1CONNECT(2) Linux Programmer's Manual CONNECT(2)
2
6 connect - initiate a connection on a socket
7
9 #include <sys/socket.h>
10 int connect(int sockfd, const struct sockaddr *addr,
12 socklen_t addrlen);
13
15 The connect() system call connects the socket referred to by the file
16 descriptor sockfd to the address specified by addr. The addrlen argu‐
17 ment specifies the size of addr. The format of the address in addr is
18 determined by the address space of the socket sockfd; see socket(2) for
19 further details.
20 If the socket sockfd is of type SOCK_DGRAM, then addr is the address to
22 which datagrams are sent by default, and the only address from which
23 datagrams are received. If the socket is of type SOCK_STREAM or
24 SOCK_SEQPACKET, this call attempts to make a connection to the socket
25 that is bound to the address specified by addr.
26 Some protocol sockets (e.g., UNIX domain stream sockets) may success‐
28 fully connect() only once.
29 Some protocol sockets (e.g., datagram sockets in the UNIX and Internet
31 domains) may use connect() multiple times to change their association.
32 Some protocol sockets (e.g., TCP sockets as well as datagram sockets in
34 the UNIX and Internet domains) may dissolve the association by connect‐
35 ing to an address with the sa_family member of sockaddr set to AF_UN‐
36 SPEC; thereafter, the socket can be connected to another address.
37 (AF_UNSPEC is supported on Linux since kernel 2.2.)
38
40 If the connection or binding succeeds, zero is returned. On error, -1
41 is returned, and errno is set to indicate the error.
42
44 The following are general socket errors only. There may be other do‐
45 main-specific error codes.
46 EACCES For UNIX domain sockets, which are identified by pathname: Write
48 permission is denied on the socket file, or search permission is
49 denied for one of the directories in the path prefix. (See also
50 path_resolution(7).)
51 EACCES, EPERM
53 The user tried to connect to a broadcast address without having
54 the socket broadcast flag enabled or the connection request
55 failed because of a local firewall rule.
56 EACCES can also be returned if an SELinux policy denied a con‐
58 nection (for example, if there is a policy saying that an HTTP
59 proxy can only connect to ports associated with HTTP servers,
60 and the proxy tries to connect to a different port). dd
61 EADDRINUSE
63 Local address is already in use.
64 EADDRNOTAVAIL
66 (Internet domain sockets) The socket referred to by sockfd had
67 not previously been bound to an address and, upon attempting to
68 bind it to an ephemeral port, it was determined that all port
69 numbers in the ephemeral port range are currently in use. See
70 the discussion of /proc/sys/net/ipv4/ip_local_port_range in
71 ip(7).
72 EAFNOSUPPORT
74 The passed address didn't have the correct address family in its
75 sa_family field.
76 EAGAIN For nonblocking UNIX domain sockets, the socket is nonblocking,
78 and the connection cannot be completed immediately. For other
79 socket families, there are insufficient entries in the routing
80 cache.
81 EALREADY
83 The socket is nonblocking and a previous connection attempt has
84 not yet been completed.
85 EBADF sockfd is not a valid open file descriptor.
87 ECONNREFUSED
89 A connect() on a stream socket found no one listening on the re‐
90 mote address.
91 EFAULT The socket structure address is outside the user's address
93 space.
94 EINPROGRESS
96 The socket is nonblocking and the connection cannot be completed
97 immediately. (UNIX domain sockets failed with EAGAIN instead.)
98 It is possible to select(2) or poll(2) for completion by select‐
99 ing the socket for writing. After select(2) indicates writabil‐
100 ity, use getsockopt(2) to read the SO_ERROR option at level
101 SOL_SOCKET to determine whether connect() completed successfully
102 (SO_ERROR is zero) or unsuccessfully (SO_ERROR is one of the
103 usual error codes listed here, explaining the reason for the
104 failure).
105 EINTR The system call was interrupted by a signal that was caught; see
107 signal(7).
108 EISCONN
110 The socket is already connected.
111 ENETUNREACH
113 Network is unreachable.
114 ENOTSOCK
116 The file descriptor sockfd does not refer to a socket.
117 EPROTOTYPE
119 The socket type does not support the requested communications
120 protocol. This error can occur, for example, on an attempt to
121 connect a UNIX domain datagram socket to a stream socket.
122 ETIMEDOUT
124 Timeout while attempting connection. The server may be too busy
125 to accept new connections. Note that for IP sockets the timeout
126 may be very long when syncookies are enabled on the server.
127
129 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD, (connect() first appeared in
130 4.2BSD).
131
133 For background on the socklen_t type, see accept(2).
134 If connect() fails, consider the state of the socket as unspecified.
136 Portable applications should close the socket and create a new one for
137 reconnecting.
138
140 An example of the use of connect() is shown in getaddrinfo(3).
141
143 accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
144 tion(7), selinux(8)
145
147 This page is part of release 5.13 of the Linux man-pages project. A
148 description of the project, information about reporting bugs, and the
149 latest version of this page, can be found at
150 https://www.kernel.org/doc/man-pages/.
151Linux 2021-03-22 CONNECT(2)