connect() - Unix, Linux System Call
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connect() - Unix, Linux System Call

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connect - initiate a connection on a socket


#include <sys/types.h> 
#include <sys/socket.h> 

int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen);


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 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.


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.

Error CodeDescription
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(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.
EADDRNOTAVAIL Non-existent interface was requested or the requested address was not local.
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 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.
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).


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).


Unconnecting a socket by calling connect() with a AF_UNSPEC address is not yet implemented.


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