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.1-2001, 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)