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 Insufficient entries in the routing cache.
68 EALREADY
70 The socket is nonblocking and a previous connection attempt has
71 not yet been completed.
72 EBADF sockfd is not a valid open file descriptor.
74 ECONNREFUSED
76 A connect() on a stream socket found no one listening on the
77 remote address.
78 EFAULT The socket structure address is outside the user's address
80 space.
81 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 EINTR The system call was interrupted by a signal that was caught; see
93 signal(7).
94 EISCONN
96 The socket is already connected.
97 ENETUNREACH
99 Network is unreachable.
100 ENOTSOCK
102 The file descriptor sockfd does not refer to a socket.
103 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 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
115 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared in
116 4.2BSD).
117
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 For background on the socklen_t type, see accept(2).
125 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
131 An example of the use of connect() is shown in getaddrinfo(3).
132
134 accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
135 tion(7)
136
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/.
142Linux 2017-09-15 CONNECT(2)