bsd/man/man2/recv.2

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  34 .\"     @(#)recv.2      8.3 (Berkeley) 2/21/94
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  36 .Dd February 21, 1994
  37 .Dt RECV 2
  38 .Os BSD 4.3r
  39 .Sh NAME
  40 .Nm recv ,
  41 .Nm recvfrom ,
  42 .Nm recvmsg
  43 .Nd receive a message from a socket
  44 .Sh SYNOPSIS
  45 .Fd #include <sys/types.h>
  46 .Fd #include <sys/socket.h>
  47 .Ft ssize_t
  48 .Fn recv "int s" "void *buf" "size_t len" "int flags"
  49 .Ft ssize_t
  50 .Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "socklen_t *fromlen"
  51 .Ft ssize_t
  52 .Fn recvmsg "int s" "struct msghdr *msg" "int flags"
  53 .Sh DESCRIPTION
  54 .Fn Recvfrom
  55 and
  56 .Fn recvmsg
  57 are used to receive messages from a socket,
  58 and may be used to receive data on a socket whether or not
  59 it is connection-oriented.
  60 .Pp
  61 If
  62 .Fa from
  63 is non-nil, and the socket is not connection-oriented,
  64 the source address of the message is filled in.
  65 .Fa Fromlen
  66 is a value-result parameter, initialized to the size of
  67 the buffer associated with
  68 .Fa from ,
  69 and modified on return to indicate the actual size of the
  70 address stored there.
  71 .Pp
  72 The
  73 .Fn recv
  74 call is normally used only on a
  75 .Em connected
  76 socket (see
  77 .Xr connect 2 )
  78 and is identical to
  79 .Fn recvfrom
  80 with a nil
  81 .Fa from
  82 parameter.
  83 As it is redundant, it may not be supported in future releases.
  84 .Pp
  85 On successful completion, all three routines return the number of
  86 message bytes read.  If a message is too long to fit in the supplied
  87 buffer, excess bytes may be discarded depending on the type of socket
  88 the message is received from (see
  89 .Xr socket 2 ) .
  90 .Pp
  91 If no messages are available at the socket, the
  92 receive call waits for a message to arrive, unless
  93 the socket is nonblocking (see
  94 .Xr fcntl 2 )
  95 in which case the value
  96 -1 is returned and the external variable
  97 .Va errno
  98 set to
  99 .Er EAGAIN .
 100 The receive calls normally return any data available,
 101 up to the requested amount,
 102 rather than waiting for receipt of the full amount requested;
 103 this behavior is affected by the socket-level options
 104 .Dv SO_RCVLOWAT
 105 and
 106 .Dv SO_RCVTIMEO
 107 described in
 108 .Xr getsockopt 2 .
 109 .Pp
 110 The
 111 .Xr select 2
 112 call may be used to determine when more data arrive.
 113 .Pp
 114 The
 115 .Fa flags
 116 argument to a recv call is formed by
 117 .Em or Ap ing
 118 one 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
 124 The
 125 .Dv MSG_OOB
 126 flag requests receipt of out-of-band data
 127 that would not be received in the normal data stream.
 128 Some protocols place expedited data at the head of the normal
 129 data queue, and thus this flag cannot be used with such protocols.
 130 The MSG_PEEK flag causes the receive operation to return data
 131 from the beginning of the receive queue without removing that
 132 data from the queue.
 133 Thus, a subsequent receive call will return the same data.
 134 The MSG_WAITALL flag requests that the operation block until
 135 the full request is satisfied.
 136 However, the call may still return less data than requested
 137 if a signal is caught, an error or disconnect occurs,
 138 or the next data to be received is of a different type than that returned.
 139 .Pp
 140 The
 141 .Fn recvmsg
 142 call uses a
 143 .Fa msghdr
 144 structure to minimize the number of directly supplied parameters.
 145 This structure has the following form, as defined in
 146 .Ao Pa sys/socket.h Ac :
 147 .Pp
 148 .Bd -literal
 149 struct msghdr {
 150         caddr_t         msg_name;       /* optional address */
 151         socklen_t       msg_namelen;    /* size of address */
 152         struct          iovec *msg_iov; /* scatter/gather array */
 153         u_int           msg_iovlen;     /* # elements in msg_iov */
 154         caddr_t         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
 160 Here
 161 .Fa msg_name
 162 and
 163 .Fa msg_namelen
 164 specify the source address if the socket is unconnected;
 165 .Fa msg_name
 166 may be given as a null pointer if no names are desired or required.
 167 .Fa Msg_iov
 168 and
 169 .Fa msg_iovlen
 170 describe scatter gather locations, as discussed in
 171 .Xr read 2 .
 172 .Fa Msg_control ,
 173 which has length
 174 .Fa msg_controllen ,
 175 points to a buffer for other protocol control related messages
 176 or other miscellaneous ancillary data.
 177 The messages are of the form:
 178 .Bd -literal
 179 struct cmsghdr {
 180         u_int   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
 187 As an example, one could use this to learn of changes in the data-stream
 188 in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
 189 a recvmsg with no data buffer provided immediately after an
 190 .Fn accept
 191 call.
 192 .Pp
 193 Open file descriptors are now passed as ancillary data for
 194 .Dv AF_UNIX
 195 domain sockets, with
 196 .Fa cmsg_level
 197 set to
 198 .Dv SOL_SOCKET
 199 and
 200 .Fa cmsg_type
 201 set to
 202 .Dv SCM_RIGHTS .
 203 .Pp
 204 The
 205 .Fa msg_flags
 206 field is set on return according to the message received.
 207 .Dv MSG_EOR
 208 indicates end-of-record;
 209 the data returned completed a record (generally used with sockets of type
 210 .Dv SOCK_SEQPACKET ) .
 211 .Dv MSG_TRUNC
 212 indicates that
 213 the trailing portion of a datagram was discarded because the datagram
 214 was larger than the buffer supplied.
 215 .Dv MSG_CTRUNC
 216 indicates that some
 217 control data were discarded due to lack of space in the buffer
 218 for ancillary data.
 219 .Dv MSG_OOB
 220 is returned to indicate that expedited or out-of-band data were received.
 221 .Pp
 222 .Sh RETURN VALUES
 223 These calls return the number of bytes received, or -1
 224 if an error occurred.
 225 .Sh ERRORS
 226 The calls fail if:
 227 .Bl -tag -width Er
 228 .It Bq Er EBADF
 229 The argument
 230 .Fa s
 231 is an invalid descriptor.
 232 .It Bq Er ENOTCONN
 233 The socket is associated with a connection-oriented protocol
 234 and has not been connected (see
 235 .Xr connect 2
 236 and
 237 .Xr accept 2 ).
 238 .It Bq Er ENOTSOCK
 239 The argument
 240 .Fa s
 241 does not refer to a socket.
 242 .It Bq Er EAGAIN
 243 The socket is marked non-blocking, and the receive operation
 244 would block, or
 245 a receive timeout had been set,
 246 and the timeout expired before data were received.
 247 .It Bq Er EINTR
 248 The receive was interrupted by delivery of a signal before
 249 any data were available.
 250 .It Bq Er EFAULT
 251 The receive buffer pointer(s) point outside the process's
 252 address space.
 253 .El
 254 .Sh SEE ALSO
 255 .Xr fcntl 2 ,
 256 .Xr read 2 ,
 257 .Xr select 2 ,
 258 .Xr getsockopt 2 ,
 259 .Xr socket 2
 260 .Sh HISTORY
 261 The
 262 .Fn recv
 263 function call appeared in
 264 .Bx 4.2 .