULL;
    msg.msg_controllen = 0;
    msg.msg_namelen = 0; 

    if (addr) {
        /* 把套接字地址从用户空间拷贝到内核空间 */
        err = move_addr_to_kernel(addr, addr_len, &address);
        if (err < 0)
            goto out_put;

        msg.msg_name = (struct sockaddr *)&address;
        msg.msg_namelen = addr_len;
    }

    /* 如果设置了非阻塞标志 */
    if (sock->file->f_flags & O_NONBLOCK)
        flags |= MSG_DONTWAIT;
    msg.msg_flags = flags;
 
    /* 调用统一的发送入口函数sock_sendmsg() */
    err = sock_sendmsg(sock , &msg, len);

out_put:
    fput_light(sock->file, fput_needed);
out:
    return err;
}
struct msghdr {
    void *msg_name; /* ptr to socket address structure */
    int msg_namelen; /* size of socket address structure */
    struct iovec *msg_iov; /* scatter/gather array,分散的数据块数组 */
    __kernel_size_t msg_iovlen; /* #elements in msg_iov,分散的数据块个数 */
    void *msg_control; /* ancillary data, 控制数据 */
    __kernel_size_t msg_controllen; /* ancillary data buffer len,控制数据的长度 */
    unsigned int msg_flags; /* flags on received message */
};

/* Structure for scatter/gather I/O. */
struct iovec {
    void *iov_base; /* Pointer to data. */
    __kernel_size_t iov_len; /* Length of data. */
};

/* For recvmmsg/ sendmmsg */
struct mmsghdr {
    struct msghdr msg_hdr;
    unsigned int msg_len;
};

sock_sendmsg()在初始化异步IO控制块后,调用__sock_sendmsg()。

int sock_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
{
    struct kiocb iocb;
    struct sock_iocb siocb;
    int ret;

    init_sync_kiocb(&iocb, NULL);
    iocb.private = &siocb;

    ret = __sock_sendmsg(&iocb, sock, msg, size);

    /* iocb queued, will get completion event */
    if (-EIOCBQUEUED == ret)
        ret = wait_on_sync_kiocb(&iocb);

    return ret;
}

/* AIO控制块 */
struct kiocb {
    struct file *ki_filp;
    struct kioctx *ki_ctx; /* NULL for sync ops,如果是同步的则为NULL */
    kiocb_cancel_fn *ki_cancel;
    void *private; /* 指向sock_iocb */
   
    union {
        void __user *user;
        struct task_struct *tsk; /* 执行io的进程 */
    } ki_obj;

    __u64 ki_user_data; /* user's data for completion */
    loff_t ki_pos;
    size_t ki_nbytes; /* copy of iocb->aio_nbytes */

    struct list_head ki_list; /* the aio core uses this for cancellation */
    /* If the aio_resfd field of the userspace iocb is not zero,
     * this is the underlying eventfd context to deliver events to.
     */
    struct eventfd_ctx *ki_eventfd;
};

__sock_sendmsg()会调用Socket层的发送函数,如果是SOCK_STREAM,

那么接着就调用inet_sendmsg()处理。

static inline int __sock_sendmsg(struct kiocb *iocb, struct socket *sock,
       struct msghdr *msg, size_t size)
{
    int err = security_socket_sendmsg(sock, msg, size);
    return err ?: __sock_sendmsg_nosec(iocb, sock, msg, size);
}

static inline int __sock_sendmsg_nosec(struct kiocb *iocb, struct socket *sock,
        struct msghdr *msg, size_t size)
{
    struct sock_iocb *si = kiocb_to_siocb(iocb);
    si->sock = sock;
    si->scm = NULL;
    si->msg = msg;
    si->size = size;

    /* 调用Socket层的操作函数,如果是SOCK_STREAM,则proto_ops为inet_stream_ops,
     * 函数指针指向inet_sendmsg()。
     */
    return sock->ops->sendmsg(iocb, sock, msg, size);
} 

sendmsg()和sendmmsg()在系统调用函数中也是拷贝用户空间的数据到内核消息头,最后调用

Socket层的发送函数inet_sendmsg()进行下一步处理,这里不再赘述。

 

Socket层

 

SOCK_STREAM套接口的socket层操作函数集实例为inet_stream_ops,其中发送函数为inet_sendmsg()。

const struct proto_ops inet_stream_ops = {
    .family = PF_INET,
    .owner = THIS_MODULE,
    ...
    .sendmsg = inet_sendmsg,
    ...
};

inet_sendmsg()主要调用TCP层的发送函数tcp_sendmsg()来处理。

int inet_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg, size_t size)
{
    struct sock *sk = sock->sk;
    sock_rps_record_flow(sk);

    /* We may need to bnd the socket.
     * 如果连接还没有分配本地端口,且允许自动绑定,那么给连接绑定一个本地端口。
     * tcp_prot的no_autobaind为true,所以TCP是不允许自动绑定端口的。
     */
    if (! inet_sk(sk)->inet_num && ! sk->sk_prot->no_autobind && inet_autobind(s))
        return -EAGAIN;

    /* 如果传输层使用的是TCP,则sk_prot为tcp_prot,sendmsg指向tcp_sendmsg() */
    return sk->sk_prot->sendmsg(iocb, sk, msg, size);
}
 
/* Automatically bind an unbound socket. */
static int inet_autobind(struct sock *sk)
{
    struct inet_sock *inet;

    /* We may need to bind the socket. */
    lock_sock(sk);

    /* 如果还没有分配本地端口 */
    if (! inet->inet_num) {

        /* SOCK_STREAM套接口的TCP操作函数集为tcp_prot,其中端口绑定函数为
         * inet_csk_get_port()。
         */
        if (sk->sk_prot->get_port(sk, 0)) {
            release_sock(sk);
            return -EAGAIN;
        }
        inet->inet_sport = htons(inet->inet_num);
    }

    release_sock(sk);
    return 0;
}

 

转载于:https://www.cnblogs.com/aiwz/p/6333237.html

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