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NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

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

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

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

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

DESCRIPTION    [Toc]    [Back]

       The recvfrom and recvmsg calls 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.

       If from is not NULL, and the socket  is	not  connection-oriented,  the
       source  address of the message is filled in.  The argument 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  (see  con-
       nect(2)) and is identical to recvfrom with a NULL from parameter.

       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 socket(2)).

       If no messages are available at the socket, the receive calls wait  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.

       The  select(2)  or poll(2) call may be used to determine when more data

       The flags argument to a recv call is formed by OR'ing one  or  more  of
       the following values:

       MSG_OOB    [Toc]    [Back]
	      This 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.

       MSG_PEEK    [Toc]    [Back]
	      This 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

       MSG_WAITALL    [Toc]    [Back]
	      This  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.

       MSG_NOSIGNAL    [Toc]    [Back]
	      This  flag  turns  off raising of SIGPIPE on stream sockets when
	      the other end disappears.

       MSG_TRUNC    [Toc]    [Back]
	      Return the real length of the packet, even when  it  was	longer
	      than the passed buffer. Only valid for packet sockets.

       MSG_ERRQUEUE    [Toc]    [Back]
	      This  flag  specifies that queued errors should be received from
	      the socket error queue.  The error is  passed  in  an  ancillary
	      message  with  a	type  dependent  on  the  protocol  (for  IPv4
	      IP_RECVERR).  The user should  supply  a	buffer	of  sufficient
	      size.  See  cmsg(3) and ip(7) for more information.  The payload
	      of the original packet that caused the error is passed as normal
	      data  via  msg_iovec.   The  original destination address of the
	      datagram that caused the error is supplied via msg_name.

	      For local errors, no address is passed (this can be checked with
	      the  cmsg_len  member  of the cmsghdr).  For error receives, the
	      MSG_ERRQUEUE is set in the msghdr.   After  an  error  has  been
	      passed,  the  pending  socket  error is regenerated based on the
	      next queued error and will be passed on the next	socket	operation.

	      The error is supplied in a sock_extended_err structure:

	      #define SO_EE_ORIGIN_NONE       0
	      #define SO_EE_ORIGIN_LOCAL      1
	      #define SO_EE_ORIGIN_ICMP       2
	      #define SO_EE_ORIGIN_ICMP6      3

	      struct sock_extended_err
		  u_int32_t	  ee_errno;   /* error number */
		  u_int8_t	  ee_origin;  /* where the error originated */
		  u_int8_t	  ee_type;    /* type */
		  u_int8_t	  ee_code;    /* code */
		  u_int8_t	  ee_pad;
		  u_int32_t	  ee_info;    /* additional information */
		  u_int32_t	  ee_data;    /* other data */
		  /* More data may follow */

	      struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

	      ee_errno contains the errno number of the queued error.  ee_ori-
	      gin is the origin code of where the error originated.  The other
	      fields are protocol specific. The macro SOCK_EE_OFFENDER returns
	      a pointer to the address of the network object where  the  error
	      originated  from	given  a pointer to the ancillary message.  If
	      this address is not known, the sa_family member of the  sockaddr
	      contains	AF_UNSPEC  and	the  other  fields of the sockaddr are
	      undefined. The payload of the packet that caused	the  error  is
	      passed as normal data.

	      For local errors, no address is passed (this can be checked with
	      the cmsg_len member of the cmsghdr).  For  error	receives,  the
	      MSG_ERRQUEUE  is	set  in  the  msghdr.  After an error has been
	      passed, the pending socket error is  regenerated	based  on  the
	      next  queued  error and will be passed on the next socket operation.

       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 */
		  size_t       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 destination  address  if  the
       socket  is  unconnected;  msg_name may be given as a null pointer if no
       names are desired or  required.	 The  fields  msg_iov  and  msg_iovlen
       describe scatter-gather locations, as discussed in readv(2).  The field
       msg_control, which has length msg_controllen, points to	a  buffer  for
       other  protocol	control  related  messages  or miscellaneous ancillary
       data. When recvmsg is called, msg_controllen should contain the	length
       of  the	available buffer in msg_control; upon return from a successful
       call it will contain the length of the control message sequence.

       The messages are of the form:

	      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[]; */

       Ancillary data should  only  be	accessed  by  the  macros  defined  in

       As  an  example,  Linux	uses  this  auxiliary  data  mechanism to pass
       extended errors, IP options or file descriptors over Unix sockets.

       The msg_flags field in the msghdr is set on return  of  recvmsg().   It
       can contain several flags:

       MSG_EOR    [Toc]    [Back]
	      indicates  end-of-record;  the  data returned completed a record
	      (generally used with sockets of type SOCK_SEQPACKET).

       MSG_TRUNC    [Toc]    [Back]
	      indicates that the trailing portion of a datagram was  discarded
	      because the datagram was larger than the buffer supplied.

       MSG_CTRUNC    [Toc]    [Back]
	      indicates  that  some control data were discarded due to lack of
	      space in the buffer for ancillary data.

       MSG_OOB    [Toc]    [Back]
	      is returned to indicate that expedited or out-of-band data  were

       MSG_ERRQUEUE    [Toc]    [Back]
	      indicates  that  no data was received but an extended error from
	      the socket error queue.

       MSG_DONTWAIT    [Toc]    [Back]
	      Enables non-blocking operation; if the  operation  would	block,
	      EAGAIN  is  returned  (this can also be enabled using the O_NON-
	      BLOCK with the F_SETFL fcntl(2)).

RETURN VALUE    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

       These  are  some  standard  errors generated by the socket layer. Additional
 errors may be generated and returned from the underlying	protocol
 modules; see their manual pages.

       EBADF  The argument s is an invalid descriptor.

       ECONNREFUSED    [Toc]    [Back]
	      A remote host refused to allow the network connection (typically
	      because it is not running the requested service).

       ENOTCONN    [Toc]    [Back]
	      The socket is associated with a connection-oriented protocol and
	      has not been connected (see connect(2) and accept(2)).

       ENOTSOCK    [Toc]    [Back]
	      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 was 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.

       EINVAL Invalid argument passed.

CONFORMING TO    [Toc]    [Back]

       4.4BSD (these function calls first appeared in 4.2BSD).

NOTE    [Toc]    [Back]

       The  prototypes	given above follow glibc2.  The Single Unix Specification
 agrees, except that it has return values of type `ssize_t'	(while
       BSD  4.*  and  libc4  and libc5 all have `int').  The flags argument is
       `int' in BSD 4.*, but `unsigned int' in libc4 and libc5.  The len argument
 is `int' in BSD 4.*, but `size_t' in libc4 and libc5.  The fromlen
       argument is  `int  *'  in  BSD  4.*,  libc4  and  libc5.   The  present
       `socklen_t *' was invented by POSIX.  See also accept(2).

SEE ALSO    [Toc]    [Back]

       fcntl(2), read(2), select(2), getsockopt(2), socket(2), cmsg(3)

Linux Man Page			  2001-06-19			       RECV(2)
[ Back ]
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