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.

       Some  protocol  sockets (e.g., UNIX domain stream sockets) may success-
       fully connect() only once.

       Some protocol sockets (e.g., datagram sockets in the UNIX and  Internet
       domains) may use connect() multiple times to change their association.

       Some protocol sockets (e.g., TCP sockets as well as datagram sockets in
       the UNIX and Internet domains) may dissolve the association by connect-
       ing  to  an address with the sa_family member of sockaddr set to AF_UN-
       SPEC; thereafter, the socket  can  be  connected  to  another  address.
       (AF_UNSPEC is 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  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

              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.

              EACCES can also be returned if an SELinux policy denied  a  con-
              nection  (for  example, if there is a policy saying that an HTTP
              proxy can only connect to ports associated  with  HTTP  servers,
              and the proxy tries to connect to a different port).  dd

              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 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

              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 re-
              mote address.

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

              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

       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-
       tion(7), selinux(8)

       This  page  is  part of release 5.07 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                             2020-04-11                        CONNECT(2)

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