NAME

send, sendto, sendmsg - 從套接字發送消息  

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 用于向另一個套接字傳遞消息. Send 僅僅用于連接套接字,而 sendto 和 sendmsg 可用于任何情況下.

目標地址用 to 指定, tolen 定義其長度.消息的長度用 len 指定. 如果消息太長不能通過下層協議,函數將返回 EMSGSIZE 錯誤,消息也不會被送出.

在數據傳送過程中所產生的錯誤不會返回給 send. 如果發生本地錯誤,則返回-1.

當要發送的消息長度大于套接字當前可用緩沖區時, send 將阻塞,除非在套接字上設置了非阻塞式輸入輸出模式. 對于非阻塞模式,這種情況下將返回 EAGAIN 錯誤. The 系統調用 select(2) 可以用來檢測何時可以發送更多的數據.

參數 flags 是一個標志字,可以包含下列標志:

對于支持帶外數據的套接字,

MSG_OOB 將送出 out-of-band (帶外)數據(比如, SOCK_STREAM 類型的套接字); 下層協議也必須支持. 帶外 數據.

MSG_DONTROUTE

在送出分組時不使用網關.只有直接連接在網絡上的主機才能接收到數據.這個標志通常僅用于診斷和路由程序. 可路由的協議族才能使用這個標志;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,將返回 EAGAIN 錯誤(也可以用 F_SETFL fcntl(2) 設置 O_NONBLOCK 實現這個功能.)

MSG_NOSIGNAL

當流式套接字的另一端中斷連接時不發送 SIGPIPE 信號,但仍然返回 EPIPE 錯誤.

MSG_CONFIRM (僅用于Linux 2.3以上版本)

通知鏈路層發生了轉發過程:得到了另一端的成功應答. 如果鏈路層沒有收到通知,它將按照常規探測網絡上的相鄰主機(比如通過免費arp). 只能用于 SOCK_DGRAM 和 SOCK_RAW 類型的套接字,且僅對IPv4和IPv6有效.詳情參見 arp(7)

結構體 msghdr 的定義如下.詳情參見 recv(2) 和下文.

struct msghdr {
    void         * msg_name;     /*地址選項*/
    socklen_t    msg_namelen;    /*地址長度*/
    struct iovec * msg_iov;      /*消息數組*/
    size_t       msg_iovlen;     /*msg_iov中的元素個數*/
    void         * msg_control;  /*輔助信息,見下文*/
    socklen_t    msg_controllen; /*輔助數據緩沖區長度*/
    int          msg_flags;      /*接收消息標志*/
};

可以使用 msg_control 和 msg_controllen 成員發送任何控制信息.內核所能處理的最大控制消息緩沖區長度由 net.core.optmem_max sysctl對每個套接字進行限定;參見 socket(7).  

返回值

成功時返回發送的字符個數,否則返回-1.  

錯誤代碼

其中一些是套接字層產生的標準錯誤.其他的是下層協議模塊產生的;參見各自的man手冊.

EBADF

指定了非法描述符.

ENOTSOCK

參數 s 不是一個套接字.

EFAULT

參數指定的用戶地址空間非法.

EMSGSIZE

消息長度越界.

EAGAIN或者EWOULDBLOCK

套接字設置為非阻塞式,但所請求的操作需要阻塞.

ENOBUFS

網絡接口輸出隊列已滿.這通常表明接口已停止發送,也有可能是暫時性的擁擠(這不會發生在linux下,當設備隊列溢出時數據報只是被簡單丟棄.

EINTR

接收到信號.

ENOMEM

沒有可用內存.

EINVAL

傳遞的參數非法.

EPIPE

連接套接字的本地端已關閉.這種情況下進程還會接收到 SIGPIPE 信號,除非設置了 MSG_NOSIGNAL

兼容于

4.4BSD,SVr4,POSIX1003.1g草案(這些系統調用首次出現于4.2BSD). MSG_CONFIRM 是Linux所做的擴展.  

注意

上面給出的函數原型遵循Single Unix Specification, glibc2也是這么做的; flags 參數在BSD4.*中是`int',但在libc4和libc5中是`unsigned int'; 參數 len 在BSD4.*和libc4中是`int',但在libc5中是'size_t'; 參數 tolen 在BSD4.*,libc4和libc5中都是`int'. 參見 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)