sendmsg 中文man页面

NAME

send, sendto, sendmsg - 从套接字发送消息  

概述

#include <sys/types.h>
#include <sys/socket.h>

int send(int s, const void *msg, size_t len, int flags);
int sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);
int sendmsg(int s, const struct msghdr *msg, int flags);  

描述

Send, sendto, 和 sendmsg 用于向另一个套接字传递消息. Send 仅仅用于连接套接字,而 sendto 和 sendmsg 可用于任何情况下.

目标地址用 to 指定, tolen 定义其长度.消息的长度用 len 指定. 如果消息太长不能通过下层协议,函数将返回 EMSGSIZE 错误,消息也不会被送出.

在数据传送过程中所产生的错误不会返回给 send. 如果发生本地错误,则返回-1.

当要发送的消息长度大于套接字当前可用缓冲区时, send 将阻塞,除非在套接字上设置了非阻塞式输入输出模式. 对于非阻塞模式,这种情况下将返回 EAGAIN 错误. The 系统调用 select(2) 可以用来检测何时可以发送更多的数据.

参数 flags 是一个标志字,可以包含下列标志:

对于支持带外数据的套接字,

MSG_OOB 将送出 out-of-band (带外)数据(比如, SOCK_STREAM 类型的套接字); 下层协议也必须支持. 带外 数据.

MSG_DONTROUTE

在送出分组时不使用网关.只有直接连接在网络上的主机才能接收到数据.这个标志通常仅用于诊断和路由程序. 可路由的协议族才能使用这个标志;包套接字不可以.

MSG_DONTWAIT

使用非阻塞式操作;如果操作需要阻塞,将返回 EAGAIN 错误(也可以用 F_SETFL fcntl(2) 设置 O_NONBLOCK 实现这个功能.)

MSG_NOSIGNAL

当流式套接字的另一端中断连接时不发送 SIGPIPE 信号,但仍然返回 EPIPE 错误.

MSG_CONFIRM (仅用于Linux 2.3以上版本)

通知链路层发生了转发过程:得到了另一端的成功应答. 如果链路层没有收到通知,它将按照常规探测网络上的相邻主机(比如通过免费arp). 只能用于 SOCK_DGRAM 和 SOCK_RAW 类型的套接字,且仅对IPv4和IPv6有效.详情参见 arp(7)

结构体 msghdr 的定义如下.详情参见 recv(2) 和下文.

struct msghdr {
    void         * msg_name;     /*地址选项*/
    socklen_t    msg_namelen;    /*地址长度*/
    struct iovec * msg_iov;      /*消息数组*/
    size_t       msg_iovlen;     /*msg_iov中的元素个数*/
    void         * msg_control;  /*辅助信息,见下文*/
    socklen_t    msg_controllen; /*辅助数据缓冲区长度*/
    int          msg_flags;      /*接收消息标志*/
};

可以使用 msg_control 和 msg_controllen 成员发送任何控制信息.内核所能处理的最大控制消息缓冲区长度由 net.core.optmem_max sysctl对每个套接字进行限定;参见 socket(7).  

返回值

成功时返回发送的字符个数,否则返回-1.  

错误代码

其中一些是套接字层产生的标准错误.其他的是下层协议模块产生的;参见各自的man手册.

EBADF

指定了非法描述符.

ENOTSOCK

参数 s 不是一个套接字.

EFAULT

参数指定的用户地址空间非法.

EMSGSIZE

消息长度越界.

EAGAIN或者EWOULDBLOCK

套接字设置为非阻塞式,但所请求的操作需要阻塞.

ENOBUFS

网络接口输出队列已满.这通常表明接口已停止发送,也有可能是暂时性的拥挤(这不会发生在linux下,当设备队列溢出时数据报只是被简单丢弃.

EINTR

接收到信号.

ENOMEM

没有可用内存.

EINVAL

传递的参数非法.

EPIPE

连接套接字的本地端已关闭.这种情况下进程还会接收到 SIGPIPE 信号,除非设置了 MSG_NOSIGNAL

兼容于

4.4BSD,SVr4,POSIX1003.1g草案(这些系统调用首次出现于4.2BSD). MSG_CONFIRM 是Linux所做的扩展.  

注意

上面给出的函数原型遵循Single Unix Specification, glibc2也是这么做的; flags 参数在BSD4.*中是`int',但在libc4和libc5中是`unsigned int'; 参数 len 在BSD4.*和libc4中是`int',但在libc5中是'size_t'; 参数 tolen 在BSD4.*,libc4和libc5中都是`int'. 参见 accept(2).  

SEE ALSO

fcntl(2), recv(2), select(2), getsockopt(2), sendfile(2), socket(2), write(2), socket(7), ip(7), tcp(7), udp(7)

#p#

NAME

send, sendto, sendmsg - send a message from a socket  

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>

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);  

DESCRIPTION

The system calls send, sendto, and sendmsg are used to transmit a message to another socket.

The send call may be used only when the socket is in a connected state (so that the intended recipient is known). The only difference between send and write is the presence of flags. With zero flags parameter, send is equivalent to write. Also, send(s,buf,len) is equivalent to sendto(s,buf,len,NULL,0).

The parameter s is the file descriptor of the sending socket.

If sendto is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the parameters to and tolen are ignored (and the error EISCONN may be returned when they are not NULL and 0), and the error ENOTCONN is returned when the socket was not actually connected. Otherwise, the address of the target is given by to with tolen specifying its size. For sendmsg, the address of the target is given by msg.msg_name, with msg.msg_namelen specifying its size.

For send and sendto, the message is found in buf and has length len. For sendmsg, the message is pointed to by the elements of the array msg.msg_iov. The sendmsg call also allows sending ancillary data (also known as control information).

If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a send. Locally detected errors are indicated by a return value of -1.

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. In non-blocking mode it would return EAGAIN in this case. The select(2) call may be used to determine when it is possible to send more data.

The flags parameter is the bitwise OR of zero or more of the following flags.

MSG_OOB

Sends out-of-band data on sockets that support this notion (e.g. of type SOCK_STREAM); the underlying protocol must also support out-of-band data.

MSG_EOR

Terminates a record (when this notion is supported, as for sockets of type SOCK_SEQPACKET).

MSG_DONTROUTE

Don't use a gateway to send out the packet, only send to hosts on directly connected networks. This is usually used only by diagnostic or routing programs. This is only defined for protocol families that route; packet sockets don't.

MSG_DONTWAIT

Enables non-blocking operation; if the operation would block, EAGAIN is returned (this can also be enabled using the O_NONBLOCK with the F_SETFL fcntl(2)).

MSG_NOSIGNAL

Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the connection. The EPIPE error is still returned.

MSG_CONFIRM (Linux 2.3+ only)

Tell the link layer that forward progress happened: you got a successful reply from the other side. If the link layer doesn't get this it'll regularly reprobe the neighbour (e.g. via a unicast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and currently only implemented for IPv4 and IPv6. See arp(7) for details.

The definition of the msghdr structure follows. See recv(2) and below for an exact description of its fields.

struct msghdr {
    void         * msg_name;     /* optional address */
    socklen_t    msg_namelen;    /* size of address */
    struct iovec * msg_iov;      /* scatter/gather array */
    size_t       msg_iovlen;     /* # elements in msg_iov */
    void         * msg_control;  /* ancillary data, see below */
    socklen_t    msg_controllen; /* ancillary data buffer len */
    int          msg_flags;      /* flags on received message */
};

You may send control information using the msg_control and msg_controllen members. The maximum control buffer length the kernel can process is limited per socket by the net.core.optmem_max sysctl; see socket(7).  

RETURN VALUE

The calls return the number of characters sent, or -1 if an error occurred.  

ERRORS

These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their respective manual pages.

EAGAIN or EWOULDBLOCK

The socket is marked non-blocking and the requested operation would block.

EBADF

An invalid descriptor was specified.

ECONNRESET

Connection reset by peer.

EDESTADDRREQ

The socket is not connection-mode, and no peer address is set.

EFAULT

An invalid user space address was specified for a parameter.

EINTR

A signal occurred before any data was transmitted.

EINVAL

Invalid argument passed.

EISCONN

The connection-mode socket was connected already but a recipient was specified. (Now either this error is returned, or the recipient specification is ignored.)

EMSGSIZE

The socket type requires that message be sent atomically, and the size of the message to be sent made this impossible.

ENOBUFS

The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion. (Normally, this does not occur in Linux. Packets are just silently dropped when a device queue overflows.)

ENOMEM

No memory available.

ENOTCONN

The socket is not connected, and no target has been given.

ENOTSOCK

The argument s is not a socket.

EOPNOTSUPP

Some bit in the flags argument is inappropriate for the socket type.

EPIPE

The local end has been shut down on a connection oriented socket. In this case the process will also receive a SIGPIPE unless MSG_NOSIGNAL is set.

CONFORMING TO

4.4BSD, SVr4, POSIX 1003.1-2001. These function calls appeared in 4.2BSD.

POSIX only describes the MSG_OOB and MSG_EOR flags. The MSG_CONFIRM flag is a Linux extension.  

NOTE

The prototypes given above follow the Single Unix Specification, as glibc2 also does; the flags argument was `int' in BSD 4.*, but `unsigned int' in libc4 and libc5; the len argument was `int' in BSD 4.* and libc4, but `size_t' in libc5; the tolen argument was `int' in BSD 4.* and libc4 and libc5. See also accept(2).  

BUGS

Linux may return EPIPE instead of ENOTCONN.  

SEE ALSO

fcntl(2), recv(2), select(2), getsockopt(2), sendfile(2), socket(2), write(2), socket(7), ip(7), tcp(7), udp(7)