1CONNECT(2)                 Linux Programmer's Manual                CONNECT(2)
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4

NAME

6       connect - initiate a connection on a socket
7

SYNOPSIS

9       #include <sys/types.h>          /* See NOTES */
10       #include <sys/socket.h>
11
12       int connect(int sockfd, const struct sockaddr *addr,
13                   socklen_t addrlen);
14

DESCRIPTION

16       The  connect()  system call connects the socket referred to by the file
17       descriptor sockfd to the address specified by addr.  The addrlen  argu‐
18       ment  specifies the size of addr.  The format of the address in addr is
19       determined by the address space of the socket sockfd; see socket(2) for
20       further details.
21
22       If the socket sockfd is of type SOCK_DGRAM, then addr is the address to
23       which datagrams are sent by default, and the only  address  from  which
24       datagrams  are  received.   If  the  socket  is  of type SOCK_STREAM or
25       SOCK_SEQPACKET, this call attempts to make a connection to  the  socket
26       that is bound to the address specified by addr.
27
28       Generally, connection-based protocol sockets may successfully connect()
29       only once; connectionless protocol sockets may use  connect()  multiple
30       times to change their association.  Connectionless sockets may dissolve
31       the association by connecting to an address with the  sa_family  member
32       of sockaddr set to AF_UNSPEC (supported on Linux since kernel 2.2).
33

RETURN VALUE

35       If  the connection or binding succeeds, zero is returned.  On error, -1
36       is returned, and errno is set appropriately.
37

ERRORS

39       The following are general socket  errors  only.   There  may  be  other
40       domain-specific error codes.
41
42       EACCES For UNIX domain sockets, which are identified by pathname: Write
43              permission is denied on the socket file, or search permission is
44              denied for one of the directories in the path prefix.  (See also
45              path_resolution(7).)
46
47       EACCES, EPERM
48              The user tried to connect to a broadcast address without  having
49              the  socket  broadcast  flag  enabled  or the connection request
50              failed because of a local firewall rule.
51
52       EADDRINUSE
53              Local address is already in use.
54
55       EADDRNOTAVAIL
56              (Internet domain sockets) The socket referred to by  sockfd  had
57              not  previously been bound to an address and, upon attempting to
58              bind it to an ephemeral port, it was determined  that  all  port
59              numbers  in  the ephemeral port range are currently in use.  See
60              the  discussion  of  /proc/sys/net/ipv4/ip_local_port_range   in
61              ip(7).
62
63       EAFNOSUPPORT
64              The passed address didn't have the correct address family in its
65              sa_family field.
66
67       EAGAIN Insufficient entries in the routing cache.
68
69       EALREADY
70              The socket is nonblocking and a previous connection attempt  has
71              not yet been completed.
72
73       EBADF  sockfd is not a valid open file descriptor.
74
75       ECONNREFUSED
76              A  connect()  on  a  stream socket found no one listening on the
77              remote address.
78
79       EFAULT The socket structure  address  is  outside  the  user's  address
80              space.
81
82       EINPROGRESS
83              The socket is nonblocking and the connection cannot be completed
84              immediately.  It is possible to select(2) or poll(2) for comple‐
85              tion by selecting the socket for writing.  After select(2) indi‐
86              cates writability, use getsockopt(2) to read the SO_ERROR option
87              at  level  SOL_SOCKET  to  determine whether connect() completed
88              successfully (SO_ERROR is zero) or unsuccessfully  (SO_ERROR  is
89              one  of the usual error codes listed here, explaining the reason
90              for the failure).
91
92       EINTR  The system call was interrupted by a signal that was caught; see
93              signal(7).
94
95       EISCONN
96              The socket is already connected.
97
98       ENETUNREACH
99              Network is unreachable.
100
101       ENOTSOCK
102              The file descriptor sockfd does not refer to a socket.
103
104       EPROTOTYPE
105              The  socket  type  does not support the requested communications
106              protocol.  This error can occur, for example, on an  attempt  to
107              connect a UNIX domain datagram socket to a stream socket.
108
109       ETIMEDOUT
110              Timeout while attempting connection.  The server may be too busy
111              to accept new connections.  Note that for IP sockets the timeout
112              may be very long when syncookies are enabled on the server.
113

CONFORMING TO

115       POSIX.1-2001,  POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared in
116       4.2BSD).
117

NOTES

119       POSIX.1 does not require  the  inclusion  of  <sys/types.h>,  and  this
120       header  file  is not required on Linux.  However, some historical (BSD)
121       implementations required this header file,  and  portable  applications
122       are probably wise to include it.
123
124       For background on the socklen_t type, see accept(2).
125
126       If  connect()  fails,  consider the state of the socket as unspecified.
127       Portable applications should close the socket and create a new one  for
128       reconnecting.
129

EXAMPLE

131       An example of the use of connect() is shown in getaddrinfo(3).
132

SEE ALSO

134       accept(2),  bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
135       tion(7)
136

COLOPHON

138       This page is part of release 4.16 of the Linux  man-pages  project.   A
139       description  of  the project, information about reporting bugs, and the
140       latest    version    of    this    page,    can     be     found     at
141       https://www.kernel.org/doc/man-pages/.
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145Linux                             2017-09-15                        CONNECT(2)
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