1.\" Copyright (c) 2013 The FreeBSD Foundation 2.\" 3.\" This documentation was written by Pawel Jakub Dawidek under sponsorship from 4.\" the FreeBSD Foundation. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND 16.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 17.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 18.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE 19.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 20.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 21.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 22.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 25.\" SUCH DAMAGE. 26.\" 27.Dd July 25, 2026 28.Dt CONNECTAT 2 29.Os 30.Sh NAME 31.Nm connectat 32.Nd initiate a connection on a socket 33.Sh LIBRARY 34.Lb libc 35.Sh SYNOPSIS 36.In sys/types.h 37.In sys/socket.h 38.Pp 39.In fcntl.h 40.Ft int 41.Fn connectat "int fd" "int s" "const struct sockaddr *name" "socklen_t namelen" 42.Sh DESCRIPTION 43The 44.Fn connectat 45system call initiates a connection on the socket 46.Fa s . 47When passed the special value 48.Dv AT_FDCWD 49in the 50.Fa fd 51parameter, the behavior is identical to a call to 52.Xr connect 2 . 53Otherwise, 54.Fn connectat 55works like the 56.Xr connect 2 57system call with two exceptions: 58.Pp 59.Bl -enum -offset indent -compact 60.It 61It is limited to sockets in the PF_LOCAL domain. 62.Pp 63.It 64If the file path stored in the 65.Fa sun_path 66field of the sockaddr_un structure is a relative path, it is located relative 67to the directory associated with the file descriptor 68.Fa fd . 69.Pp 70.It 71If the 72.Fa sun_path 73field is empty, that is, 74.Fa namelen 75equals 76.Li offsetof(struct sockaddr_un, sun_path) , 77then 78.Fa fd 79does not resolve a path but instead names the peer socket directly. 80.Pp 81The descriptor may be the peer socket itself, or a descriptor for a file, such 82as an 83.Dv O_PATH 84handle opened with 85.Xr open 2 . 86In the latter case, the 87.Dv O_PATH 88file description may point either to a traditional socket file bound to the 89peer, or to a 90.Pa /dev/fd/N 91node. 92The 93.Pa /dev/fd/N 94node must name the socket descriptor itself; the named descriptor is resolved a 95single level and not chased further. 96With a standard 97.Xr fdescfs 5 98mount this means a node naming a traditional socket file, or another 99.Pa /dev/fd/N 100node, does not resolve to a peer. 101.Pp 102In all cases, 103.Fa fd 104must carry the 105.Dv CAP_CONNECTAT 106capability right, and the target of a 107.Pa /dev/fd/N 108node must also carry that right; see 109.Xr unix 4 110for the details of this form. 111.El 112.Sh RETURN VALUES 113.Rv -std connectat 114.Sh ERRORS 115The 116.Fn connectat 117system call may fail with the same errors as the 118.Xr connect 2 119system call or with the following errors: 120.Bl -tag -width Er 121.It Bq Er EBADF 122The 123.Fa sun_path 124field does not specify an absolute path and the 125.Fa fd 126argument is neither 127.Dv AT_FDCWD 128nor a valid file descriptor. 129.It Bq Er ENOTDIR 130The 131.Fa sun_path 132field is not an absolute path and 133.Fa fd 134is neither 135.Dv AT_FDCWD 136nor a file descriptor associated with a directory. 137.It Bq Er EINVAL 138The 139.Fa sun_path 140field is empty but 141.Fa fd 142is 143.Dv AT_FDCWD , 144so no descriptor names the peer. 145.El 146.Sh SEE ALSO 147.Xr bindat 2 , 148.Xr connect 2 , 149.Xr socket 2 , 150.Xr unix 4 151.Sh AUTHORS 152The 153.Nm 154was developed by 155.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net 156under sponsorship from the FreeBSD Foundation. 157