man2/connect.2

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  63 .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
  64 .\" Modified 1998, 1999 by Andi Kleen
  65 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  66 .\"
  67 .TH CONNECT 2 2016-12-12 "Linux" "Linux Programmer's Manual"
  68 .SH NAME
  69 connect \- initiate a connection on a socket
  70 .SH SYNOPSIS
  71 .nf
  72 .BR "#include <sys/types.h>" "          /* See NOTES */"
  73 .br
  74 .B #include <sys/socket.h>
  75 .sp
  76 .BI "int connect(int " sockfd ", const struct sockaddr *" addr ,
  77 .BI "            socklen_t " addrlen );
  78 .fi
  79 .SH DESCRIPTION
  80 The
  81 .BR connect ()
  82 system call connects the socket referred to by the file descriptor
  83 .I sockfd
  84 to the address specified by
  85 .IR addr .
  86 The
  87 .I addrlen
  88 argument specifies the size of
  89 .IR addr .
  90 The format of the address in
  91 .I addr
  92 is determined by the address space of the socket
  93 .IR sockfd ;
  94 see
  95 .BR socket (2)
  96 for further details.
  97
  98 If the socket
  99 .I sockfd
 100 is of type
 101 .BR SOCK_DGRAM ,
 102 then
 103 .I addr
 104 is the address to which datagrams are sent by default, and the only
 105 address from which datagrams are received.
 106 If the socket is of type
 107 .B SOCK_STREAM
 108 or
 109 .BR SOCK_SEQPACKET ,
 110 this call attempts to make a connection to the socket that is bound
 111 to the address specified by
 112 .IR addr .
 113 .PP
 114 Generally, connection-based protocol sockets may successfully
 115 .BR connect ()
 116 only once; connectionless protocol sockets may use
 117 .BR connect ()
 118 multiple times to change their association.
 119 Connectionless sockets may
 120 dissolve the association by connecting to an address with the
 121 .I sa_family
 122 member of
 123 .I sockaddr
 124 set to
 125 .BR AF_UNSPEC
 126 (supported on Linux since kernel 2.2).
 127 .SH RETURN VALUE
 128 If the connection or binding succeeds, zero is returned.
 129 On error, \-1 is returned, and
 130 .I errno
 131 is set appropriately.
 132 .SH ERRORS
 133 The following are general socket errors only.
 134 There may be other domain-specific error codes.
 135 .TP
 136 .B EACCES
 137 For UNIX domain sockets, which are identified by pathname:
 138 Write permission is denied on the socket file,
 139 or search permission is denied for one of the directories
 140 in the path prefix.
 141 (See also
 142 .BR path_resolution (7).)
 143 .TP
 144 .BR EACCES ", " EPERM
 145 The user tried to connect to a broadcast address without having the socket
 146 broadcast flag enabled or the connection request failed because of a local
 147 firewall rule.
 148 .TP
 149 .B EADDRINUSE
 150 Local address is already in use.
 151 .TP
 152 .B EADDRNOTAVAIL
 153 (Internet domain sockets)
 154 The socket referred to by
 155 .I sockfd
 156 had not previously been bound to an address and,
 157 upon attempting to bind it to an ephemeral port,
 158 it was determined that all port numbers in the ephemeral port range
 159 are currently in use.
 160 See the discussion of
 161 .I /proc/sys/net/ipv4/ip_local_port_range
 162 in
 163 .BR ip (7).
 164 .TP
 165 .B EAFNOSUPPORT
 166 The passed address didn't have the correct address family in its
 167 .I sa_family
 168 field.
 169 .TP
 170 .B EAGAIN
 171 Insufficient entries in the routing cache.
 172 .TP
 173 .B EALREADY
 174 The socket is nonblocking and a previous connection attempt has not yet
 175 been completed.
 176 .TP
 177 .B EBADF
 178 .I sockfd
 179 is not a valid open file descriptor.
 180 .TP
 181 .B ECONNREFUSED
 182 No-one listening on the remote address.
 183 .TP
 184 .B EFAULT
 185 The socket structure address is outside the user's address space.
 186 .TP
 187 .B EINPROGRESS
 188 The socket is nonblocking and the connection cannot be completed
 189 immediately.
 190 It is possible to
 191 .BR select (2)
 192 or
 193 .BR poll (2)
 194 for completion by selecting the socket for writing.
 195 After
 196 .BR select (2)
 197 indicates writability, use
 198 .BR getsockopt (2)
 199 to read the
 200 .B SO_ERROR
 201 option at level
 202 .B SOL_SOCKET
 203 to determine whether
 204 .BR connect ()
 205 completed successfully
 206 .RB ( SO_ERROR
 207 is zero) or unsuccessfully
 208 .RB ( SO_ERROR
 209 is one of the usual error codes listed here,
 210 explaining the reason for the failure).
 211 .TP
 212 .B EINTR
 213 The system call was interrupted by a signal that was caught; see
 214 .BR signal (7).
 215 .\" For TCP, the connection will complete asynchronously.
 216 .\" See http://lkml.org/lkml/2005/7/12/254
 217 .TP
 218 .B EISCONN
 219 The socket is already connected.
 220 .TP
 221 .B ENETUNREACH
 222 Network is unreachable.
 223 .TP
 224 .B ENOTSOCK
 225 The file descriptor
 226 .I sockfd
 227 does not refer to a socket.
 228 .TP
 229 .B EPROTOTYPE
 230 The socket type does not support the requested communications protocol.
 231 This error can occur, for example,
 232 on an attempt to connect a UNIX domain datagram socket to a stream socket.
 233 .TP
 234 .B ETIMEDOUT
 235 Timeout while attempting connection.
 236 The server may be too
 237 busy to accept new connections.
 238 Note that for IP sockets the timeout may
 239 be very long when syncookies are enabled on the server.
 240 .SH CONFORMING TO
 241 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (the
 242 .BR connect ()
 243 function first appeared in 4.2BSD).
 244 .\" SVr4 documents the additional
 245 .\" general error codes
 246 .\" .BR EADDRNOTAVAIL ,
 247 .\" .BR EINVAL ,
 248 .\" .BR EAFNOSUPPORT ,
 249 .\" .BR EALREADY ,
 250 .\" .BR EINTR ,
 251 .\" .BR EPROTOTYPE ,
 252 .\" and
 253 .\" .BR ENOSR .
 254 .\" It also
 255 .\" documents many additional error conditions not described here.
 256 .SH NOTES
 257 POSIX.1 does not require the inclusion of
 258 .IR <sys/types.h> ,
 259 and this header file is not required on Linux.
 260 However, some historical (BSD) implementations required this header
 261 file, and portable applications are probably wise to include it.
 262
 263 For background on the
 264 .I socklen_t
 265 type, see
 266 .BR accept (2).
 267
 268 If
 269 .BR connect ()
 270 fails, consider the state of the socket as unspecified.
 271 Portable applications should close the socket and create a new one for
 272 reconnecting.
 273 .SH EXAMPLE
 274 An example of the use of
 275 .BR connect ()
 276 is shown in
 277 .BR getaddrinfo (3).
 278 .SH SEE ALSO
 279 .BR accept (2),
 280 .BR bind (2),
 281 .BR getsockname (2),
 282 .BR listen (2),
 283 .BR socket (2),
 284 .BR path_resolution (7)