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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
connect(2) System Calls Manualconnect(2)connect - initiate a connection on a socket
Standard C library (libc,-lc)
#include <sys/socket.h>int connect(intsockfd, const struct sockaddr *addr,socklen_taddrlen);
Theconnect() system call connects the socket referred to by the file descriptorsockfd to the address specified byaddr. Theaddrlen argument specifies the size ofaddr. The format of the address inaddr is determined by the address space of the socketsockfd; seesocket(2) for further details. If the socketsockfd is of typeSOCK_DGRAM, thenaddr is the address to which datagrams are sent by default, and the only address from which datagrams are received. If the socket is of typeSOCK_STREAMorSOCK_SEQPACKET, this call attempts to make a connection to the socket that is bound to the address specified byaddr. Some protocol sockets (e.g., UNIX domain stream sockets) may successfullyconnect() only once. Some protocol sockets (e.g., datagram sockets in the UNIX and Internet domains) may useconnect() 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 thesa_family member ofsockaddr set toAF_UNSPEC; thereafter, the socket can be connected to another address. (AF_UNSPECis supported since Linux 2.2.)
If the connection or binding succeeds, zero is returned. On error, -1 is returned, anderrno is set to indicate the error.
The following are general socket errors only. There may be other domain-specific error codes.EACCESFor 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 alsopath_resolution(7).)EACCESEPERMThe 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.EACCESIt 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 bysockfd 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 inip(7).EAFNOSUPPORT The passed address didn't have the correct address family in itssa_family field.EAGAINFor 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.EBADFsockfd is not a valid open file descriptor.ECONNREFUSED Aconnect() on a stream socket found no one listening on the remote address.EFAULTThe 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 withEAGAINinstead.) It is possible toselect(2) orpoll(2) for completion by selecting the socket for writing. Afterselect(2) indicates writability, usegetsockopt(2) to read theSO_ERRORoption at levelSOL_SOCKETto determine whetherconnect() completed successfully (SO_ERRORis zero) or unsuccessfully (SO_ERRORis one of the usual error codes listed here, explaining the reason for the failure).EINTRThe system call was interrupted by a signal that was caught; seesignal(7).EISCONN The socket is already connected.ENETUNREACH Network is unreachable.ENOTSOCK The file descriptorsockfd 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.
POSIX.1-2008.
POSIX.1-2001, SVr4, 4.4BSD, (connect() first appeared in 4.2BSD).
Ifconnect() fails, consider the state of the socket as unspecified. Portable applications should close the socket and create a new one for reconnecting.
An example of the use ofconnect() is shown ingetaddrinfo(3).
accept(2),bind(2),getsockname(2),listen(2),socket(2),path_resolution(7),selinux(8)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17connect(2)Pages that refer to this page:telnet-probe(1), accept(2), bind(2), getpeername(2), io_uring_enter2(2), io_uring_enter(2), listen(2), recv(2), select(2), select_tut(2), send(2), shutdown(2), socket(2), socketcall(2), syscalls(2), write(2), getaddrinfo(3), io_uring_prep_connect(3), ldap_get_option(3), rtime(3), sockaddr(3type), ldap.conf(5), slapd-asyncmeta(5), slapd-ldap(5), slapd-meta(5), ddp(7), ip(7), landlock(7), netlink(7), packet(7), sctp(7), signal(7), signal-safety(7), sock_diag(7), socket(7), tcp(7), udp(7), unix(7), vsock(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |