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

       connect - initiate a connection on a socket

       #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 argument
       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 successfully
       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 connecting
       to an address with the sa_family member of sockaddr set to AF_UNSPEC;
       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 to indicate the error.

       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.

              EACCES can also be returned if an SELinux policy denied a
              connection (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 ip(7).

              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
              remote address.

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

              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 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 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

       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_resolution(7), selinux(8)

       This page is part of release 5.13 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                              2021-03-22                         CONNECT(2)