1connect(2) System Calls Manual connect(2)
2
6 connect - initiate a connection on a socket
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9 Standard C library (libc, -lc)
10
12 #include <sys/socket.h>
13 int connect(int sockfd, const struct sockaddr *addr,
15 socklen_t addrlen);
16
18 The connect() system call connects the socket referred to by the file
19 descriptor sockfd to the address specified by addr. The addrlen argu‐
20 ment specifies the size of addr. The format of the address in addr is
21 determined by the address space of the socket sockfd; see socket(2) for
22 further details.
23 If the socket sockfd is of type SOCK_DGRAM, then addr is the address to
25 which datagrams are sent by default, and the only address from which
26 datagrams are received. If the socket is of type SOCK_STREAM or
27 SOCK_SEQPACKET, this call attempts to make a connection to the socket
28 that is bound to the address specified by addr.
29 Some protocol sockets (e.g., UNIX domain stream sockets) may success‐
31 fully connect() only once.
32 Some protocol sockets (e.g., datagram sockets in the UNIX and Internet
34 domains) may use connect() multiple times to change their association.
35 Some protocol sockets (e.g., TCP sockets as well as datagram sockets in
37 the UNIX and Internet domains) may dissolve the association by connect‐
38 ing to an address with the sa_family member of sockaddr set to AF_UN‐
39 SPEC; thereafter, the socket can be connected to another address.
40 (AF_UNSPEC is supported since Linux 2.2.)
41
43 If the connection or binding succeeds, zero is returned. On error, -1
44 is returned, and errno is set to indicate the error.
45
47 The following are general socket errors only. There may be other do‐
48 main-specific error codes.
49 EACCES For UNIX domain sockets, which are identified by pathname: Write
51 permission is denied on the socket file, or search permission is
52 denied for one of the directories in the path prefix. (See also
53 path_resolution(7).)
54 EACCES, EPERM
56 The user tried to connect to a broadcast address without having
57 the socket broadcast flag enabled or the connection request
58 failed because of a local firewall rule.
59 EACCES It can also be returned if an SELinux policy denied a connection
61 (for example, if there is a policy saying that an HTTP proxy can
62 only connect to ports associated with HTTP servers, and the
63 proxy tries to connect to a different port).
64 EADDRINUSE
66 Local address is already in use.
67 EADDRNOTAVAIL
69 (Internet domain sockets) The socket referred to by sockfd had
70 not previously been bound to an address and, upon attempting to
71 bind it to an ephemeral port, it was determined that all port
72 numbers in the ephemeral port range are currently in use. See
73 the discussion of /proc/sys/net/ipv4/ip_local_port_range in
74 ip(7).
75 EAFNOSUPPORT
77 The passed address didn't have the correct address family in its
78 sa_family field.
79 EAGAIN For nonblocking UNIX domain sockets, the socket is nonblocking,
81 and the connection cannot be completed immediately. For other
82 socket families, there are insufficient entries in the routing
83 cache.
84 EALREADY
86 The socket is nonblocking and a previous connection attempt has
87 not yet been completed.
88 EBADF sockfd is not a valid open file descriptor.
90 ECONNREFUSED
92 A connect() on a stream socket found no one listening on the re‐
93 mote address.
94 EFAULT The socket structure address is outside the user's address
96 space.
97 EINPROGRESS
99 The socket is nonblocking and the connection cannot be completed
100 immediately. (UNIX domain sockets failed with EAGAIN instead.)
101 It is possible to select(2) or poll(2) for completion by select‐
102 ing the socket for writing. After select(2) indicates writabil‐
103 ity, use getsockopt(2) to read the SO_ERROR option at level
104 SOL_SOCKET to determine whether connect() completed successfully
105 (SO_ERROR is zero) or unsuccessfully (SO_ERROR is one of the
106 usual error codes listed here, explaining the reason for the
107 failure).
108 EINTR The system call was interrupted by a signal that was caught; see
110 signal(7).
111 EISCONN
113 The socket is already connected.
114 ENETUNREACH
116 Network is unreachable.
117 ENOTSOCK
119 The file descriptor sockfd does not refer to a socket.
120 EPROTOTYPE
122 The socket type does not support the requested communications
123 protocol. This error can occur, for example, on an attempt to
124 connect a UNIX domain datagram socket to a stream socket.
125 ETIMEDOUT
127 Timeout while attempting connection. The server may be too busy
128 to accept new connections. Note that for IP sockets the timeout
129 may be very long when syncookies are enabled on the server.
130
132 POSIX.1-2008.
133
135 POSIX.1-2001, SVr4, 4.4BSD, (connect() first appeared in 4.2BSD).
136
138 If connect() fails, consider the state of the socket as unspecified.
139 Portable applications should close the socket and create a new one for
140 reconnecting.
141
143 An example of the use of connect() is shown in getaddrinfo(3).
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146 accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolu‐
147 tion(7), selinux(8)
148Linux man-pages 6.04 2023-03-30 connect(2)