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connect - initiate a connection on a socket
Standard C Library (libc, -lc)
connect(int s, const struct sockaddr *name, socklen_t namelen);
The s argument is a socket. If it is of type SOCK_DGRAM, this call
specifies the peer with which the socket is to be associated; this
address is that to which datagrams are to be sent, and the only address
from which datagrams are to be received. If the socket is of type
SOCK_STREAM, this call attempts to make a connection to another socket.
The other socket is specified by name, which is an address in the
communications space of the socket. Each communications space interprets
the name argument in its own way. Generally, stream sockets may
successfully connect() only once; datagram sockets may use connect()
multiple times to change their association. Datagram sockets may
dissolve the association by connecting to an invalid address, such as a
The connect() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The connect() system call fails if:
[EBADF] The s argument is not a valid descriptor.
[ENOTSOCK] The s argument is a descriptor for a file, not a
[EADDRNOTAVAIL] The specified address is not available on this
[EAFNOSUPPORT] Addresses in the specified address family cannot be
used with this socket.
[EISCONN] The socket is already connected.
[ETIMEDOUT] Connection establishment timed out without
establishing a connection.
[ECONNREFUSED] The attempt to connect was forcefully rejected.
[ENETUNREACH] The network is not reachable from this host.
[EHOSTUNREACH] The remote host is not reachable from this host.
[EADDRINUSE] The address is already in use.
[EFAULT] The name argument specifies an area outside the
process address space.
[EINPROGRESS] The socket is non-blocking and the connection cannot
be completed immediately. It is possible to select(2)
for completion by selecting the socket for writing.
[EINTR] The connection attempt was interrupted by the delivery
of a signal. The connection will be established in
the background, as in the case of EINPROGRESS.
[EALREADY] A previous connection attempt has not yet been
[EACCES] An attempt is made to connect to a broadcast address
(obtained through the INADDR_BROADCAST constant or the
INADDR_NONE return value) through a socket that does
not provide broadcast functionality.
[EAGAIN] An auto-assigned port number was requested but no
auto-assigned ports are available. Increasing the
port range specified by sysctl(3) MIB variables
net.inet.ip.portrange.last may alleviate the problem.
The following errors are specific to connecting names in the UNIX domain.
These errors may not apply in future versions of the UNIX IPC domain.
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or
an entire path name exceeded 1023 characters.
[ENOENT] The named socket does not exist.
[EACCES] Search permission is denied for a component of the
[EACCES] Write access to the named socket is denied.
[ELOOP] Too many symbolic links were encountered in
translating the pathname.
accept(2), getpeername(2), getsockname(2), select(2), socket(2),
The connect() system call appeared in 4.2BSD.