connect(2) System Calls Manual connect(2)
NAME
connect - initiate a connection on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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.
Some protocol sockets (e.g., UNIX domain stream sockets) may success-
fully 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 connect-
ing to an address with the sa_family member of sockaddr set to AF_UN-
SPEC; thereafter, the socket can be connected to another address.
(AF_UNSPEC is supported since Linux 2.2.)
RETURN VALUE
If the connection or binding succeeds, zero is returned. On error, -1
is returned, and errno is set to indicate the error.
ERRORS
The following are general socket errors only. There may be other do-
main-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 re-
mote 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 select-
ing the socket for writing. After select(2) indicates writabil-
ity, 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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.4BSD, (connect() first appeared in 4.2BSD).
NOTES
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).
SEE ALSO
accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu-
tion(7), selinux(8)
Linux man-pages 6.7 2023-11-01 connect(2)
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