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


     connect - initiate a connection on a socket


     #include <sys/types.h>          /* See NOTES */
     #include <sys/socket.h>
     int connect(int sockfd, const struct sockaddr *addr,
                 socklen_t addrlen);


     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).


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


     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
            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.
            Local address is already in use.
            (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
            The passed address didn't have the correct address family in its
            sa_family field.
     EAGAIN Insufficient entries in the routing cache.
            The socket is nonblocking and a previous connection attempt  has
            not yet been completed.
     EBADF  sockfd is not a valid open file descriptor.
            A  connect()  on  a  stream socket found no one listening on the
            remote address.
     EFAULT The socket structure  address  is  outside  the  user's  address
            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
            The socket is already connected.
            Network is unreachable.
            The file descriptor sockfd does not refer to a socket.
            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.
            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.


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


     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


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


     accept(2),  bind(2), getsockname(2), listen(2), socket(2), path_resolu-


     This page is part of release 4.16 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

Linux 2017-09-15 CONNECT(2)

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