bsd/man/man2/recv.2

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  32 .\"     @(#)recv.2      8.3 (Berkeley) 2/21/94
  33 .\"
  34 .Dd March 18, 2015
  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 or
  99 .Xr connectx 2 )
 100 and is identical to
 101 .Fn recvfrom
 102 with a
 103 null pointer passed as its
 104 .Fa address
 105 argument.
 106 As it is redundant, it may not be supported in future releases.
 107 .Pp
 108 All three routines return the length of the message on successful
 109 completion.
 110 If a message is too long to fit in the supplied buffer,
 111 excess bytes may be discarded depending on the type of socket
 112 the message is received from (see
 113 .Xr socket 2 ) .
 114 .Pp
 115 If no messages are available at the socket, the
 116 receive call waits for a message to arrive, unless
 117 the socket is nonblocking (see
 118 .Xr fcntl 2 )
 119 in which case the value
 120 -1 is returned and the external variable
 121 .Va errno
 122 set to
 123 .Er EAGAIN .
 124 The receive calls normally return any data available,
 125 up to the requested amount,
 126 rather than waiting for receipt of the full amount requested;
 127 this behavior is affected by the socket-level options
 128 .Dv SO_RCVLOWAT
 129 and
 130 .Dv SO_RCVTIMEO
 131 described in
 132 .Xr getsockopt 2 .
 133 .Pp
 134 The
 135 .Xr select 2
 136 system call may be used to determine when more data arrive.
 137 .Pp
 138 If no messages are available to be received and the peer has
 139 performed an orderly shutdown, the value 0 is returned.
 140 .Pp
 141 The
 142 .Fa flags
 143 argument to a
 144 .Fn recv
 145 function is formed by
 146 .Em or Ap ing
 147 one or more of the values:
 148 .Bl -column MSG_WAITALL -offset indent
 149 .It Dv MSG_OOB Ta process out-of-band data
 150 .It Dv MSG_PEEK Ta peek at incoming message
 151 .It Dv MSG_WAITALL Ta wait for full request or error
 152 .El
 153 .Pp
 154 The
 155 .Dv MSG_OOB
 156 flag requests receipt of out-of-band data
 157 that would not be received in the normal data stream.
 158 Some protocols place expedited data at the head of the normal
 159 data queue, and thus this flag cannot be used with such protocols.
 160 The
 161 .Dv MSG_PEEK
 162 flag causes the receive operation to return data
 163 from the beginning of the receive queue without removing that
 164 data from the queue.
 165 Thus, a subsequent receive call will return the same data.
 166 The
 167 .Dv MSG_WAITALL
 168 flag requests that the operation block until
 169 the full request is satisfied.
 170 However, the call may still return less data than requested
 171 if a signal is caught, an error or disconnect occurs,
 172 or the next data to be received is of a different type than that returned.
 173 .Pp
 174 The
 175 .Fn recvmsg
 176 system call uses a
 177 .Fa msghdr
 178 structure to minimize the number of directly supplied arguments.
 179 This structure has the following form, as defined in
 180 .In sys/socket.h :
 181 .Pp
 182 .Bd -literal
 183 struct msghdr {
 184         void            *msg_name;      /* optional address */
 185         socklen_t       msg_namelen;    /* size of address */
 186         struct          iovec *msg_iov; /* scatter/gather array */
 187         int             msg_iovlen;     /* # elements in msg_iov */
 188         void            *msg_control;   /* ancillary data, see below */
 189         socklen_t       msg_controllen; /* ancillary data buffer len */
 190         int             msg_flags;      /* flags on received message */
 191 };
 192 .Ed
 193 .Pp
 194 Here
 195 .Fa msg_name
 196 and
 197 .Fa msg_namelen
 198 specify the destination address if the socket is unconnected;
 199 .Fa msg_name
 200 may be given as a null pointer if no names are desired or required.
 201 .Pp
 202 The
 203 .Fa msg_iov
 204 and
 205 .Fa msg_iovlen
 206 arguments
 207 describe scatter gather locations, as discussed in
 208 .Xr read 2 .
 209 .Fa msg_iovlen
 210 shall be set to the dimension of this array. In each
 211 .Fa iovec
 212 structure, the
 213 .Fa iov_base
 214 field specifies a storage area and the
 215 .Fa iov_len
 216 field gives its size in bytes. Each storage area indicated by
 217 .Fa msg_iov
 218 is filled with received data in turn until all of the received data
 219 is stored or all of the areas have been filled.
 220 .Pp
 221 The
 222 .Fa msg_control
 223 argument,
 224 which has length
 225 .Fa msg_controllen ,
 226 points to a buffer for other protocol control related messages
 227 or other miscellaneous ancillary data.
 228 The messages are of the form:
 229 .Bd -literal
 230 struct cmsghdr {
 231         u_int   cmsg_len;       /* data byte count, including hdr */
 232         int     cmsg_level;     /* originating protocol */
 233         int     cmsg_type;      /* protocol-specific type */
 234 /* followed by
 235         u_char  cmsg_data[]; */
 236 };
 237 .Ed
 238 .Pp
 239 As an example, one could use this to learn of changes
 240 in the data-stream in XNS/SPP,
 241 or in ISO, to obtain user-connection-request data by requesting a
 242 .Fn recvmsg
 243 with no data buffer provided immediately after an
 244 .Fn accept
 245 system call.
 246 .Pp
 247 Open file descriptors are now passed as ancillary data for
 248 .Dv AF_UNIX
 249 domain sockets, with
 250 .Fa cmsg_level
 251 set to
 252 .Dv SOL_SOCKET
 253 and
 254 .Fa cmsg_type
 255 set to
 256 .Dv SCM_RIGHTS .
 257 .Pp
 258 The
 259 .Fa msg_flags
 260 field is set on return according to the message received.
 261 .Dv MSG_EOR
 262 indicates end-of-record;
 263 the data returned completed a record.
 264 .\" (generally used with sockets of type
 265 .\".Dv SOCK_SEQPACKET ) .
 266 .Dv MSG_TRUNC
 267 indicates that
 268 the trailing portion of a datagram was discarded
 269 because the datagram was larger than the buffer supplied.
 270 .Dv MSG_CTRUNC
 271 indicates that some control data were discarded
 272 due to lack of space in the buffer for ancillary data.
 273 .Dv MSG_OOB
 274 is returned to indicate that expedited or out-of-band data were received.
 275 .Sh RETURN VALUES
 276 These calls return the number of bytes received, or -1
 277 if an error occurred.
 278 .Pp
 279 For TCP sockets, the return value 0 means the peer has closed its
 280 half side of the connection.
 281 .Sh ERRORS
 282 The calls fail if:
 283 .Bl -tag -width Er
 284 .\" ===========
 285 .It Bq Er EAGAIN
 286 The socket is marked non-blocking, and the receive operation
 287 would block, or
 288 a receive timeout had been set,
 289 and the timeout expired before data were received.
 290 .\" ===========
 291 .It Bq Er EBADF
 292 The argument
 293 .Fa socket
 294 is an invalid descriptor.
 295 .\" ===========
 296 .It Bq Er ECONNRESET
 297 The connection is closed by the peer
 298 during a receive attempt on a socket.
 299 .\" ===========
 300 .It Bq Er EFAULT
 301 The receive buffer pointer(s) point outside the process's
 302 address space.
 303 .\" ===========
 304 .It Bq Er EINTR
 305 The receive was interrupted by delivery of a signal before
 306 any data were available.
 307 .\" ===========
 308 .It Bq Er EINVAL
 309 MSG_OOB is set, but no out-of-band data is available.
 310 .\" ===========
 311 .It Bq Er ENOBUFS
 312 An attempt to allocate a memory buffer fails.
 313 .\" ===========
 314 .It Bq Er ENOTCONN
 315 The socket is associated with a connection-oriented protocol
 316 and has not been connected (see
 317 .Xr connect 2,
 318 .Xr connectx 2,
 319 and
 320 .Xr accept 2 ) .
 321 .\" ===========
 322 .It Bq Er ENOTSOCK
 323 The argument
 324 .Fa socket
 325 does not refer to a socket.
 326 .\" ===========
 327 .It Bq Er EOPNOTSUPP
 328 The type and/or protocol of
 329 .Fa socket
 330 do not support the option(s) specified in
 331 .Fa flags .
 332 .\" ===========
 333 .It Bq Er ETIMEDOUT
 334 The connection timed out.
 335 .El
 336 .Pp
 337 The
 338 .Fn recvfrom
 339 call may also fail if:
 340 .Bl -tag -width Er
 341 .\" ===========
 342 .It Bq Er EINVAL
 343 The total of the iov_len values overflows a ssize_t.
 344 .El
 345 .Pp
 346 The
 347 .Fn recvmsg
 348 call may also fail if:
 349 .Bl -tag -width Er
 350 .\" ===========
 351 .It Bq Er EMSGSIZE
 352 The
 353 .Fa msg_iovlen
 354 member of the
 355 .Fa msghdr
 356 structure pointed to by message is less than or equal to 0, or is greater than
 357 .Dv IOV_MAX .
 358 .\" ===========
 359 .It Bq Er ENOMEM
 360 Insufficient memory is available.
 361 .El
 362 .Sh SEE ALSO
 363 .Xr fcntl 2 ,
 364 .Xr getsockopt 2 ,
 365 .Xr read 2 ,
 366 .Xr select 2 ,
 367 .Xr socket 2
 368 .Sh HISTORY
 369 The
 370 .Fn recv
 371 function appeared in
 372 .Bx 4.2 .