.\" $NetBSD: recv.2,v 1.36.20.1 2018/04/09 13:34:10 bouyer Exp $ .\" .\" Copyright (c) 1983, 1990, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)recv.2 8.3 (Berkeley) 2/21/94 .\" .Dd March 19, 2018 .Dt RECV 2 .Os .Sh NAME .Nm recv , .Nm recvfrom , .Nm recvmsg , .Nm recvmmsg .Nd receive a message from a socket .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/socket.h .Ft ssize_t .Fn recv "int s" "void *buf" "size_t len" "int flags" .Ft ssize_t .Fn recvfrom "int s" "void * restrict buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen" .Ft ssize_t .Fn recvmsg "int s" "struct msghdr *msg" "int flags" .Ft int .Fn recvmmsg "int s" "struct mmsghdr *mmsg" "unsigned int vlen" "unsigned int flags" "struct timespec *timeout" .Sh DESCRIPTION .Fn recvfrom , .Fn recvmsg and .Fn recvmmsg are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented. .Pp If .Fa from is non-nil, and the socket is not connection-oriented, the source address of the message is filled in. .Fa fromlen is a value-result parameter, initialized to the size of the buffer associated with .Fa from , and modified on return to indicate the actual size of the address stored there. .Pp The .Fn recvmmsg call be used to receive multiple messages in the same call using an array of .Fa mmsghdr elements with the following form, as defined in .Ao Pa sys/socket.h Ac : .Pp .Bd -literal struct mmsghdr { struct msghdr msg_hdr; /* the message to be sent */ unsigned int msg_len; /* number of bytes received */ }; .Ed .Pp The .Fa msg_len member contains the number of bytes received for each .Fa msg_hdr member. The array has .Fa vlen elements, which is limited to .Dv 1024 . If there is an error, a number fewer than .Fa vlen may be returned, and the error may be retrieved using .Xr getsockopt 2 with .Dv SO_ERROR . If the flag .Dv MSG_WAITFORONE is set in .Fa flags then the .Fn recvmmsg call will wait for one message, and set .Dv MSG_DONTWAIT for the rest. If the .Fa timeout parameter is not .Dv NULL , then .Fn recvmmsg will return if that time is exceeded. .Pp The .Fn recv call is normally used only on a .Em connected socket (see .Xr connect 2 ) and is identical to .Fn recvfrom with a nil .Fa from parameter. As it is redundant, it may not be supported in future releases. .Pp .Fn recv , .Fn recvfrom and .Fn recvmsg routines return the length of the message on successful completion. .Fn recvmmsg returns the number of messages received. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see .Xr socket 2 ) . .Pp If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see .Xr fcntl 2 ) in which case the value \-1 is returned and the external variable .Va errno set to .Er EAGAIN . If no data is available and the remote peer was shut down, 0 is returned. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options .Dv SO_RCVLOWAT and .Dv SO_RCVTIMEO described in .Xr getsockopt 2 . .Pp The .Xr select 2 or .Xr poll 2 call may be used to determine when more data arrive. .Pp The .Fa flags argument to a recv call is formed by .Em or Ap ing one or more of the values: .Bl -column MSG_CMSG_CLOEXEC -offset indent .It Dv MSG_CMSG_CLOEXEC Ta set the close on exec property for passed file descriptors .It Dv MSG_OOB Ta process out-of-band data .It Dv MSG_PEEK Ta peek at incoming message .It Dv MSG_WAITALL Ta wait for full request or error .El The .Dv MSG_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The .Dv MSG_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. The .Dv MSG_WAITALL flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. .Pp The .Fn recvmsg call uses a .Fa msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in .Ao Pa sys/socket.h Ac : .Pp .Bd -literal struct msghdr { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ socklen_t msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; .Ed .Pp Here .Fa msg_name and .Fa msg_namelen specify the source address if the socket is unconnected; .Fa msg_name may be given as a null pointer if no names are desired or required. If the socket is connected, .Fa msg_name and .Fa msg_namelen are ignored. .Fa msg_iov and .Fa msg_iovlen describe scatter gather locations, as discussed in .Xr read 2 . .Fa msg_control , which has length .Fa msg_controllen , points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form: .Bd -literal struct cmsghdr { socklen_t cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ }; .Ed As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a recvmsg with no data buffer provided immediately after an .Fn accept call. .Pp Open file descriptors are now passed as ancillary data for .Dv AF_LOCAL domain sockets, with .Fa cmsg_level set to .Dv SOL_SOCKET and .Fa cmsg_type set to .Dv SCM_RIGHTS . .Pp The .Fa msg_flags field is set on return according to the message received. .Dv MSG_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type .Dv SOCK_SEQPACKET ) . .Dv MSG_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. .Dv MSG_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. .Dv MSG_OOB is returned to indicate that expedited or out-of-band data were received. .Sh RETURN VALUES The .Fn recv , .Fn recvfrom and .Fn recvmsg calls return the number of bytes received, or \-1 if an error occurred. For connected sockets whose remote peer was shut down, 0 is returned when no more data is available. The .Fn recvmmsg calls return the number of messages received, or \-1 if an error occurred. .Sh ERRORS The calls fail if: .Bl -tag -width Er .It Bq Er EAGAIN The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the timeout expired before data were received. .It Bq Er EBADF The argument .Fa s is an invalid descriptor. .It Bq Er EFAULT The receive buffer pointer(s) point outside the process's address space. .It Bq Er EINTR The receive was interrupted by delivery of a signal before any data were available. .It Bq Er EINVAL The total length of the I/O is more than can be expressed by the ssize_t return value. .It Bq Er ENOBUFS A message was not delivered because it would have overflowed the buffer. .It Bq Er ENOTCONN The socket is associated with a connection-oriented protocol and has not been connected (see .Xr connect 2 and .Xr accept 2 ) . .It Bq Er ENOTSOCK The argument .Fa s does not refer to a socket. .El .Pp .Fn recvmsg will also fail if: .Bl -tag -width Er .It Bq Er EMSGSIZE The .Fa msg_iovlen member of the .Fa msg structure is less than or equal to 0 or is greater than .Brq Dv IOV_MAX . .El .Sh SEE ALSO .Xr fcntl 2 , .Xr getsockopt 2 , .Xr poll 2 , .Xr read 2 , .Xr select 2 , .Xr socket 2 .Sh HISTORY The .Fn recv function call appeared in .Bx 4.2 . The .Fn recvmmsg function call appeared in .Tn Linux 2.6.32 and .Nx 7.0 .