# connect

## NAME



           connect - initiate a connection on a socket
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/socket.h>**
    
           **int connect(int** _sockfd_**, const struct sockaddr ***_addr_**,**
                       **socklen_t** _addrlen_**);**
    


## DESCRIPTION



           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) 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 _sa_family_ 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



           If the connection or binding succeeds, zero is returned.  On
           error, -1 is returned, and _errno_ is set to indicate the error.
    


## ERRORS



           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).)
    
           **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/ip_local_port_range_ in ip(7).
    
           **EAFNOSUPPORT**
                  The passed address didn't have the correct address family
                  in its _sa_family_ 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) or poll(2)
                  for completion by selecting the socket for writing.  After
                  select(2) indicates writability, use getsockopt(2) 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).
    
           **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



           POSIX.1-2008.
    


## NOTES



           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



           An example of the use of **connect**() is shown in getaddrinfo(3).
    


* * *