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connect - initiate a connection on a socket
#include <sys/types.h> /* See NOTES */ #include <sys/socket.h> int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
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. Generally, connection-based protocol sockets may successfully connect() only once; connectionless protocol sockets may use connect() multiple times to change their association. Connectionless sockets may dissolve the association by connecting to an address with the sa_family member of sockaddr set to AF_UNSPEC (supported on Linux since kernel 2.2).
If the connection or binding succeeds, zero is returned. On error, -1 is returned, and errno is set appropriately.
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. 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 AF_INET see the description of /proc/sys/net/ipv4/ip_local_port_range ip(7) for information on how to increase the number of local ports. EALREADY The socket is nonblocking 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 nonblocking 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 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 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.
SVr4, 4.4BSD, (the connect() function first appeared in 4.2BSD), POSIX.1-2001.
POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it. 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).
An example of the use of connect() is shown in getaddrinfo(3).
accept(2), bind(2), getsockname(2), listen(2), socket(2), path_resolution(7)
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