1CONNECT(2)                 Linux Programmer's Manual                CONNECT(2)
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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       Some  protocol  sockets (e.g., UNIX domain stream sockets) may success‐
29       fully connect() only once.
30
31       Some protocol sockets (e.g., datagram sockets in the UNIX and  Internet
32       domains) may use connect() multiple times to change their association.
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34       Some protocol sockets (e.g., TCP sockets as well as datagram sockets in
35       the UNIX and Internet domains) may dissolve the association by connect‐
36       ing  to  an  address  with  the  sa_family  member  of  sockaddr set to
37       AF_UNSPEC; thereafter, the socket can be connected to another  address.
38       (AF_UNSPEC is supported on Linux since kernel 2.2.)
39

RETURN VALUE

41       If  the connection or binding succeeds, zero is returned.  On error, -1
42       is returned, and errno is set appropriately.
43

ERRORS

45       The following are general socket  errors  only.   There  may  be  other
46       domain-specific error codes.
47
48       EACCES For UNIX domain sockets, which are identified by pathname: Write
49              permission is denied on the socket file, or search permission is
50              denied for one of the directories in the path prefix.  (See also
51              path_resolution(7).)
52
53       EACCES, EPERM
54              The user tried to connect to a broadcast address without  having
55              the  socket  broadcast  flag  enabled  or the connection request
56              failed because of a local firewall rule.
57
58              EACCES can also be returned if an SELinux policy denied  a  con‐
59              nection  (for  example, if there is a policy saying that an HTTP
60              proxy can only connect to ports associated  with  HTTP  servers,
61              and the proxy tries to connect to a different port).  dd
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63       EADDRINUSE
64              Local address is already in use.
65
66       EADDRNOTAVAIL
67              (Internet  domain  sockets) The socket referred to by sockfd had
68              not previously been bound to an address and, upon attempting  to
69              bind  it  to  an ephemeral port, it was determined that all port
70              numbers in the ephemeral port range are currently in  use.   See
71              the   discussion  of  /proc/sys/net/ipv4/ip_local_port_range  in
72              ip(7).
73
74       EAFNOSUPPORT
75              The passed address didn't have the correct address family in its
76              sa_family field.
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78       EAGAIN For  nonblocking UNIX domain sockets, the socket is nonblocking,
79              and the connection cannot be completed immediately.   For  other
80              socket  families,  there are insufficient entries in the routing
81              cache.
82
83       EALREADY
84              The socket is nonblocking and a previous connection attempt  has
85              not yet been completed.
86
87       EBADF  sockfd is not a valid open file descriptor.
88
89       ECONNREFUSED
90              A  connect()  on  a  stream socket found no one listening on the
91              remote address.
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93       EFAULT The socket structure  address  is  outside  the  user's  address
94              space.
95
96       EINPROGRESS
97              The socket is nonblocking and the connection cannot be completed
98              immediately.  (UNIX domain sockets failed with EAGAIN  instead.)
99              It is possible to select(2) or poll(2) for completion by select‐
100              ing the socket for writing.  After select(2) indicates writabil‐
101              ity,  use  getsockopt(2)  to  read  the SO_ERROR option at level
102              SOL_SOCKET to determine whether connect() completed successfully
103              (SO_ERROR  is  zero)  or  unsuccessfully (SO_ERROR is one of the
104              usual error codes listed here, explaining  the  reason  for  the
105              failure).
106
107       EINTR  The system call was interrupted by a signal that was caught; see
108              signal(7).
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110       EISCONN
111              The socket is already connected.
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113       ENETUNREACH
114              Network is unreachable.
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116       ENOTSOCK
117              The file descriptor sockfd does not refer to a socket.
118
119       EPROTOTYPE
120              The socket type does not support  the  requested  communications
121              protocol.   This  error can occur, for example, on an attempt to
122              connect a UNIX domain datagram socket to a stream socket.
123
124       ETIMEDOUT
125              Timeout while attempting connection.  The server may be too busy
126              to accept new connections.  Note that for IP sockets the timeout
127              may be very long when syncookies are enabled on the server.
128

CONFORMING TO

130       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared  in
131       4.2BSD).
132

NOTES

134       POSIX.1  does  not  require  the  inclusion  of <sys/types.h>, and this
135       header file is not required on Linux.  However, some  historical  (BSD)
136       implementations  required  this  header file, and portable applications
137       are probably wise to include it.
138
139       For background on the socklen_t type, see accept(2).
140
141       If connect() fails, consider the state of the  socket  as  unspecified.
142       Portable  applications should close the socket and create a new one for
143       reconnecting.
144

EXAMPLES

146       An example of the use of connect() is shown in getaddrinfo(3).
147

SEE ALSO

149       accept(2), bind(2), getsockname(2), listen(2), socket(2),  path_resolu‐
150       tion(7), selinux(8)
151

COLOPHON

153       This  page  is  part of release 5.07 of the Linux man-pages project.  A
154       description of the project, information about reporting bugs,  and  the
155       latest     version     of     this    page,    can    be    found    at
156       https://www.kernel.org/doc/man-pages/.
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160Linux                             2020-04-11                        CONNECT(2)
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