connect(2)

NAME

   connect - initiate a connection on a socket

SYNOPSIS

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

   int connect(int sockfd, const struct sockaddr *addr,
               socklen_t addrlen);

DESCRIPTION

   The  connect()  system call connects the socket referred to by the file
   descriptor sockfd to  the  address  specified  by  addr.   The  addrlen
   argument specifies the size of addr.  The format of the address in addr
   is determined by the address space of the socket sockfd; see  socket(2)
   for further details.

   If the socket sockfd is of type SOCK_DGRAM, then addr is the address to
   which datagrams are sent by default, and the only  address  from  which
   datagrams  are  received.   If  the  socket  is  of type SOCK_STREAM or
   SOCK_SEQPACKET, this call attempts to make a connection to  the  socket
   that is bound to the address specified by addr.

   Generally, connection-based protocol sockets may successfully connect()
   only once; connectionless protocol sockets may use  connect()  multiple
   times to change their association.  Connectionless sockets may dissolve
   the association by connecting to an address with the  sa_family  member
   of sockaddr set to AF_UNSPEC (supported on Linux since kernel 2.2).

RETURN VALUE

   If  the connection or binding succeeds, zero is returned.  On error, -1
   is returned, and errno is set appropriately.

ERRORS

   The following are general socket  errors  only.   There  may  be  other
   domain-specific error codes.

   EACCES For UNIX domain sockets, which are identified by pathname: Write
          permission is denied on the socket file, or search permission is
          denied for one of the directories in the path prefix.  (See also
          path_resolution(7).)

   EACCES, EPERM
          The user tried to connect to a broadcast address without  having
          the  socket  broadcast  flag  enabled  or the connection request
          failed because of a local firewall rule.

   EADDRINUSE
          Local address is already in use.

   EADDRNOTAVAIL
          (Internet domain sockets) The socket referred to by  sockfd  had
          not  previously been bound to an address and, upon attempting to
          bind it to an ephemeral port, it was determined  that  all  port
          numbers  in  the ephemeral port range are currently in use.  See
          the  discussion  of  /proc/sys/net/ipv4/ip_local_port_range   in
          ip(7).

   EAFNOSUPPORT
          The passed address didn't have the correct address family in its
          sa_family field.

   EAGAIN Insufficient entries in the routing cache.

   EALREADY
          The socket is nonblocking and a previous connection attempt  has
          not yet been completed.

   EBADF  sockfd is not a valid open file descriptor.

   ECONNREFUSED
          No-one listening on the remote address.

   EFAULT The  socket  structure  address  is  outside  the user's address
          space.

   EINPROGRESS
          The socket is nonblocking and the connection cannot be completed
          immediately.   It  is  possible  to  select(2)  or  poll(2)  for
          completion by selecting the socket for writing.  After select(2)
          indicates  writability,  use  getsockopt(2) to read the SO_ERROR
          option  at  level  SOL_SOCKET  to  determine  whether  connect()
          completed  successfully  (SO_ERROR  is  zero)  or unsuccessfully
          (SO_ERROR  is  one  of  the  usual  error  codes  listed   here,
          explaining the reason for the failure).

   EINTR  The system call was interrupted by a signal that was caught; see
          signal(7).

   EISCONN
          The socket is already connected.

   ENETUNREACH
          Network is unreachable.

   ENOTSOCK
          The file descriptor sockfd does not refer to a socket.

   EPROTOTYPE
          The socket type does not support  the  requested  communications
          protocol.   This  error can occur, for example, on an attempt to
          connect a UNIX domain datagram socket to a stream socket.

   ETIMEDOUT
          Timeout while attempting connection.  The server may be too busy
          to accept new connections.  Note that for IP sockets the timeout
          may be very long when syncookies are enabled on the server.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (the connect() function first
   appeared in 4.2BSD).

NOTES

   POSIX.1  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.

   For background on the socklen_t type, see accept(2).

   If connect() fails, consider the state of the  socket  as  unspecified.
   Portable  applications should close the socket and create a new one for
   reconnecting.

EXAMPLE

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

SEE ALSO

   accept(2),    bind(2),    getsockname(2),     listen(2),     socket(2),
   path_resolution(7)

COLOPHON

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.

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