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67 .TH CONNECT 2 2021-03-22 "Linux" "Linux Programmer's Manual"
69 connect \- initiate a connection on a socket
72 .B #include <sys/socket.h>
74 .BI "int connect(int " sockfd ", const struct sockaddr *" addr ,
75 .BI " socklen_t " addrlen );
80 system call connects the socket referred to by the file descriptor
82 to the address specified by
86 argument specifies the size of
88 The format of the address in
90 is determined by the address space of the socket
102 is the address to which datagrams are sent by default, and the only
103 address from which datagrams are received.
104 If the socket is of type
108 this call attempts to make a connection to the socket that is bound
109 to the address specified by
112 Some protocol sockets (e.g., UNIX domain stream sockets)
117 Some protocol sockets
118 (e.g., datagram sockets in the UNIX and Internet domains)
121 multiple times to change their association.
123 Some protocol sockets
124 (e.g., TCP sockets as well as datagram sockets in the UNIX and
126 may dissolve the association by connecting to an address with the
132 thereafter, the socket can be connected to another address.
134 is supported on Linux since kernel 2.2.)
136 If the connection or binding succeeds, zero is returned.
137 On error, \-1 is returned, and
139 is set to indicate the error.
141 The following are general socket errors only.
142 There may be other domain-specific error codes.
145 For UNIX domain sockets, which are identified by pathname:
146 Write permission is denied on the socket file,
147 or search permission is denied for one of the directories
150 .BR path_resolution (7).)
152 .BR EACCES ", " EPERM
153 The user tried to connect to a broadcast address without having the socket
154 broadcast flag enabled or the connection request failed because of a local
158 can also be returned if an SELinux policy denied a connection (for
159 example, if there is a policy saying that an HTTP proxy can only
160 connect to ports associated with HTTP servers, and the proxy tries to
161 connect to a different port).
165 Local address is already in use.
168 (Internet domain sockets)
169 The socket referred to by
171 had not previously been bound to an address and,
172 upon attempting to bind it to an ephemeral port,
173 it was determined that all port numbers in the ephemeral port range
174 are currently in use.
175 See the discussion of
176 .I /proc/sys/net/ipv4/ip_local_port_range
181 The passed address didn't have the correct address family in its
186 For nonblocking UNIX domain sockets, the socket is nonblocking, and the
187 connection cannot be completed immediately.
188 For other socket families, there are insufficient entries in the routing cache.
191 The socket is nonblocking and a previous connection attempt has not yet
196 is not a valid open file descriptor.
201 on a stream socket found no one listening on the remote address.
204 The socket structure address is outside the user's address space.
207 The socket is nonblocking and the connection cannot be completed immediately.
208 (UNIX domain sockets failed with
215 for completion by selecting the socket for writing.
218 indicates writability, use
226 completed successfully
228 is zero) or unsuccessfully
230 is one of the usual error codes listed here,
231 explaining the reason for the failure).
234 The system call was interrupted by a signal that was caught; see
236 .\" For TCP, the connection will complete asynchronously.
237 .\" See http://lkml.org/lkml/2005/7/12/254
240 The socket is already connected.
243 Network is unreachable.
248 does not refer to a socket.
251 The socket type does not support the requested communications protocol.
252 This error can occur, for example,
253 on an attempt to connect a UNIX domain datagram socket to a stream socket.
256 Timeout while attempting connection.
257 The server may be too
258 busy to accept new connections.
259 Note that for IP sockets the timeout may
260 be very long when syncookies are enabled on the server.
262 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD,
264 first appeared in 4.2BSD).
265 .\" SVr4 documents the additional
266 .\" general error codes
267 .\" .BR EADDRNOTAVAIL ,
269 .\" .BR EAFNOSUPPORT ,
276 .\" documents many additional error conditions not described here.
278 For background on the
285 fails, consider the state of the socket as unspecified.
286 Portable applications should close the socket and create a new one for
289 An example of the use of
299 .BR path_resolution (7),