网络编程

http://linux.die.net/man/7/socket

#include <sys/types.h>

#include<sys/socket.h>

PF_INET

AF_INET

PF_INET6

PF_UNIX

SOCK_STREAM ---------- 需要自己对报文进行分解，一次可能读出部分，或可能读出多个报文。

SOCK_DGRAM ---------- 一次只读出一个报文

SOCK_SEQPACKET ---------- 一次只读出一个报文

INADDR_ANY 本地任意地址

socket

bind

listen ： 对SOCK_DGRAM操作返回错误；

accept

connect：不能用于UDP，否则会有很多副作用。

write

read

send

sendto

recvfrom  --- 连接，非连接都可以；在连接状态下，返回0，表示连接断开；非阻塞模式下，当read、write返回EAGAIN时，表示无数据可读了；

recvmsg ---  同上。

recv --- 连接。返回0，则表示对方连接断开！

shutdown

套接字选项：
setsockopt：分SOL_SOCKET, IPPROTO_P, IPPROTO_TCP三个级别设置属性！！！

fcntl:

ioctl:

Name

socket - Linux socket interface

Synopsis

#include <sys/socket.h>

sockfd = socket(int socket_family, int socket_type, int protocol);

Description

This manual page describes the Linux networking socket layer user interface. The BSD compatible sockets are the uniform interface between the user process and the network protocol stacks in the kernel. The protocol modules are grouped into protocol familieslike AF_INET, AF_IPX, AF_PACKET and socket types like SOCK_STREAM orSOCK_DGRAM. See socket(2) for more information on families and types.

Socket-layer functions

These functions are used by the user process to send or receive packets and to do other socket operations. For more information see their respective manual pages.

socket(2) creates a socket, connect(2) connects a socket to a remote socket address, the bind(2) function binds a socket to a local socket address, listen(2) tells the socket that new connections shall be accepted, and accept(2) is used to get a new socket with a new incoming connection. socketpair(2) returns two connected anonymous sockets (only implemented for a few local families like AF_UNIX)

send(2), sendto(2), and sendmsg(2) send data over a socket, and recv(2), recvfrom(2), recvmsg(2) receive data from a socket. poll(2) and select(2) wait for arriving data or a readiness to send data. In addition, the standard I/O operations like write(2), writev(2), sendfile(2), read(2), and readv(2) can be used to read and write data.

getsockname(2) returns the local socket address and getpeername(2) returns the remote socket address. getsockopt(2) and setsockopt(2) are used to set or get socket layer or protocol options. ioctl(2) can be used to set or read some other options.

close(2) is used to close a socket. shutdown(2) closes parts of a full-duplex socket connection.

Seeking, or calling pread(2) or pwrite(2) with a nonzero position is not supported on sockets.

It is possible to do nonblocking I/O on sockets by setting the O_NONBLOCK flag on a socket file descriptor using fcntl(2). Then all operations that would block will (usually) return with EAGAIN (operation should be retried later); connect(2) will return EINPROGRESS error. The user can then wait for various events via poll(2) or select(2).

An alternative to poll(2) and select(2) is to let the kernel inform the application about events via a SIGIO signal. For that the O_ASYNC flag must be set on a socket file descriptor via fcntl(2) and a valid signal handler for SIGIO must be installed via sigaction(2). See the Signals discussion below.

Socket address structures

Each socket domain has its own format for socket addresses, with a domain-specific address structure. Each of these structures begins with an integer "family" field (typed as sa_family_t) that indicates the type of the address structure. This allows the various system calls (e.g.,  connect(2),  bind(2),  accept(2),  getsockname(2),  getpeername(2)), which are generic to all socket domains, to determine the domain of a particular socket address.

To allow any type of socket address to be passed to interfaces in the sockets API, the type struct sockaddr is defined. The purpose of this type is purely to allow casting of domain-specific socket address types to a "generic" type, so as to avoid compiler warnings about type mismatches in calls to the sockets API.

In addition, the sockets API provides the data type struct sockaddr_storage. This type is suitable to accommodate all supported domain-specific socket address structures; it is large enough and is aligned properly. (In particular, it is large enough to hold IPv6 socket addresses.) The structure includes the following field, which can be used to identify the type of socket address actually stored in the structure:

sa_family_t ss_family;

The  sockaddr_storage structure is useful in programs that must handle socket addresses in a generic way (e.g., programs that must deal with both IPv4 and IPv6 socket addresses).

Socket options

These socket options can be set by using  setsockopt(2) and read with  getsockopt(2) with the socket level set to  SOL_SOCKET for all sockets: SO_ACCEPTCONN

Returns a value indicating whether or not this socket has been marked to accept connections with  listen(2). The value 0 indicates that this is not a listening socket, the value 1 indicates that this is a listening socket. This socket option is read-only.

SO_BINDTODEVICE

Bind this socket to a particular device like "eth0", as specified in the passed interface name. If the name is an empty string or the option length is zero, the socket device binding is removed. The passed option is a variable-length null-terminated interface name string with the maximum size of  IFNAMSIZ. If a socket is bound to an interface, only packets received from that particular interface are processed by the socket. Note that this only works for some socket types, particularly  AF_INET sockets. It is not supported for packet sockets (use normal  bind(2) there).

Before Linux 3.8, this socket option could be set, but could not retrieved with getsockopt(2). Since Linux 3.8, it is readable. The optlen argument should contain the buffer size available to receive the device name and is recommended to be IFNAMSZ bytes. The real device name length is reported back in the optlen argument.

SO_BROADCAST

Set or get the broadcast flag. When enabled, datagram sockets are allowed to send packets to a broadcast address. This option has no effect on stream-oriented sockets.

SO_BSDCOMPAT

Enable BSD bug-to-bug compatibility. This is used by the UDP protocol module in Linux 2.0 and 2.2. If enabled ICMP errors received for a UDP socket will not be passed to the user program. In later kernel versions, support for this option has been phased out: Linux 2.4 silently ignores it, and Linux 2.6 generates a kernel warning (printk()) if a program uses this option. Linux 2.0 also enabled BSD bug-to-bug compatibility options (random header changing, skipping of the broadcast flag) for raw sockets with this option, but that was removed in Linux 2.2.

SO_DEBUG

Enable socket debugging. Only allowed for processes with the  CAP_NET_ADMIN capability or an effective user ID of 0.

SO_DOMAIN (since Linux 2.6.32)

Retrieves the socket domain as an integer, returning a value such as  AF_INET6. See  socket(2) for details. This socket option is read-only.

SO_ERROR

Get and clear the pending socket error. This socket option is read-only. Expects an integer.

SO_DONTROUTE

Don't send via a gateway, only send to directly connected hosts. The same effect can be achieved by setting the  MSG_DONTROUTE flag on a socket  send(2) operation. Expects an integer boolean flag.

SO_KEEPALIVE

Enable sending of keep-alive messages on connection-oriented sockets. Expects an integer boolean flag.

SO_LINGER

Sets or gets the  SO_LINGER option. The argument is a  linger structure.

struct linger {
    int l_onoff;    /* linger active */
    int l_linger;   /* how many seconds to linger for */
};

When enabled, a close(2) or shutdown(2) will not return until all queued messages for the socket have been successfully sent or the linger timeout has been reached. Otherwise, the call returns immediately and the closing is done in the background. When the socket is closed as part of exit(2), it always lingers in the background.

TCP连接断开的时候调用closesocket函数，有优雅的断开和强制断开两种方式.
那么如何设置断开连接的方式呢？是通过设置socket描述符一个linger结构体属性。
linger结构体数据结构如下：
struct linger
{
int l_onoff;
int l_linger;
};
有三种组合方式：
第一种
l_onoff = 0;
l_linger忽略
这种方式下，就是在closesocket的时候立刻返回，底层会将未发送完的数据发送完成后再释放资源，也就
是优雅的退出。
第二种
l_onoff非零
l_linger = 0;
这种方式下，在调用closesocket的时候同样会立刻返回，但不会发送未发送完成的数据，而是通过一个REST包强制的关闭socket描述符，也就是强制的退出。
第三种
l_onoff非零
l_linger > 0
这种方式下，在调用closesocket的时候不会立刻返回，内核会延迟一段时间，这个时间就由l_linger得值来决定。如果超时时间到达之前，发送完未发送的数据(包括FIN包)并得到另一端的确认，closesocket会返回正确，socket描述符优雅性退出。否则，closesocket会直接返回错误值，未发送数据丢失，socket描述符被强制性退出。需要注意的时，如果socket描述符被设置为非堵塞型，则closesocket会直接返回值。
Linux下需要包含头文件<arpa/inet.h>

SO_MARK (since Linux 2.6.25)

Set the mark for each packet sent through this socket (similar to the netfilter MARK target but socket-based). Changing the mark can be used for mark-based routing without netfilter or for packet filtering. Setting this option requires the  CAP_NET_ADMIN capability.

SO_OOBINLINE

If this option is enabled, out-of-band data is directly placed into the receive data stream. Otherwise out-of-band data is only passed when the  MSG_OOB flag is set during receiving.

SO_PASSCRED

Enable or disable the receiving of the  SCM_CREDENTIALS control message. For more information see  unix(7).

SO_PEERCRED

Return the credentials of the foreign process connected to this socket. This is only possible for connected  AF_UNIX stream sockets and  AF_UNIX stream and datagram socket pairs created using  socketpair(2); see  unix(7). The returned credentials are those that were in effect at the time of the call to  connect(2) or  socketpair(2). The argument is a  ucred structure; define the  GNU_SOURCE feature test macro to obtain the definition of that structure from  <sys/socket.h>. This socket option is read-only.

SO_PRIORITY

Set the protocol-defined priority for all packets to be sent on this socket. Linux uses this value to order the networking queues: packets with a higher priority may be processed first depending on the selected device queueing discipline. For  ip(7), this also sets the IP type-of-service (TOS) field for outgoing packets. Setting a priority outside the range 0 to 6 requires the  CAP_NET_ADMIN capability.

SO_PROTOCOL (since Linux 2.6.32)

Retrieves the socket protocol as an integer, returning a value such as  IPPROTO_SCTP. See  socket(2) for details. This socket option is read-only.

SO_RCVBUF

Sets or gets the maximum socket receive buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using  setsockopt(2), and this doubled value is returned by  getsockopt(2). The default value is set by the  /proc/sys/net/core/rmem_default file, and the maximum allowed value is set by the /proc/sys/net/core/rmem_max file. The minimum (doubled) value for this option is 256.

SO_RCVBUFFORCE (since Linux 2.6.14)

Using this socket option, a privileged ( CAP_NET_ADMIN) process can perform the same task as  SO_RCVBUF, but the  rmem_max limit can be overridden.

SO_RCVLOWAT and  SO_SNDLOWAT

Specify the minimum number of bytes in the buffer until the socket layer will pass the data to the protocol ( SO_SNDLOWAT) or the user on receiving ( SO_RCVLOWAT). These two values are initialized to 1.  SO_SNDLOWAT is not changeable on Linux ( setsockopt(2) fails with the error  ENOPROTOOPT).  SO_RCVLOWAT is changeable only since Linux 2.4. The  select(2) and  poll(2) system calls currently do not respect the  SO_RCVLOWAT setting on Linux, and mark a socket readable when even a single byte of data is available. A subsequent read from the socket will block until  SO_RCVLOWAT bytes are available.

SO_RCVTIMEO and  SO_SNDTIMEO

Specify the receiving or sending timeouts until reporting an error. The argument is a  struct timeval. If an input or output function blocks for this period of time, and data has been sent or received, the return value of that function will be the amount of data transferred; if no data has been transferred and the timeout has been reached then -1 is returned with  errno set to  EAGAIN or  EWOULDBLOCK, or  EINPROGRESS (for  connect(2)) just as if the socket was specified to be nonblocking. If the timeout is set to zero (the default) then the operation will never timeout. Timeouts only have effect for system calls that perform socket I/O (e.g.,  read(2),  recvmsg(2),  send(2), sendmsg(2)); timeouts have no effect for  select(2),  poll(2),  epoll_wait(2), and so on.

SO_REUSEADDR

Indicates that the rules used in validating addresses supplied in a  bind(2) call should allow reuse of local addresses. For  AF_INET sockets this means that a socket may bind, except when there is an active listening socket bound to the address. When the listening socket is bound to  INADDR_ANY with a specific port then it is not possible to bind to this port for any local address. Argument is an integer boolean flag.

SO_SNDBUF

Sets or gets the maximum socket send buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using  setsockopt(2), and this doubled value is returned by  getsockopt(2). The default value is set by the  /proc/sys/net/core/wmem_default file and the maximum allowed value is set by the /proc/sys/net/core/wmem_max file. The minimum (doubled) value for this option is 2048.

SO_SNDBUFFORCE (since Linux 2.6.14)

Using this socket option, a privileged ( CAP_NET_ADMIN) process can perform the same task as  SO_SNDBUF, but the  wmem_max limit can be overridden.

SO_TIMESTAMP

Enable or disable the receiving of the  SO_TIMESTAMP control message. The timestamp control message is sent with level  SOL_SOCKET and the  cmsg_data field is a struct timeval indicating the reception time of the last packet passed to the user in this call. See  cmsg(3) for details on control messages.

SO_TYPE

Gets the socket type as an integer (e.g.,  SOCK_STREAM). This socket option is read-only.

Signals

When writing onto a connection-oriented socket that has been shut down (by the local or the remote end)  SIGPIPE is sent to the writing process and  EPIPE is returned. The signal is not sent when the write call specified the  MSG_NOSIGNAL flag.

When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO is sent when an I/O event occurs. It is possible to use poll(2) or select(2) in the signal handler to find out which socket the event occurred on. An alternative (in Linux 2.2) is to set a real-time signal using the F_SETSIG fcntl(2); the handler of the real time signal will be called with the file descriptor in the si_fd field of its siginfo_t. See fcntl(2) for more information.

Under some circumstances (e.g., multiple processes accessing a single socket), the condition that caused the SIGIO may have already disappeared when the process reacts to the signal. If this happens, the process should wait again because Linux will resend the signal later.

/proc interfaces

The core socket networking parameters can be accessed via files in the directory  /proc/sys/net/core/. rmem_default

contains the default setting in bytes of the socket receive buffer.

rmem_max

contains the maximum socket receive buffer size in bytes which a user may set by using the  SO_RCVBUF socket option.

wmem_default

contains the default setting in bytes of the socket send buffer.

wmem_max

contains the maximum socket send buffer size in bytes which a user may set by using the  SO_SNDBUF socket option.

message_cost and  message_burst

configure the token bucket filter used to load limit warning messages caused by external network events.

netdev_max_backlog

Maximum number of packets in the global input queue.

optmem_max

Maximum length of ancillary data and user control data like the iovecs per socket.

Ioctls

These operations can be accessed using  ioctl(2):

error = ioctl(ip_socket, ioctl_type, &value_result);

SIOCGSTAMP

Return a  struct timeval with the receive timestamp of the last packet passed to the user. This is useful for accurate round trip time measurements. See  setitimer(2) for a description of  struct timeval. This ioctl should only be used if the socket option  SO_TIMESTAMP is not set on the socket. Otherwise, it returns the timestamp of the last packet that was received while  SO_TIMESTAMP was not set, or it fails if no such packet has been received, (i.e.,  ioctl(2) returns -1 with  errno set to  ENOENT).

SIOCSPGRP

Set the process or process group to send  SIGIO or  SIGURG signals to when an asynchronous I/O operation has finished or urgent data is available. The argument is a pointer to a  pid_t. If the argument is positive, send the signals to that process. If the argument is negative, send the signals to the process group with the ID of the absolute value of the argument. The process may only choose itself or its own process group to receive signals unless it has the  CAP_KILL capability or an effective UID of 0.

FIOASYNC

Change the  O_ASYNC flag to enable or disable asynchronous I/O mode of the socket. Asynchronous I/O mode means that the  SIGIO signal or the signal set with F_SETSIG is raised when a new I/O event occurs.

Argument is an integer boolean flag. (This operation is synonymous with the use of  fcntl(2) to set the  O_ASYNC flag.)

SIOCGPGRP

Get the current process or process group that receives  SIGIO or  SIGURG signals, or 0 when none is set.

Valid  fcntl(2) operations: FIOGETOWN

The same as the  SIOCGPGRP ioctl(2).

FIOSETOWN

The same as the  SIOCSPGRP ioctl(2).

Versions

SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASSCRED is new in Linux 2.2. The /proc interfaces was introduced in Linux 2.2. SO_RCVTIMEO andSO_SNDTIMEO are supported since Linux 2.3.41. Earlier, timeouts were fixed to a protocol-specific setting, and could not be read or written.

Notes

Linux assumes that half of the send/receive buffer is used for internal kernel structures; thus the values in the corresponding /proc files are twice what can be observed on the wire.

Linux will only allow port reuse with the SO_REUSEADDR option when this option was set both in the previous program that performed a bind(2) to the port and in the program that wants to reuse the port. This differs from some implementations (e.g., FreeBSD) where only the later program needs to set the SO_REUSEADDR option. Typically this difference is invisible, since, for example, a server program is designed to always set this option.

Bugs

The CONFIG_FILTER socket options SO_ATTACH_FILTER and SO_DETACH_FILTER are not documented. The suggested interface to use them is via the libpcap library.

See Also

getsockopt(2), connect(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7)

Referenced By

accept4 (2),  echoping (1),  intro (2),  ipv6 (7),  irda (7),  raw (7),  recvmmsg (2),  sctp (7),  sendmmsg (2),  tbench_srv (1),  udplite (7),  x25 (7)