1.\" $NetBSD: recv.2,v 1.26 2004/05/13 10:20:58 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 11, 2003 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_WAITALL -offset indent 120.It Dv MSG_OOB Ta process out-of-band data 121.It Dv MSG_PEEK Ta peek at incoming message 122.It Dv MSG_WAITALL Ta wait for full request or error 123.El 124The 125.Dv MSG_OOB 126flag requests receipt of out-of-band data 127that would not be received in the normal data stream. 128Some protocols place expedited data at the head of the normal 129data queue, and thus this flag cannot be used with such protocols. 130The MSG_PEEK flag causes the receive operation to return data 131from the beginning of the receive queue without removing that 132data from the queue. 133Thus, a subsequent receive call will return the same data. 134The MSG_WAITALL flag requests that the operation block until 135the full request is satisfied. 136However, the call may still return less data than requested 137if a signal is caught, an error or disconnect occurs, 138or the next data to be received is of a different type than that returned. 139.Pp 140The 141.Fn recvmsg 142call uses a 143.Fa msghdr 144structure to minimize the number of directly supplied parameters. 145This structure has the following form, as defined in 146.Ao Pa sys/socket.h Ac : 147.Pp 148.Bd -literal 149struct msghdr { 150 void *msg_name; /* optional address */ 151 socklen_t msg_namelen; /* size of address */ 152 struct iovec *msg_iov; /* scatter/gather array */ 153 int msg_iovlen; /* # elements in msg_iov */ 154 void *msg_control; /* ancillary data, see below */ 155 socklen_t msg_controllen; /* ancillary data buffer len */ 156 int msg_flags; /* flags on received message */ 157}; 158.Ed 159.Pp 160Here 161.Fa msg_name 162and 163.Fa msg_namelen 164specify the destination address if the socket is unconnected; 165.Fa msg_name 166may be given as a null pointer if no names are desired or required. 167.Fa msg_iov 168and 169.Fa msg_iovlen 170describe scatter gather locations, as discussed in 171.Xr read 2 . 172.Fa msg_control , 173which has length 174.Fa msg_controllen , 175points to a buffer for other protocol control related messages 176or other miscellaneous ancillary data. 177The messages are of the form: 178.Bd -literal 179struct cmsghdr { 180 socklen_t cmsg_len; /* data byte count, including hdr */ 181 int cmsg_level; /* originating protocol */ 182 int cmsg_type; /* protocol-specific type */ 183/* followed by 184 u_char cmsg_data[]; */ 185}; 186.Ed 187As an example, one could use this to learn of changes in the data-stream 188in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting 189a recvmsg with no data buffer provided immediately after an 190.Fn accept 191call. 192.Pp 193Open file descriptors are now passed as ancillary data for 194.Dv AF_LOCAL 195domain sockets, with 196.Fa cmsg_level 197set to 198.Dv SOL_SOCKET 199and 200.Fa cmsg_type 201set to 202.Dv SCM_RIGHTS . 203.Pp 204The 205.Fa msg_flags 206field is set on return according to the message received. 207.Dv MSG_EOR 208indicates end-of-record; 209the data returned completed a record (generally used with sockets of type 210.Dv SOCK_SEQPACKET ) . 211.Dv MSG_TRUNC 212indicates that 213the trailing portion of a datagram was discarded because the datagram 214was larger than the buffer supplied. 215.Dv MSG_CTRUNC 216indicates that some 217control data were discarded due to lack of space in the buffer 218for ancillary data. 219.Dv MSG_OOB 220is returned to indicate that expedited or out-of-band data were received. 221.Sh RETURN VALUES 222These calls return the number of bytes received, or \-1 223if an error occurred. 224.Sh ERRORS 225The calls fail if: 226.Bl -tag -width Er 227.It Bq Er EBADF 228The argument 229.Fa s 230is an invalid descriptor. 231.It Bq Er ENOTCONN 232The socket is associated with a connection-oriented protocol 233and has not been connected (see 234.Xr connect 2 235and 236.Xr accept 2 ) . 237.It Bq Er ENOTSOCK 238The argument 239.Fa s 240does not refer to a socket. 241.It Bq Er EAGAIN 242The socket is marked non-blocking, and the receive operation 243would block, or 244a receive timeout had been set, 245and the timeout expired before data were received. 246.It Bq Er EINTR 247The receive was interrupted by delivery of a signal before 248any data were available. 249.It Bq Er EFAULT 250The receive buffer pointer(s) point outside the process's 251address space. 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.El 256.Pp 257.Fn recvmsg 258will also fail if: 259.Bl -tag -width Er 260.It Bq Er EMSGSIZE 261The 262.Fa msg_iovlen 263member of the 264.Fa msg 265structure is less than or equal to 0 266or is greater than 267.Dv {IOV_MAX} . 268.El 269.Sh SEE ALSO 270.Xr fcntl 2 , 271.Xr getsockopt 2 , 272.Xr poll 2 , 273.Xr read 2 , 274.Xr select 2 , 275.Xr socket 2 276.Sh HISTORY 277The 278.Fn recv 279function call appeared in 280.Bx 4.2 . 281