.\" $NetBSD: recv.2,v 1.16 1999/12/02 21:42:38 kleink 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. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. 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 February 21, 1994 .Dt RECV 2 .Os .Sh NAME .Nm recv , .Nm recvfrom , .Nm recvmsg .Nd receive a message from a socket .Sh LIBRARY .Lb libc .Sh SYNOPSIS .Fd #include .Fd #include .Ft ssize_t .Fn recv "int s" "void *buf" "size_t len" "int flags" .Ft ssize_t .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "socklen_t *fromlen" .Ft ssize_t .Fn recvmsg "int s" "struct msghdr *msg" "int flags" .Sh DESCRIPTION .Fn recvfrom and .Fn recvmsg 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 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 All three routines return the length of the message on successful completion. 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 . 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_WAITALL -offset indent .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 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 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 { caddr_t msg_name; /* optional address */ u_int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ u_int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* ancillary data, see below */ u_int 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 destination address if the socket is unconnected; .Fa msg_name may be given as a null pointer if no names are desired or required. .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 { u_int 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. .Pp .Sh RETURN VALUES These calls return the number of bytes received, or -1 if an error occurred. .Sh ERRORS The calls fail if: .Bl -tag -width Er .It Bq Er EBADF The argument .Fa s is an invalid descriptor. .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. .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 EINTR The receive was interrupted by delivery of a signal before any data were available. .It Bq Er EFAULT The receive buffer pointer(s) point outside the process's address space. .It Bq Er EINVAL The total length of the I/O is more than can be expressed by the ssize_t return value. .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 .Dv {IOV_MAX} . .El .Sh SEE ALSO .Xr fcntl 2 , .Xr read 2 , .Xr poll 2 , .Xr select 2 , .Xr getsockopt 2 , .Xr socket 2 .Sh HISTORY The .Fn recv function call appeared in .Bx 4.2 .