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

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.

Linux                                              2020-04-11                                         CONNECT(2)