1CONNECT(2) Linux Programmer's Manual CONNECT(2)
2
6 connect - initiate a connection on a socket
7
9 #include <sys/types.h> /* See NOTES */
10 #include <sys/socket.h>
11 int connect(int sockfd, const struct sockaddr *addr,
13 socklen_t addrlen);
14
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 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 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
35 If the connection or binding succeeds, zero is returned. On error, -1
36 is returned, and errno is set appropriately.
37
39 The following are general socket errors only. There may be other
40 domain-specific error codes.
41 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 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 EADDRINUSE
53 Local address is already in use.
54 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 EAFNOSUPPORT
64 The passed address didn't have the correct address family in its
65 sa_family field.
66 EAGAIN For nonblocking UNIX domain sockets, the socket is nonblocking,
68 and the connection cannot be completed immediately. For other
69 socket families, there are insufficient entries in the routing
70 cache.
71 EALREADY
73 The socket is nonblocking and a previous connection attempt has
74 not yet been completed.
75 EBADF sockfd is not a valid open file descriptor.
77 ECONNREFUSED
79 A connect() on a stream socket found no one listening on the
80 remote address.
81 EFAULT The socket structure address is outside the user's address
83 space.
84 EINPROGRESS
86 The socket is nonblocking and the connection cannot be completed
87 immediately. (UNIX domain sockets failed with EAGAIN instead.)
88 It is possible to select(2) or poll(2) for completion by select‐
89 ing the socket for writing. After select(2) indicates writabil‐
90 ity, use getsockopt(2) to read the SO_ERROR option at level
91 SOL_SOCKET to determine whether connect() completed successfully
92 (SO_ERROR is zero) or unsuccessfully (SO_ERROR is one of the
93 usual error codes listed here, explaining the reason for the
94 failure).
95 EINTR The system call was interrupted by a signal that was caught; see
97 signal(7).
98 EISCONN
100 The socket is already connected.
101 ENETUNREACH
103 Network is unreachable.
104 ENOTSOCK
106 The file descriptor sockfd does not refer to a socket.
107 EPROTOTYPE
109 The socket type does not support the requested communications
110 protocol. This error can occur, for example, on an attempt to
111 connect a UNIX domain datagram socket to a stream socket.
112 ETIMEDOUT
114 Timeout while attempting connection. The server may be too busy
115 to accept new connections. Note that for IP sockets the timeout
116 may be very long when syncookies are enabled on the server.
117
119 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared in
120 4.2BSD).
121
123 POSIX.1 does not require the inclusion of <sys/types.h>, and this
124 header file is not required on Linux. However, some historical (BSD)
125 implementations required this header file, and portable applications
126 are probably wise to include it.
127 For background on the socklen_t type, see accept(2).
129 If connect() fails, consider the state of the socket as unspecified.
131 Portable applications should close the socket and create a new one for
132 reconnecting.
133
135 An example of the use of connect() is shown in getaddrinfo(3).
136
138 accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
139 tion(7)
140
142 This page is part of release 5.02 of the Linux man-pages project. A
143 description of the project, information about reporting bugs, and the
144 latest version of this page, can be found at
145 https://www.kernel.org/doc/man-pages/.
146Linux 2019-03-06 CONNECT(2)