send和recv解释

int send(int s , const void * msg , size_t len , int flags );
flags取值有：
0： 与write()无异
MSG_DONTROUTE:告诉内核，目标主机在本地网络，不用查路由表
MSG_DONTWAIT:将单个I／O操作设置为非阻塞模式
MSG_OOB:指明发送的是带外信息

int recv(int s, void *buf, size_t len, int flags);
flags取值有：
0：常规操作，与read()相同
MSG_DONTWAIT:将单个I／O操作设置为非阻塞模式
MSG_OOB:指明发送的是带外信息
MSG_PEEK:可以查看可读的信息，在接收数据后不会将这些数据丢失
MSG_WAITALL:通知内核直到读到请求的数据字节数时，才返回。

int recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen);
flags取值有：
0：常规操作，与read()相同
MSG_OOB:指明发送的是带外信息
MSG_PEEK:可以查看可读的信息，在接收数据后不会将这些数据丢失

int sendto(int s , const void * msg , size_t len , int flags , const struct sockaddr * to , socklen_t tolen );
flags取值有：
0： 与write()无异
MSG_DONTROUTE:告诉内核，目标主机在本地网络，不用查路由表
MSG_OOB:指明发送的是带外信息

在UDP下，发送或者接收长度为0的数据报是允许的。仅包含IP头部、UDP头部的数据。


#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);  

DESCRIPTION

Send, sendto, and sendmsg are used to transmit a message to another socket. Send may be used only when the socket is in a connected state, while sendto and sendmsg may be used at any time.
send,sendto,sendmsgke可用于传输消息给另一个socket，send一般仅用于连接状态的，如TCP，sendto,sendmsg能用于任何情况下。

The address of the target is given by to with tolen specifying its size. The length of the message is given by len. If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

tolen是接收地址的长度，即sizeof(struct sockaddr),len标识的是所传送数据的长度，如果太长以至于无法在该协议下通过，则会返回EMSGSIZE错误，数据也将不被传送。

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

在send中，会隐式传送没有迹象的错误，通常，当发现错误时，通过返回－1来表明。

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.

当发送缓存的大小无法满足数据的存放时，send会阻塞，除非socket已经被设置为非阻塞I／O模式.在非阻塞模式下，将返回EAGAIN。系统调用select()也许能用于发现一次数据发送过多的情况。

The flags parameter is a flagword and can contain the following flags:

MSG_OOB

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

      传送的是带外信息。

MSG_DONTROUTE

Dont'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.

      从送数据包时不用网关、路由，发送到网络上与自己直接相连的主机。

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)).

      非阻塞操作，如果操作将要进入阻塞状态，则EAGAIN错误将会返回。

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.

      在流式socket中，当另一端断开连接时，不用返回SIGPIPE错误。EPIPE错误还是要返回的。

MSG_CONFIRM (Linux 2.3+ only)

Tell the link layer that forward process 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.

EBADF

An invalid descriptor was specified.

     指定了一个不合法的描述符。

ENOTSOCK

The argument s is not a socket.

     参数s不是一个socket套接字。

EFAULT

An invalid user space address was specified for a parameter.

      有参数被指定到了一个非法的用户地址空间。

EMSGSIZE

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

EAGAIN or EWOULDBLOCK

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

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. (This cannot occur in Linux, packets are just silently dropped when a device queue overflows.)

EINTR

A signal occurred.

ENOMEM

No memory available.

EINVAL

Invalid argument passed.

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.

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

int recv(int s, void *buf, size_t len, int flags);

int recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen);

int recvmsg(int s, struct msghdr *msg, int flags);  

DESCRIPTION

The recvfrom and recvmsg calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If from is not NULL, and the socket is not connection-oriented, the source address of the message is filled in. The argument fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.

The recv call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom with a NULL from parameter.

All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket(2)).

If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external variable errno set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.

The select(2) or poll(2) call may be used to determine when more data arrives.

The flags argument to a recv call is formed by OR'ing one or more of the following values:

MSG_OOB

This flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols.

MSG_PEEK

This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.

MSG_WAITALL

This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned.

MSG_NOSIGNAL

This flag turns off raising of SIGPIPE on stream sockets when the other end disappears.

MSG_TRUNC

Return the real length of the packet, even when it was longer than the passed buffer. Only valid for packet sockets.

MSG_ERRQUEUE

This flag specifies that queued errors should be received from the socket error queue. The error is passed in an ancillary message with a type dependent on the protocol (for IPv4 IP_RECVERR). The user should supply a buffer of sufficient size. See cmsg(3) and ip(7) for more information. The payload of the original packet that caused the error is passed as normal data via msg_iovec. The original destination address of the datagram that caused the error is supplied via msg_name.

For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE is set in the msghdr. After an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation.

The error is supplied in a sock_extended_err structure:

#define SO_EE_ORIGIN_NONE       0
#define SO_EE_ORIGIN_LOCAL      1
#define SO_EE_ORIGIN_ICMP       2
#define SO_EE_ORIGIN_ICMP6      3

struct sock_extended_err
{
    u_int32_t       ee_errno;   /* error number */
    u_int8_t        ee_origin;  /* where the error originated */ 
    u_int8_t        ee_type;    /* type */
    u_int8_t        ee_code;    /* code */
    u_int8_t        ee_pad;
    u_int32_t       ee_info;    /* additional information */
    u_int32_t       ee_data;    /* other data */  
    /* More data may follow */ 
};

struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol specific. The macro SOCK_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the ancillary message. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined. The payload of the packet that caused the error is passed as normal data.

For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE is set in the msghdr. After an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation.

The recvmsg call uses a msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in <sys/socket.h>:

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 */
};

Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. The fields msg_iov and msg_iovlen describe scatter-gather locations, as discussed in readv(2). The field msg_control, which has length msg_controllen, points to a buffer for other protocol control related messages or miscellaneous ancillary data. When recvmsg is called, msg_controllen should contain the length of the available buffer in msg_control; upon return from a successful call it will contain the length of the control message sequence.

The messages are of the form:

 

struct cmsghdr {
    socklen_t   cmsg_len;   /* data byte count, including hdr */
    int         cmsg_level; /* originating protocol */
    int         cmsg_type;  /* protocol-specific type */
/* followed by
    u_char      cmsg_data[]; */
};

Ancillary data should only be accessed by the macros defined in cmsg(3).

As an example, Linux uses this auxiliary data mechanism to pass extended errors, IP options or file descriptors over Unix sockets.

The msg_flags field in the msghdr is set on return of recvmsg(). It can contain several flags:

MSG_EOR

indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).

MSG_TRUNC

indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.

MSG_CTRUNC

indicates that some control data were discarded due to lack of space in the buffer for ancillary data.

MSG_OOB

is returned to indicate that expedited or out-of-band data were received.

MSG_ERRQUEUE

indicates that no data was received but an extended error from the socket error queue.

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)).

 

RETURN VALUE

These calls return the number of bytes received, 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 manual pages.

EBADF

The argument s is an invalid descriptor.

ECONNREFUSED

A remote host refused to allow the network connection (typically because it is not running the requested service).

ENOTCONN

The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).

ENOTSOCK

The argument s does not refer to a socket.

EAGAIN

The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received.

EINTR

The receive was interrupted by delivery of a signal before any data were available.

EFAULT

The receive buffer pointer(s) point outside the process's address space.

EINVAL

Invalid argument passed.