Ubuntu Manpage: connect - initiate a connection on a socket

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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 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.)

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 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).)

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.

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

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 remote 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 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).

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.

EXAMPLES

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

COLOPHON

       This  page  is  part of release 5.10 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/.