user-0 提问: 请教linux下socket编程中send()函数如何强制其将数据发出去？

回复:
user: user-1
content: 都是做异步网络发送： https://github.com/owenliang/simple_kit/tree/master/simple_io

user: user-2
content: SEND Section: Linux Programmer's Manual (2) Updated: 2004-07-01  --------------------------------------------------------------------------------    NAME send, sendto, sendmsg - send a message on 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.  MSG_MORE (Since Linux 2.4.4)  The caller has more data to send. This flag is used with TCP sockets to obtain the same effect as the TCP_CORK socket option (see tcp(7)), with the difference that this flag can be set on a per-call basis. Since Linux 2.6, this flag is also supported for UDP sockets, and informs the kernel to package all of the data sent in calls with this flag set into a single datagram which is only transmitted when a call is performed that does not specify this flag.  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 On success, these calls return the number of characters sent. On error, -1 is returned, and errno is set appropriately.    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.  EACCES  (For Unix domain sockets, which are identified by pathname) Write permission is denied on the destination socket file, or search permission is denied for one of the directories the path prefix. (See path_resolution(2).)  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), getsockopt(2), recv(2), select(2), sendfile(2), socket(2), write(2), ip(7), socket(7), tcp(7), udp(7)  --------------------------------------------------------------------------------

user: user-3
content: 网卡的发送需要排队的吧，使用send，是用户区的，从网卡上发出去是内核做的；应该没有办法使用send将数据立即发送到网络上，因为你不能直接操作网卡；

user: user-0
content: 引用 1 楼 a991556 的回复:强制发送？没听说过 socket应该有个缓冲区吧，应该就是刷新缓冲区的意思

user: user-4
content: 有个No Delay设置，会立马发送，具体可以网上自己搜一下。

user: user-5
content: 强制发送？没听说过