Ubuntu Manpage: connect - initiate a connection on a socket

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.

       Generally,  connection-based  protocol  sockets  may  successfully  connect()  only  once; connectionless
       protocol sockets may use connect() multiple times to change their  association.   Connectionless  sockets
       may  dissolve  the  association  by connecting to an address with the sa_family member of sockaddr set to
       AF_UNSPEC (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.

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.

EXAMPLE

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

COLOPHON

       This  page  is  part  of  release  5.05  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/.

Linux                                              2019-03-06                                         CONNECT(2)