From Linux Man Pages
connect - initiate a connection on a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen);
DESCRIPTION
The connect() system call connects the socket referred to by the file descriptor sockfd to the address specified
by serv_addr. The addrlen argument specifies the size of serv_addr. The format of the address in serv_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 serv_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 serv_addr.
Generally, connection-based protocol sockets may successfully connect() only once; connectionless protocol sock-
ets may use connect() multiple times to change their association. Connectionless sockets may dissolve the asso-
ciation by connecting to an address with the sa_family member of sockaddr set to AF_UNSPEC.
RETURN VALUE
If the connection or binding succeeds, zero is returned. On error, -1 is returned, and errno is set appropri-
ately.
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_resolu-
tion(2).)
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.
EADDRINUSE
Local address is already in use.
EAFNOSUPPORT
The passed address didn't have the correct address family in its sa_family field.
EAGAIN No more free local ports or insufficient entries in the routing cache. For PF_INET see the
net.ipv4.ip_local_port_range sysctl in ip(7) on how to increase the number of local ports.
EALREADY
The socket is non-blocking and a previous connection attempt has not yet been completed.
EBADF The file descriptor is not a valid index in the descriptor table.
ECONNREFUSED
No one listening on the remote address.
EFAULT The socket structure address is outside the user's address space.
EINPROGRESS
The socket is non-blocking and the connection cannot be completed immediately. 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 con-
nect() 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.
EISCONN
The socket is already connected.
ENETUNREACH
Network is unreachable.
ENOTSOCK
The file descriptor is not associated with a 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.
CONFORMING TO
SVr4, 4.4BSD (the connect() function first appeared in 4.2BSD).
NOTE
The third argument of connect() is in reality an int (and this is what 4.x BSD and libc4 and libc5 have). Some
POSIX confusion resulted in the present socklen_t, also used by glibc. See also accept(2).
BUGS
Unconnecting a socket by calling connect() with a AF_UNSPEC address is not yet implemented.
RELATED
accept(2), bind(2), getsockname(2), listen(2), path_resolution(2), socket(2)
CATEGORY