CONNECT(2) Linux Programmer's Manual CONNECT(2)NAMEconnect - 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 argu‐
ment 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 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. It is possible to select(2) or poll(2) for comple‐
tion by selecting the socket for writing. After select(2) indi‐
cates 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).
SEE ALSOaccept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
tion(7)COLOPHON
This page is part of release 4.14 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 2017-09-15 CONNECT(2)
