1.\" $NetBSD: recv.2,v 1.29 2011/06/27 08:21:08 wiz Exp $ 2.\" 3.\" Copyright (c) 1983, 1990, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 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.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)recv.2 8.3 (Berkeley) 2/21/94 31.\" 32.Dd June 24, 2011 33.Dt RECV 2 34.Os 35.Sh NAME 36.Nm recv , 37.Nm recvfrom , 38.Nm recvmsg 39.Nd receive a message from a socket 40.Sh LIBRARY 41.Lb libc 42.Sh SYNOPSIS 43.In sys/socket.h 44.Ft ssize_t 45.Fn recv "int s" "void *buf" "size_t len" "int flags" 46.Ft ssize_t 47.Fn recvfrom "int s" "void * restrict buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen" 48.Ft ssize_t 49.Fn recvmsg "int s" "struct msghdr *msg" "int flags" 50.Sh DESCRIPTION 51.Fn recvfrom 52and 53.Fn recvmsg 54are used to receive messages from a socket, 55and may be used to receive data on a socket whether or not 56it is connection-oriented. 57.Pp 58If 59.Fa from 60is non-nil, and the socket is not connection-oriented, 61the source address of the message is filled in. 62.Fa fromlen 63is a value-result parameter, initialized to the size of 64the buffer associated with 65.Fa from , 66and modified on return to indicate the actual size of the 67address stored there. 68.Pp 69The 70.Fn recv 71call is normally used only on a 72.Em connected 73socket (see 74.Xr connect 2 ) 75and is identical to 76.Fn recvfrom 77with a nil 78.Fa from 79parameter. 80As it is redundant, it may not be supported in future releases. 81.Pp 82All three routines return the length of the message on successful 83completion. 84If a message is too long to fit in the supplied buffer, 85excess bytes may be discarded depending on the type of socket 86the message is received from (see 87.Xr socket 2 ) . 88.Pp 89If no messages are available at the socket, the 90receive call waits for a message to arrive, unless 91the socket is nonblocking (see 92.Xr fcntl 2 ) 93in which case the value 94\-1 is returned and the external variable 95.Va errno 96set to 97.Er EAGAIN . 98The receive calls normally return any data available, 99up to the requested amount, 100rather than waiting for receipt of the full amount requested; 101this behavior is affected by the socket-level options 102.Dv SO_RCVLOWAT 103and 104.Dv SO_RCVTIMEO 105described in 106.Xr getsockopt 2 . 107.Pp 108The 109.Xr select 2 110or 111.Xr poll 2 112call may be used to determine when more data arrive. 113.Pp 114The 115.Fa flags 116argument to a recv call is formed by 117.Em or Ap ing 118one or more of the values: 119.Bl -column MSG_CMSG_CLOEXEC -offset indent 120.It Dv MSG_CMSG_CLOEXEC Ta set the close on exec property for passed file descriptors 121.It Dv MSG_OOB Ta process out-of-band data 122.It Dv MSG_PEEK Ta peek at incoming message 123.It Dv MSG_WAITALL Ta wait for full request or error 124.El 125The 126.Dv MSG_OOB 127flag requests receipt of out-of-band data 128that would not be received in the normal data stream. 129Some protocols place expedited data at the head of the normal 130data queue, and thus this flag cannot be used with such protocols. 131The 132.Dv MSG_PEEK 133flag causes the receive operation to return data 134from the beginning of the receive queue without removing that 135data from the queue. 136Thus, a subsequent receive call will return the same data. 137The 138.Dv MSG_WAITALL 139flag requests that the operation block until 140the full request is satisfied. 141However, the call may still return less data than requested 142if a signal is caught, an error or disconnect occurs, 143or the next data to be received is of a different type than that returned. 144.Pp 145The 146.Fn recvmsg 147call uses a 148.Fa msghdr 149structure to minimize the number of directly supplied parameters. 150This structure has the following form, as defined in 151.Ao Pa sys/socket.h Ac : 152.Pp 153.Bd -literal 154struct msghdr { 155 void *msg_name; /* optional address */ 156 socklen_t msg_namelen; /* size of address */ 157 struct iovec *msg_iov; /* scatter/gather array */ 158 int msg_iovlen; /* # elements in msg_iov */ 159 void *msg_control; /* ancillary data, see below */ 160 socklen_t msg_controllen; /* ancillary data buffer len */ 161 int msg_flags; /* flags on received message */ 162}; 163.Ed 164.Pp 165Here 166.Fa msg_name 167and 168.Fa msg_namelen 169specify the source address if the socket is unconnected; 170.Fa msg_name 171may be given as a null pointer if no names are desired or required. 172If the socket is connected, 173.Fa msg_name 174and 175.Fa msg_namelen 176are ignored. 177.Fa msg_iov 178and 179.Fa msg_iovlen 180describe scatter gather locations, as discussed in 181.Xr read 2 . 182.Fa msg_control , 183which has length 184.Fa msg_controllen , 185points to a buffer for other protocol control related messages 186or other miscellaneous ancillary data. 187The messages are of the form: 188.Bd -literal 189struct cmsghdr { 190 socklen_t cmsg_len; /* data byte count, including hdr */ 191 int cmsg_level; /* originating protocol */ 192 int cmsg_type; /* protocol-specific type */ 193/* followed by 194 u_char cmsg_data[]; */ 195}; 196.Ed 197As an example, one could use this to learn of changes in the data-stream 198in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting 199a recvmsg with no data buffer provided immediately after an 200.Fn accept 201call. 202.Pp 203Open file descriptors are now passed as ancillary data for 204.Dv AF_LOCAL 205domain sockets, with 206.Fa cmsg_level 207set to 208.Dv SOL_SOCKET 209and 210.Fa cmsg_type 211set to 212.Dv SCM_RIGHTS . 213.Pp 214The 215.Fa msg_flags 216field is set on return according to the message received. 217.Dv MSG_EOR 218indicates end-of-record; 219the data returned completed a record (generally used with sockets of type 220.Dv SOCK_SEQPACKET ) . 221.Dv MSG_TRUNC 222indicates that 223the trailing portion of a datagram was discarded because the datagram 224was larger than the buffer supplied. 225.Dv MSG_CTRUNC 226indicates that some 227control data were discarded due to lack of space in the buffer 228for ancillary data. 229.Dv MSG_OOB 230is returned to indicate that expedited or out-of-band data were received. 231.Sh RETURN VALUES 232These calls return the number of bytes received, or \-1 233if an error occurred. 234.Sh ERRORS 235The calls fail if: 236.Bl -tag -width Er 237.It Bq Er EAGAIN 238The socket is marked non-blocking, and the receive operation 239would block, or 240a receive timeout had been set, 241and the timeout expired before data were received. 242.It Bq Er EBADF 243The argument 244.Fa s 245is an invalid descriptor. 246.It Bq Er EFAULT 247The receive buffer pointer(s) point outside the process's 248address space. 249.It Bq Er EINTR 250The receive was interrupted by delivery of a signal before 251any data were available. 252.It Bq Er EINVAL 253The total length of the I/O is more than can be expressed by the ssize_t 254return value. 255.It Bq Er ENOTCONN 256The socket is associated with a connection-oriented protocol 257and has not been connected (see 258.Xr connect 2 259and 260.Xr accept 2 ) . 261.It Bq Er ENOTSOCK 262The argument 263.Fa s 264does not refer to a socket. 265.El 266.Pp 267.Fn recvmsg 268will also fail if: 269.Bl -tag -width Er 270.It Bq Er EMSGSIZE 271The 272.Fa msg_iovlen 273member of the 274.Fa msg 275structure is less than or equal to 0 276or is greater than 277.Dv {IOV_MAX} . 278.El 279.Sh SEE ALSO 280.Xr fcntl 2 , 281.Xr getsockopt 2 , 282.Xr poll 2 , 283.Xr read 2 , 284.Xr select 2 , 285.Xr socket 2 286.Sh HISTORY 287The 288.Fn recv 289function call appeared in 290.Bx 4.2 . 291