connect(2) — Linux manual page


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

NAME         top

       connect - initiate a connection on a socket

SYNOPSIS         top

       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int connect(int sockfd, const struct sockaddr *addr,
                   socklen_t addrlen);

DESCRIPTION         top

       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

       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.)

RETURN VALUE         top

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

ERRORS         top

       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).)

              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).

              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 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.  (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 signal(7).

              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

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared
       in 4.2BSD).

NOTES         top

       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.

EXAMPLES         top

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

SEE ALSO         top

       accept(2), bind(2), getsockname(2), listen(2), socket(2),
       path_resolution(7), selinux(8)

COLOPHON         top

       This page is part of release 5.08 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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