CONNECT(2)                          Linux Programmer's Manual                          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 for‐
       mat  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; connec‐
       tionless 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 directo‐
              ries in the path prefix.  (See also path_resolution(7).)

       EACCES, EPERM
              The user tried to connect to a broadcast address without having the  socket  broad‐
              cast  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 com‐
              pleted.

       EBADF  The file descriptor is not a valid index in the descriptor table.

       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 suc‐
              cessfully (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 is not associated with 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 con‐
              nections.  Note that for IP sockets the timeout may be very  long  when  syncookies
              are enabled on the server.

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

NOTES
       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  third  argument of connect() is in reality an int (and this is what 4.x BSD and libc4
       and libc5 have).  Some POSIX confusion resulted in the present  socklen_t,  also  used  by
       glibc.  See also accept(2).

       If  connect()  fails,  consider the state of the socket as unspecified.  Portable applica‐
       tions 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 3.74 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                                       2014-09-06                                 CONNECT(2)