bsd/man/man2/recv.2

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  32 .\"     @(#)recv.2      8.3 (Berkeley) 2/21/94
  33 .\"
  34 .Dd May 15, 2006
  35 .Dt RECV 2
  36 .Os
  37 .Sh NAME
  38 .Nm recv ,
  39 .Nm recvfrom ,
  40 .Nm recvmsg
  41 .Nd receive a message from a socket
  42 .Sh LIBRARY
  43 .Lb libc
  44 .Sh SYNOPSIS
  45 .In sys/socket.h
  46 .Ft ssize_t
  47 .Fo recv
  48 .Fa "int socket"
  49 .Fa "void *buffer"
  50 .Fa "size_t length"
  51 .Fa "int flags"
  52 .Fc
  53 .Ft ssize_t
  54 .Fo recvfrom
  55 .Fa "int socket"
  56 .Fa "void *restrict buffer"
  57 .Fa "size_t length"
  58 .Fa "int flags"
  59 .Fa "struct sockaddr *restrict address"
  60 .Fa "socklen_t *restrict address_len"
  61 .Fc
  62 .Ft ssize_t
  63 .Fo recvmsg
  64 .Fa "int socket"
  65 .Fa "struct msghdr *message"
  66 .Fa "int flags"
  67 .Fc
  68 .Sh DESCRIPTION
  69 The
  70 .Fn recvfrom
  71 and
  72 .Fn recvmsg
  73 system calls
  74 are used to receive messages from a socket,
  75 and may be used to receive data on a socket whether or not
  76 it is connection-oriented.
  77 .Pp
  78 If
  79 .Fa address
  80 is not a null pointer
  81 and the socket is not connection-oriented,
  82 the source address of the message is filled in.
  83 The
  84 .Fa address_len
  85 argument
  86 is a value-result argument, initialized to the size of
  87 the buffer associated with
  88 .Fa address ,
  89 and modified on return to indicate the actual size of the
  90 address stored there.
  91 .Pp
  92 The
  93 .Fn recv
  94 function is normally used only on a
  95 .Em connected
  96 socket (see
  97 .Xr connect 2 )
  98 and is identical to
  99 .Fn recvfrom
 100 with a
 101 null pointer passed as its
 102 .Fa address
 103 argument.
 104 As it is redundant, it may not be supported in future releases.
 105 .Pp
 106 All three routines return the length of the message on successful
 107 completion.
 108 If a message is too long to fit in the supplied buffer,
 109 excess bytes may be discarded depending on the type of socket
 110 the message is received from (see
 111 .Xr socket 2 ) .
 112 .Pp
 113 If no messages are available at the socket, the
 114 receive call waits for a message to arrive, unless
 115 the socket is nonblocking (see
 116 .Xr fcntl 2 )
 117 in which case the value
 118 -1 is returned and the external variable
 119 .Va errno
 120 set to
 121 .Er EAGAIN .
 122 The receive calls normally return any data available,
 123 up to the requested amount,
 124 rather than waiting for receipt of the full amount requested;
 125 this behavior is affected by the socket-level options
 126 .Dv SO_RCVLOWAT
 127 and
 128 .Dv SO_RCVTIMEO
 129 described in
 130 .Xr getsockopt 2 .
 131 .Pp
 132 The
 133 .Xr select 2
 134 system call may be used to determine when more data arrive.
 135 .Pp
 136 If no messages are available to be received and the peer has
 137 performed an orderly shutdown, the value 0 is returned.
 138 .Pp
 139 The
 140 .Fa flags
 141 argument to a
 142 .Fn recv
 143 function is formed by
 144 .Em or Ap ing
 145 one or more of the values:
 146 .Bl -column MSG_WAITALL -offset indent
 147 .It Dv MSG_OOB Ta process out-of-band data
 148 .It Dv MSG_PEEK Ta peek at incoming message
 149 .It Dv MSG_WAITALL Ta wait for full request or error
 150 .El
 151 .Pp
 152 The
 153 .Dv MSG_OOB
 154 flag requests receipt of out-of-band data
 155 that would not be received in the normal data stream.
 156 Some protocols place expedited data at the head of the normal
 157 data queue, and thus this flag cannot be used with such protocols.
 158 The
 159 .Dv MSG_PEEK
 160 flag causes the receive operation to return data
 161 from the beginning of the receive queue without removing that
 162 data from the queue.
 163 Thus, a subsequent receive call will return the same data.
 164 The
 165 .Dv MSG_WAITALL
 166 flag requests that the operation block until
 167 the full request is satisfied.
 168 However, the call may still return less data than requested
 169 if a signal is caught, an error or disconnect occurs,
 170 or the next data to be received is of a different type than that returned.
 171 .Pp
 172 The
 173 .Fn recvmsg
 174 system call uses a
 175 .Fa msghdr
 176 structure to minimize the number of directly supplied arguments.
 177 This structure has the following form, as defined in
 178 .In sys/socket.h :
 179 .Pp
 180 .Bd -literal
 181 struct msghdr {
 182         void            *msg_name;      /* optional address */
 183         socklen_t       msg_namelen;    /* size of address */
 184         struct          iovec *msg_iov; /* scatter/gather array */
 185         int             msg_iovlen;     /* # elements in msg_iov */
 186         void            *msg_control;   /* ancillary data, see below */
 187         socklen_t       msg_controllen; /* ancillary data buffer len */
 188         int             msg_flags;      /* flags on received message */
 189 };
 190 .Ed
 191 .Pp
 192 Here
 193 .Fa msg_name
 194 and
 195 .Fa msg_namelen
 196 specify the destination address if the socket is unconnected;
 197 .Fa msg_name
 198 may be given as a null pointer if no names are desired or required.
 199 The
 200 .Fa msg_iov
 201 and
 202 .Fa msg_iovlen
 203 arguments
 204 describe scatter gather locations, as discussed in
 205 .Xr read 2 .
 206 The
 207 .Fa msg_control
 208 argument,
 209 which has length
 210 .Fa msg_controllen ,
 211 points to a buffer for other protocol control related messages
 212 or other miscellaneous ancillary data.
 213 The messages are of the form:
 214 .Bd -literal
 215 struct cmsghdr {
 216         u_int   cmsg_len;       /* data byte count, including hdr */
 217         int     cmsg_level;     /* originating protocol */
 218         int     cmsg_type;      /* protocol-specific type */
 219 /* followed by
 220         u_char  cmsg_data[]; */
 221 };
 222 .Ed
 223 .Pp
 224 As an example, one could use this to learn of changes
 225 in the data-stream in XNS/SPP,
 226 or in ISO, to obtain user-connection-request data by requesting a
 227 .Fn recvmsg
 228 with no data buffer provided immediately after an
 229 .Fn accept
 230 system call.
 231 .Pp
 232 Open file descriptors are now passed as ancillary data for
 233 .Dv AF_UNIX
 234 domain sockets, with
 235 .Fa cmsg_level
 236 set to
 237 .Dv SOL_SOCKET
 238 and
 239 .Fa cmsg_type
 240 set to
 241 .Dv SCM_RIGHTS .
 242 .Pp
 243 The
 244 .Fa msg_flags
 245 field is set on return according to the message received.
 246 .Dv MSG_EOR
 247 indicates end-of-record;
 248 the data returned completed a record (generally used with sockets of type
 249 .Dv SOCK_SEQPACKET ) .
 250 .Dv MSG_TRUNC
 251 indicates that
 252 the trailing portion of a datagram was discarded
 253 because the datagram was larger than the buffer supplied.
 254 .Dv MSG_CTRUNC
 255 indicates that some control data were discarded
 256 due to lack of space in the buffer for ancillary data.
 257 .Dv MSG_OOB
 258 is returned to indicate that expedited or out-of-band data were received.
 259 .Sh RETURN VALUES
 260 These calls return the number of bytes received, or -1
 261 if an error occurred.
 262 .Pp
 263 For TCP sockets, the return value 0 means the peer has closed its
 264 half side of the connection.
 265 .Sh ERRORS
 266 The calls fail if:
 267 .Bl -tag -width Er
 268 .\" ===========
 269 .It Bq Er EAGAIN
 270 The socket is marked non-blocking, and the receive operation
 271 would block, or
 272 a receive timeout had been set,
 273 and the timeout expired before data were received.
 274 .\" ===========
 275 .It Bq Er EBADF
 276 The argument
 277 .Fa socket
 278 is an invalid descriptor.
 279 .\" ===========
 280 .It Bq Er ECONNRESET
 281 The connection is closed by the peer
 282 during a receive attempt on a socket.
 283 .\" ===========
 284 .It Bq Er EFAULT
 285 The receive buffer pointer(s) point outside the process's
 286 address space.
 287 .\" ===========
 288 .It Bq Er EINTR
 289 The receive was interrupted by delivery of a signal before
 290 any data were available.
 291 .\" ===========
 292 .It Bq Er EINVAL
 293 MSG_OOB is set, but no out-of-band data is available.
 294 .\" ===========
 295 .It Bq Er ENOBUFS
 296 An attempt to allocate a memory buffer fails.
 297 .\" ===========
 298 .It Bq Er ENOTCONN
 299 The socket is associated with a connection-oriented protocol
 300 and has not been connected (see
 301 .Xr connect 2
 302 and
 303 .Xr accept 2 ) .
 304 .\" ===========
 305 .It Bq Er ENOTSOCK
 306 The argument
 307 .Fa socket
 308 does not refer to a socket.
 309 .\" ===========
 310 .It Bq Er EOPNOTSUPP
 311 The type and/or protocol of
 312 .Fa socket
 313 do not support the option(s) specified in
 314 .Fa flags .
 315 .\" ===========
 316 .It Bq Er ETIMEDOUT
 317 The connection timed out.
 318 .El
 319 .Pp
 320 The
 321 .Fn recvfrom
 322 call may also fail if:
 323 .Bl -tag -width Er
 324 .\" ===========
 325 .It Bq Er EINVAL
 326 The total of the iov_len values overflows a ssize_t.
 327 .El
 328 .Pp
 329 The
 330 .Fn recvmsg
 331 call may also fail if:
 332 .Bl -tag -width Er
 333 .\" ===========
 334 .It Bq Er EMSGSIZE
 335 The requested message size is invalid.
 336 .\" ===========
 337 .It Bq Er ENOMEM
 338 Insufficient memory is available.
 339 .El
 340 .Sh SEE ALSO
 341 .Xr fcntl 2 ,
 342 .Xr getsockopt 2 ,
 343 .Xr read 2 ,
 344 .Xr select 2 ,
 345 .Xr socket 2
 346 .Sh HISTORY
 347 The
 348 .Fn recv
 349 function appeared in
 350 .Bx 4.2 .