connect(2) - Linux manual page (original) (raw)
connect(2) System Calls Manual connect(2)
NAME top
connect - initiate a connection on a socketLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/socket.h>**
**int connect(int** _sockfd_**, const struct sockaddr ***_addr_**,**
**socklen_t** _addrlen_**);**DESCRIPTION top
The **connect**() system call connects the socket referred to by the
file descriptor _sockfd_ to the address specified by _addr_. The
_addrlen_ argument specifies the size of _addr_. The format of the
address in _addr_ is determined by the address space of the socket
_sockfd_; see [socket(2)](../man2/socket.2.html) for further details.
If the socket _sockfd_ is of type **SOCK_DGRAM**, then _addr_ is the
address to which datagrams are sent by default, and the only
address from which datagrams are received. If the socket is of
type **SOCK_STREAM** or **SOCK_SEQPACKET**, this call attempts to make a
connection to the socket that is bound to the address specified by
_addr_.
Some protocol sockets (e.g., UNIX domain stream sockets) may
successfully **connect**() only once.
Some protocol sockets (e.g., datagram sockets in the UNIX and
Internet domains) may use **connect**() multiple times to change their
association.
Some protocol sockets (e.g., TCP sockets as well as datagram
sockets in the UNIX and Internet domains) may dissolve the
association by connecting to an address with the _safamily_ member
of _sockaddr_ set to **AF_UNSPEC**; thereafter, the socket can be
connected to another address. (**AF_UNSPEC** is supported since Linux
2.2.)RETURN VALUE top
If the connection or binding succeeds, zero is returned. On
error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
The following are general socket errors only. There may be other
domain-specific error codes.
**EACCES** For UNIX domain sockets, which are identified by pathname:
Write permission is denied on the socket file, or search
permission is denied for one of the directories in the path
prefix. (See also [path_resolution(7)](../man7/path%5Fresolution.7.html).)
**EACCES**
**EPERM** The user tried to connect to a broadcast address without
having the socket broadcast flag enabled or the connection
request failed because of a local firewall rule.
**EACCES** It can also be returned if an SELinux policy denied a
connection (for example, if there is a policy saying that
an HTTP proxy can only connect to ports associated with
HTTP servers, and the proxy tries to connect to a different
port).
**EADDRINUSE**
Local address is already in use.
**EADDRNOTAVAIL**
(Internet domain sockets) The socket referred to by _sockfd_
had not previously been bound to an address and, upon
attempting to bind it to an ephemeral port, it was
determined that all port numbers in the ephemeral port
range are currently in use. See the discussion of
_/proc/sys/net/ipv4/iplocalportrange_ in [ip(7)](../man7/ip.7.html).
**EAFNOSUPPORT**
The passed address didn't have the correct address family
in its _safamily_ field.
**EAGAIN** For nonblocking UNIX domain sockets, the socket is
nonblocking, and the connection cannot be completed
immediately. For other socket families, there are
insufficient entries in the routing cache.
**EALREADY**
The socket is nonblocking and a previous connection attempt
has not yet been completed.
**EBADF** _sockfd_ is not a valid open file descriptor.
**ECONNREFUSED**
A **connect**() on a stream socket found no one listening on
the remote address.
**EFAULT** The socket structure address is outside the user's address
space.
**EINPROGRESS**
The socket is nonblocking and the connection cannot be
completed immediately. (UNIX domain sockets failed with
**EAGAIN** instead.) It is possible to [select(2)](../man2/select.2.html) or [poll(2)](../man2/poll.2.html)
for completion by selecting the socket for writing. After
[select(2)](../man2/select.2.html) indicates writability, use [getsockopt(2)](../man2/getsockopt.2.html) to read
the **SO_ERROR** option at level **SOL_SOCKET** to determine
whether **connect**() completed successfully (**SO_ERROR** is zero)
or unsuccessfully (**SO_ERROR** is one of the usual error codes
listed here, explaining the reason for the failure).
**EINTR** The system call was interrupted by a signal that was
caught; see [signal(7)](../man7/signal.7.html).
**EISCONN**
The socket is already connected.
**ENETUNREACH**
Network is unreachable.
**ENOTSOCK**
The file descriptor _sockfd_ does not refer to a socket.
**EPROTOTYPE**
The socket type does not support the requested
communications protocol. This error can occur, for
example, on an attempt to connect a UNIX domain datagram
socket to a stream socket.
**ETIMEDOUT**
Timeout while attempting connection. The server may be too
busy to accept new connections. Note that for IP sockets
the timeout may be very long when syncookies are enabled on
the server.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001, SVr4, 4.4BSD, (**connect**() first appeared in 4.2BSD).NOTES top
If **connect**() fails, consider the state of the socket as
unspecified. Portable applications should close the socket and
create a new one for reconnecting.EXAMPLES top
An example of the use of **connect**() is shown in [getaddrinfo(3)](../man3/getaddrinfo.3.html).SEE ALSO top
[accept(2)](../man2/accept.2.html), [bind(2)](../man2/bind.2.html), [getsockname(2)](../man2/getsockname.2.html), [listen(2)](../man2/listen.2.html), [socket(2)](../man2/socket.2.html),
[path_resolution(7)](../man7/path%5Fresolution.7.html), [selinux(8)](../man8/selinux.8.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 connect(2)
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