connect(2) — Linux manual page


connect(2)                 System Calls Manual                connect(2)

NAME         top

       connect - initiate a connection on a socket

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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 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 since
       Linux 2.2.)

RETURN VALUE         top

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

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).)

       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.

       EACCES It 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

       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 server.

STANDARDS         top


HISTORY         top

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

NOTES         top

       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

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       user-space interface documentation) project.  Information about
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       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
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       2024-06-26.  If you discover any rendering problems in this HTML
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       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-05-02                     connect(2)

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