*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->OpenBSD man pages -> recvmsg (2)              



NAME    [Toc]    [Back]

     recv, recvfrom, recvmsg - receive a message from a socket

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/socket.h>

     recv(int s, void *buf, size_t len, int flags);

     recvfrom(int s, void *buf, size_t  len,  int  flags,  struct
sockaddr *from,
             socklen_t *fromlen);

     recvmsg(int s, struct msghdr *msg, int flags);

DESCRIPTION    [Toc]    [Back]

     recvfrom() and recvmsg() are used to receive messages from a
socket, and
     may be used to receive data on a socket whether or not it is

     If from is non-null and the socket is not connection-oriented, the source
     address of the message is filled in.  fromlen is a value-result parameter,
  initialized  to the size of the buffer associated with
from, and modified
 on return to indicate the actual size of  the  address
stored there.

     The  recv() call is normally used only on a connected socket
     connect(2)) and is identical to recvfrom() with a null  from
     As  it  is  redundant, it may not be supported in future releases.

     On successful completion, all three routines return the number of message
     bytes read.  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 socket(2)).

     If no messages are available at the socket, the receive call
waits for a
     message to arrive, unless the socket is nonblocking (see fcntl(2)) in
     which  case  the value -1 is returned and the external variable errno set
     to 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
     SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).

     The  select(2) or poll(2) system calls may be used to determine when more
     data arrive.

     The flags argument to a recv call is formed by ORing one  or
more of the

           MSG_OOB         process out-of-band data
           MSG_PEEK        peek at incoming message
           MSG_WAITALL     wait for full request or error
           MSG_DONTWAIT    don't block

     The  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.
   The  MSG_DONTWAIT flag requests the call to return
when it would
     block otherwise.  If no data is available, errno is  set  to
     flag  is  not  available  in  strict ANSI or C99 compilation

     The recvmsg() call uses a msghdr structure to  minimize  the
number of directly
  supplied parameters.  This structure has the following form, as
     defined in <sys/socket.h>:

     struct msghdr {
             void            *msg_name;      /* optional  address
             socklen_t        msg_namelen;     /* size of address
             struct          iovec  *msg_iov;  /*  scatter/gather
array */
             unsigned  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 */

     Here  msg_name and msg_namelen specify the source address if
the socket is
     unconnected; msg_name may be given as a null pointer  if  no
names are desired
  or required.  msg_iov and msg_iovlen describe scatter
gather locations,
 as discussed  in  read(2).   msg_control,  which  has
     msg_controllen,  points  to a buffer for other protocol control related
     messages or other miscellaneous ancillary  data.   The  messages are of the

     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[]; */

     As an example, one could use this to learn of changes in the
     in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
 a recvmsg with no data buffer provided immediately after
an accept()

     Open  file  descriptors are now passed as ancillary data for
AF_UNIX domain
     and socketpair(2) sockets, with cmsg_level set to SOL_SOCKET
     cmsg_type set to SCM_RIGHTS.

     The  msg_flags  field is set on return according to the message received.
     It will contain zero or more of the following values:

           MSG_OOB     Returned to  indicate  that  expedited  or
out-of-band data
                       was received.
           MSG_EOR     Indicates end-of-record; the data returned
completed a
                       record (generally  used  with  sockets  of
           MSG_TRUNC    Indicates  that the trailing portion of a
datagram was
                       discarded because the datagram was  larger
than the
                       buffer supplied.
           MSG_CTRUNC  Indicates that some control data were discarded due to
                       lack of space in the buffer for  ancillary
           MSG_BCAST    Indicates that the packet was received as
           MSG_MCAST   Indicates that the packet was received  as

RETURN VALUES    [Toc]    [Back]

     These calls return the number of bytes received, or -1 if an
error occurred.

ERRORS    [Toc]    [Back]

     recv(), recvfrom(), and recvmsg() fail if:

     [EBADF]         The argument s is an invalid descriptor.

     [ENOTCONN]      The socket is associated with a  connectionoriented protocol
  and  has not been connected (see connect(2) and

     [ENOTSOCK]      The argument s does not refer to a socket.

     [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.

     [EINTR]         The receive was interrupted by delivery of a
signal before
 any data were available.

     [EFAULT]        The receive buffer pointer(s) point  outside
the process's
                     address space.

     [EHOSTUNREACH]   A  socket operation was attempted to an unreachable host.

     [EHOSTDOWN]     A socket operation failed because the destination host
                     was down.

     [ENETDOWN]       A  socket operation encountered a dead network.

     In addition, recv() and recvfrom() may return the  following

     [EINVAL]      len was larger than SSIZE_MAX.

     Also, recv() may return the following error:

     [ECONNREFUSED]   The socket is associated with a connectionoriented protocol
 and the connection was forcefully  rejected (see

     And recvmsg() may return one of the following errors:

     [EINVAL]       The  sum of the iov_len values in the msg_iov
array overflowed
 an ssize_t.

     [EMSGSIZE]    The msg_iovlen member of msg was less  than  0
or larger than

SEE ALSO    [Toc]    [Back]

     connect(2),  fcntl(2),  getsockopt(2), poll(2), read(2), select(2),
     socket(2), socketpair(2)

HISTORY    [Toc]    [Back]

     The recv() function call appeared in 4.2BSD.

OpenBSD     3.6                        February     15,      1999
[ Back ]
 Similar pages
Name OS Title
recvmsg Tru64 Receive a message from a socket using a message structure
tsix_recvfrom_mac IRIX receive a message and a MAC label from a socket
shutdown Tru64 Shut down socket send and receive operations
msgrcv OpenBSD receive a message from a message queue
msgrcv NetBSD receive a message from a message queue
msgrcv FreeBSD receive a message from a message queue
mq_receive HP-UX receive a message from a message queue
msgrcv Tru64 Receive a message from a message queue
sendmsg Tru64 Send a message from a socket using a message structure
tt_message_receive HP-UX receive a message
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service