connect

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  do-
       main-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 For nonblocking UNIX domain sockets, the socket is  nonblocking,
              and  the  connection cannot be completed immediately.  For other
              socket families, there are 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
              A connect() on a stream socket found no one listening on the re-
              mote 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.   (UNIX domain sockets failed with EAGAIN instead.)
              It is possible to select(2) or poll(2) for completion by select-
              ing the socket for writing.  After select(2) indicates writabil-
              ity, 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, (connect() 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_resolu-
       tion(7)

COLOPHON
       This page is part of release 5.05 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                             2019-03-06                        CONNECT(2)