connect(2)



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  argu-
       ment  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 comple-
              tion by selecting the socket for writing.  After select(2) indi-
              cates 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.

       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  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_resolu-
       tion(7)

COLOPHON
       This  page  is  part of release 4.06 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/.

Linux                             2016-03-15                        CONNECT(2)

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