send+recv

zlm8715

                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
#include
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
#include
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
:
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.
               
               
               
               
               

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