NAME
connect
—
initiate a connection on a
socket
SYNOPSIS
#include
<sys/socket.h>
int
connect
(int
s, const struct sockaddr
*name, socklen_t
namelen);
DESCRIPTION
The parameter s is a socket. If it is of
type SOCK_DGRAM
, this call specifies the peer with
which the socket is to be associated; this address is that to which
datagrams are to be sent, and the only address from which datagrams are to
be received. If the socket is of type SOCK_STREAM
,
this call attempts to make a connection to another socket. The other socket
is specified by name, which is an address in the
communications space of the socket. namelen indicates
the amount of space pointed to by name, in bytes; the
sa_len member of name is
ignored. Each communications space interprets the name
parameter in its own way. Generally, stream sockets may use
connect
()
only once; datagram sockets may use connect
()
multiple times to change their association. Datagram sockets may dissolve
the association by connecting to an invalid address, such as a null
address.
If the socket is in non-blocking mode and the
connection cannot be completed immediately, or if it is interrupted by a
signal,
connect
()
will return an error and the connection attempt will proceed asynchronously.
Subsequent calls to connect
() will fail with errno
set to EALREADY
. It is possible to use
select(2) or
poll(2) to determine when the connect operation has completed by
checking the socket for writability. The success or failure of the
connection attempt may be determined by using
getsockopt(2) to check the socket error status with the
SO_ERROR
option at the
SOL_SOCKET
level. If the connection was successful,
the error value will be zero. Otherwise, it will be one of the error values
listed below.
RETURN VALUES
If the connection or binding succeeds, 0 is returned. Otherwise a -1 is returned, and a more specific error code is stored in errno.
EXAMPLES
The following code connects to the host described by
name and handles the case where
connect
() is interrupted by a signal.
#include <sys/socket.h> #include <poll.h> #include <errno.h> #include <err.h> int connect_wait(int s) { struct pollfd pfd[1]; int error = 0; socklen_t len = sizeof(error); pfd[0].fd = s; pfd[0].events = POLLOUT; if (poll(pfd, 1, -1) == -1) return -1; if (getsockopt(s, SOL_SOCKET, SO_ERROR, &error, &len) < 0) return -1; if (error != 0) { errno = error; return -1; } return 0; } ... int retcode; ... for (retcode = connect(s, name, namelen); retcode != 0 && errno == EINTR; retcode = connect_wait(s)) continue; if (retcode == -1) err(1, "connect");
ERRORS
The connect
() call fails if:
- [
EBADF
] - s is not a valid descriptor.
- [
ENOTSOCK
] - s is not a socket.
- [
EADDRNOTAVAIL
] - No suitable address is available on the local machine.
- [
EAFNOSUPPORT
] - Addresses in the specified address family cannot be used with this socket.
- [
EISCONN
] - The socket is already connected.
- [
ETIMEDOUT
] - Connection establishment timed out without establishing a connection.
- [
EINVAL
] - A TCP connection with a local broadcast, the all-ones or a multicast address as the peer was attempted.
- [
ECONNREFUSED
] - The attempt to connect was forcefully rejected.
- [
EHOSTUNREACH
] - The destination address specified an unreachable host.
- [
EINTR
] - The connection attempt was interrupted by a signal. The attempt will continue asynchronously as if the socket was non-blocking.
- [
ENETUNREACH
] - The network isn't reachable from this host.
- [
EADDRINUSE
] - The address is already in use.
- [
EFAULT
] - The name parameter specifies an area outside the process address space.
- [
EINPROGRESS
] - The socket is non-blocking and the connection cannot be completed immediately.
- [
EALREADY
] - Either the socket is non-blocking or a previous call to
connect
() was interrupted by a signal, and the connection attempt has not yet been completed. - [
EPERM
] - A TCP connection on a socket with socket option TCP_MD5SIG was attempted without configuring the security parameters correctly.
The following errors are specific to connecting names in the UNIX-domain. These errors may not apply in future versions of the UNIX IPC domain.
- [
ENOTDIR
] - A component of the path prefix is not a directory.
- [
ENAMETOOLONG
] - A component of a pathname exceeded
NAME_MAX
characters, or an entire pathname (including the terminating NUL) exceededPATH_MAX
bytes. - [
ENOENT
] - The named socket does not exist.
- [
EACCES
] - Search permission is denied for a component of the path prefix.
- [
EACCES
] - Write access to the named socket is denied.
- [
ELOOP
] - Too many symbolic links were encountered in translating the pathname.
- [
EPROTOTYPE
] - The file described by name is of a different type
than s. E.g., s may be of type
SOCK_STREAM
whereas name may refer to a socket of typeSOCK_DGRAM
.
SEE ALSO
accept(2), getsockname(2), getsockopt(2), poll(2), select(2), socket(2)
STANDARDS
The connect
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
The connect
() system call first appeared
in 4.1cBSD.