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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
CONNECT(2) Linux Programmer's Manual CONNECT(2)
connect - initiate a connection on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int connect(int sockfd, const struct sockaddr *addr,
socklen_t addrlen);
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.)
If the connection or binding succeeds, zero is returned. On error,
-1 is returned, and errno is set appropriately.
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.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared
in 4.2BSD).
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.
An example of the use of connect() is shown in getaddrinfo(3).
accept(2), bind(2), getsockname(2), listen(2), socket(2),
path_resolution(7), selinux(8)
This page is part of release 5.08 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 2020-04-11 CONNECT(2)
Pages that refer to this page: telnet-probe(1) , accept(2) , accept4(2) , bind(2) , getpeername(2) , listen(2) , _newselect(2) , pselect(2) , pselect6(2) , recv(2) , recvfrom(2) , recvmsg(2) , select(2) , select_tut(2) , shutdown(2) , socket(2) , socketcall(2) , syscalls(2) , write(2) , fd_clr(3) , FD_CLR(3) , fd_isset(3) , FD_ISSET(3) , fd_set(3) , FD_SET(3) , fd_zero(3) , FD_ZERO(3) , freeaddrinfo(3) , gai_strerror(3) , getaddrinfo(3) , ldap_get_option(3) , ldap_set_option(3) , rtime(3) , slapd-asyncmeta(5) , slapd-ldap(5) , slapd-meta(5) , ddp(7) , ip(7) , netlink(7) , packet(7) , sctp(7) , signal(7) , signal-safety(7) , sock_diag(7) , socket(7) , tcp(7) , udp(7) , unix(7) , vsock(7)
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