SOCKET(7)                           Linux Programmer's Manual                           SOCKET(7)

       socket - Linux socket interface


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

       This manual page describes the Linux networking socket layer user interface.  The BSD com‐
       patible sockets are the uniform interface between the user process and the network  proto‐
       col stacks in the kernel.  The protocol modules are grouped into protocol families such as
       AF_INET, AF_IPX, and AF_PACKET, and socket types such as SOCK_STREAM or  SOCK_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 (implemented
       only 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 con‐

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

       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).

       │                            I/O events                              │
       │Event      │ Poll flag │ Occurrence                                 │
       │Read       │ POLLIN    │ New data arrived.                          │
       │Read       │ POLLIN    │ A connection setup has been completed (for │
       │           │           │ connection-oriented sockets)               │
       │Read       │ POLLHUP   │ A disconnection request has been initiated │
       │           │           │ by the other end.                          │
       │Read       │ POLLHUP   │ A connection is broken (only  for  connec‐ │
       │           │           │ tion-oriented protocols).  When the socket │
       │           │           │ is written SIGPIPE is also sent.           │
       │Write      │ POLLOUT   │ Socket has enough send  buffer  space  for │
       │           │           │ writing new data.                          │
       │Read/Write │ POLLIN|   │ An outgoing connect(2) finished.           │
       │           │ POLLOUT   │                                            │
       │Read/Write │ POLLERR   │ An asynchronous error occurred.            │
       │Read/Write │ POLLHUP   │ The other end has shut down one direction. │
       │Exception  │ POLLPRI   │ Urgent data arrived.  SIGURG is sent then. │
       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 sigac‐
       tion(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 iden‐
       tify 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
       The socket options listed below can be set by using setsockopt(2) and read  with  getsock‐
       opt(2)  with  the socket level set to SOL_SOCKET for all sockets.  Unless otherwise noted,
       optval is a pointer to an int.

              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.

              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-ter‐
              minated 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 works only for some socket types, particu‐
              larly  AF_INET sockets.  It is not supported for packet sockets (use normal bind(2)

              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 con‐
              tain 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 argu‐

              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 sock‐

              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.

              Enable socket debugging.  Only allowed for processes with the  CAP_NET_ADMIN  capa‐
              bility 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.

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

              Don't  send  via a gateway, send only 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.

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

              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.

       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  rout‐
              ing  without  netfilter  or for packet filtering.  Setting this option requires the
              CAP_NET_ADMIN capability.

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

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

       SO_PEEK_OFF (since Linux 3.4)
              This  option, which is currently supported only for unix(7) sockets, sets the value
              of the "peek offset" for the recv(2) system call when used with MSG_PEEK flag.

              When this option is set to a negative value (it is set to -1 for all new  sockets),
              traditional  behavior  is  provided:  recv(2) with the MSG_PEEK flag will peek data
              from the front of the queue.

              When the option is set to a value greater than or equal to zero, then the next peek
              at  data queued in the socket will occur at the byte offset specified by the option
              value.  At the same time, the "peek offset" will be incremented by  the  number  of
              bytes  that  were  peeked from the queue, so that a subsequent peek will return the
              next data in the queue.

              If data is removed from the front of the queue via a call to recv(2)  (or  similar)
              without  the  MSG_PEEK  flag,  the "peek offset" will be decreased by the number of
              bytes removed.  In other words, receiving data without the MSG_PEEK flag will cause
              the  "peek  offset" to be adjusted to maintain the correct relative position in the
              queued data, so that a subsequent peek will retrieve the data that would have  been
              retrieved had the data not been removed.

              For  datagram  sockets,  if the "peek offset" points to the middle of a packet, the
              data returned will be marked with the MSG_TRUNC flag.

              The following example serves to illustrate  the  use  of  SO_PEEK_OFF.   Suppose  a
              stream socket has the following queued input data:


              The  following  sequence  of  recv(2) calls would have the effect noted in the com‐

                  int ov = 4;                  // Set peek offset to 4
                  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));

                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
                  recv(fd, buf, 2, 0);         // Reads "aa"; offset set to 6
                  recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8

              Return the credentials of the foreign process connected to this  socket.   This  is
              possible  only 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 socket‐
              pair(2).  The argument is a ucred structure; define the  _GNU_SOURCE  feature  test
              macro  to obtain the definition of that structure from .  This socket
              option is read-only.

              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.  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.

              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.

              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.

              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 EIN‐
              PROGRESS (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.

              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_REUSEPORT (since Linux 3.9)
              Permits  multiple  AF_INET  or  AF_INET6 sockets to be bound to an identical socket
              address.  This option must be set on each socket (including the first socket) prior
              to  calling bind(2) on the socket.  To prevent port hijacking, all of the processes
              binding to the same address must have the same effective UID.  This option  can  be
              employed with both TCP and UDP sockets.

              For TCP sockets, this option allows accept(2) load distribution in a multi-threaded
              server to be improved by using a distinct listener socket for  each  thread.   This
              provides  improved  load  distribution  as  compared to traditional techniques such
              using a single accept(2)ing thread that distributes connections, or having multiple
              threads that compete to accept(2) from the same socket.

              For UDP sockets, the use of this option can provide better distribution of incoming
              datagrams to multiple processes (or threads) as compared to the  traditional  tech‐
              nique of having multiple processes compete to receive datagrams on the same socket.

       SO_RXQ_OVFL (since Linux 2.6.33)
              Indicates that an unsigned 32-bit value ancillary message (cmsg) should be attached
              to received skbs indicating the number of packets dropped by the socket between the
              last received packet and this received packet.

              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.

              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.

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

       SO_BUSY_POLL (since Linux 3.11)
              Sets  the  approximate time in microseconds to busy poll on a blocking receive when
              there is no data.  Increasing this value requires CAP_NET_ADMIN.  The  default  for
              this option is controlled by the /proc/sys/net/core/busy_read file.

              The  value  in  the /proc/sys/net/core/busy_poll file determines how long select(2)
              and poll(2) will busy poll when they operate on sockets with SO_BUSY_POLL  set  and
              no events to report are found.

              In  both  cases,  busy polling will only be done when the socket last received data
              from a network device that supports this option.

              While busy polling may improve latency of some applications,  care  must  be  taken
              when using it since this will increase both CPU utilization and power usage.

       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  condi‐
       tion  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 sig‐
       nal later.

   /proc interfaces
       The  core  socket  networking  parameters  can  be  accessed  via  files  in the directory

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

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

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

              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.

              Maximum number of packets in the global input queue.

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

       These operations can be accessed using ioctl(2):

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

              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 be used only 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

              Set  the  process or process group to send SIGIO or SIGURG signals to when an asyn‐
              chronous 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 effec‐
              tive UID of 0.

              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.)

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

       Valid fcntl(2) operations:

              The same as the SIOCGPGRP ioctl(2).

              The same as the SIOCSPGRP ioctl(2).

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

       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

       Linux will allow port reuse only 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.

       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.

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

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-05-07                                  SOCKET(7)


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