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SOCKET(2)		   Linux Programmer's Manual		     SOCKET(2)

       socket - create an endpoint for communication

       #include <sys/types.h>	       /* See NOTES */
       #include <sys/socket.h>

       int socket(int domain, int type, int protocol);

       socket()	 creates  an endpoint for communication and returns a descrip‐

       The domain argument specifies a communication domain; this selects  the
       protocol	 family	 which will be used for communication.	These families
       are  defined  in	 <sys/socket.h>.   The	currently  understood  formats

       Name		   Purpose			    Man page
       AF_UNIX, AF_LOCAL   Local communication		    unix(7)
       AF_INET		   IPv4 Internet protocols	    ip(7)
       AF_INET6		   IPv6 Internet protocols	    ipv6(7)
       AF_IPX		   IPX - Novell protocols
       AF_NETLINK	   Kernel user interface device	    netlink(7)
       AF_X25		   ITU-T X.25 / ISO-8208 protocol   x25(7)
       AF_AX25		   Amateur radio AX.25 protocol
       AF_ATMPVC	   Access to raw ATM PVCs
       AF_APPLETALK	   Appletalk			    ddp(7)
       AF_PACKET	   Low level packet interface	    packet(7)

       The  socket  has	 the indicated type, which specifies the communication
       semantics.  Currently defined types are:

       SOCK_STREAM     Provides sequenced, reliable, two-way, connection-based
		       byte  streams.  An out-of-band data transmission mecha‐
		       nism may be supported.

       SOCK_DGRAM      Supports datagrams (connectionless, unreliable messages
		       of a fixed maximum length).

       SOCK_SEQPACKET  Provides	 a  sequenced,	reliable,  two-way connection-
		       based data transmission path  for  datagrams  of	 fixed
		       maximum	length;	 a  consumer  is  required  to read an
		       entire packet with each input system call.

       SOCK_RAW	       Provides raw network protocol access.

       SOCK_RDM	       Provides a reliable datagram layer that does not	 guar‐
		       antee ordering.

       SOCK_PACKET     Obsolete	 and  should  not be used in new programs; see

       Some socket types may not be implemented by all protocol families;  for
       example, SOCK_SEQPACKET is not implemented for AF_INET.

       Since Linux 2.6.27, the type argument serves a second purpose: in addi‐
       tion to specifying a socket type, it may include the bitwise OR of  any
       of the following values, to modify the behavior of socket():

       SOCK_NONBLOCK   Set  the	 O_NONBLOCK  file  status flag on the new open
		       file description.  Using this flag saves extra calls to
		       fcntl(2) to achieve the same result.

       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
		       descriptor.  See the description of the O_CLOEXEC  flag
		       in open(2) for reasons why this may be useful.

       The  protocol  specifies	 a  particular	protocol  to  be used with the
       socket.	Normally only a single protocol exists to support a particular
       socket  type within a given protocol family, in which case protocol can
       be specified as 0.  However, it is possible  that  many	protocols  may
       exist,  in  which  case a particular protocol must be specified in this
       manner.	The protocol number to use is specific to  the	“communication
       domain” in which communication is to take place; see protocols(5).  See
       getprotoent(3) on how to map protocol name strings to protocol numbers.

       Sockets of type SOCK_STREAM are full-duplex byte	 streams,  similar  to
       pipes.	They  do not preserve record boundaries.  A stream socket must
       be in a connected state before any data may be sent or received on  it.
       A connection to another socket is created with a connect(2) call.  Once
       connected, data may be transferred using read(2) and write(2) calls  or
       some variant of the send(2) and recv(2) calls.  When a session has been
       completed a close(2) may be performed.  Out-of-band data	 may  also  be
       transmitted  as	described  in  send(2)	and  received  as described in

       The communications protocols which implement a SOCK_STREAM ensure  that
       data  is not lost or duplicated.	 If a piece of data for which the peer
       protocol has buffer space cannot be successfully transmitted  within  a
       reasonable  length  of  time,  then  the connection is considered to be
       dead.  When SO_KEEPALIVE is enabled on the socket the  protocol	checks
       in  a protocol-specific manner if the other end is still alive.	A SIG‐
       PIPE signal is raised if a  process  sends  or  receives	 on  a	broken
       stream; this causes naive processes, which do not handle the signal, to
       exit.   SOCK_SEQPACKET  sockets	employ	the  same  system   calls   as
       SOCK_STREAM  sockets.   The  only difference is that read(2) calls will
       return only the amount of data requested, and any data remaining in the
       arriving	 packet	 will  be  discarded.	Also all message boundaries in
       incoming datagrams are preserved.

       SOCK_DGRAM and SOCK_RAW sockets allow sending of	 datagrams  to	corre‐
       spondents  named	 in sendto(2) calls.  Datagrams are generally received
       with recvfrom(2), which	returns	 the  next  datagram  along  with  the
       address of its sender.

       SOCK_PACKET  is an obsolete socket type to receive raw packets directly
       from the device driver.	Use packet(7) instead.

       An fcntl(2) F_SETOWN operation can be used  to  specify	a  process  or
       process	group  to  receive  a  SIGURG signal when the out-of-band data
       arrives or SIGPIPE signal when a SOCK_STREAM  connection	 breaks	 unex‐
       pectedly.   This	 operation  may	 also  be  used	 to set the process or
       process group that receives the I/O and	asynchronous  notification  of
       I/O events via SIGIO.  Using F_SETOWN is equivalent to an ioctl(2) call
       with the FIOSETOWN or SIOCSPGRP argument.

       When the network signals an error  condition  to	 the  protocol	module
       (e.g.,  using  a ICMP message for IP) the pending error flag is set for
       the socket.  The next operation on this socket will  return  the	 error
       code of the pending error.  For some protocols it is possible to enable
       a per-socket error queue to retrieve  detailed  information  about  the
       error; see IP_RECVERR in ip(7).

       The  operation of sockets is controlled by socket level options.	 These
       options are defined in <sys/socket.h>.  The functions setsockopt(2) and
       getsockopt(2) are used to set and get options, respectively.

       On  success,  a	file  descriptor  for  the new socket is returned.  On
       error, -1 is returned, and errno is set appropriately.

       EACCES Permission to create a socket of the specified type and/or  pro‐
	      tocol is denied.

	      The  implementation  does not support the specified address fam‐

       EINVAL Unknown protocol, or protocol family not available.

       EINVAL Invalid flags in type.

       EMFILE Process file table overflow.

       ENFILE The system limit on the total number  of	open  files  has  been

	      Insufficient  memory is available.  The socket cannot be created
	      until sufficient resources are freed.

	      The protocol type or the specified  protocol  is	not  supported
	      within this domain.

       Other errors may be generated by the underlying protocol modules.

       4.4BSD, POSIX.1-2001.

       The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.

       socket()	 appeared in 4.2BSD.  It is generally portable to/from non-BSD
       systems supporting clones of the BSD socket layer (including  System  V

       POSIX.1-2001  does not require the inclusion of <sys/types.h>, and this
       header file is not required on Linux.  However, some  historical	 (BSD)
       implementations	required  this	header file, and portable applications
       are probably wise to include it.

       The manifest constants used under 4.x BSD  for  protocol	 families  are
       PF_UNIX,	 PF_INET,  etc., while AF_UNIX etc. are used for address fami‐
       lies.  However, already the BSD man page promises: "The protocol family
       generally  is the same as the address family", and subsequent standards
       use AF_* everywhere.

       An example of the use of socket() is shown in getaddrinfo(3).

       accept(2),  bind(2),  connect(2),  fcntl(2),  getpeername(2),  getsock‐
       name(2),	  getsockopt(2),   ioctl(2),   listen(2),   read(2),  recv(2),
       select(2),  send(2),  shutdown(2),  socketpair(2),  write(2),   getpro‐
       toent(3), ip(7), socket(7), tcp(7), udp(7), unix(7)

       “An   Introductory   4.3BSD  Interprocess  Communication	 Tutorial”  is
       reprinted in UNIX Programmer's Supplementary Documents Volume 1.

       “BSD Interprocess Communication Tutorial” is reprinted in UNIX Program‐
       mer's Supplementary Documents Volume 1.

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2009-01-19			     SOCKET(2)

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