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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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connect(2) System Calls Manual connect(2)
connect - initiate a connection on a socket
Standard C library (libc, -lc)
#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 since Linux
2.2.)
If the connection or binding succeeds, zero is returned. On
error, -1 is returned, and errno is set to indicate the error.
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 It 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).
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.1-2008.
POSIX.1-2001, SVr4, 4.4BSD, (connect() first appeared in 4.2BSD).
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 connect(2)
Pages that refer to this page: telnet-probe(1), accept(2), bind(2), getpeername(2), io_uring_enter2(2), io_uring_enter(2), listen(2), recv(2), select(2), select_tut(2), send(2), shutdown(2), socket(2), socketcall(2), syscalls(2), write(2), getaddrinfo(3), io_uring_prep_connect(3), ldap_get_option(3), rtime(3), sockaddr(3type), ldap.conf(5), slapd-asyncmeta(5), slapd-ldap(5), slapd-meta(5), ddp(7), ip(7), landlock(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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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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