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RECV(2)			   Linux Programmer's Manual		       RECV(2)

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

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

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

       ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
			struct sockaddr *src_addr, socklen_t *addrlen);

       ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

       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  src_addr  is	 not  NULL,  and  the underlying protocol provides the
       source address, this source address is filled  in.   When  src_addr  is
       NULL,  nothing  is  filled  in;	in this case, addrlen is not used, and
       should also be NULL.  The argument addrlen is a value-result  argument,
       which  the  caller should initialize before the call to the size of the
       buffer associated with src_addr, and modified on return to indicate the
       actual  size  of the source address.  The returned address is truncated
       if the buffer provided is too small; in this case, addrlen will	return
       a value greater than was supplied to the call.

       The  recv()  call is normally used only on a connected socket (see con‐
       nect(2)) and is identical to recvfrom() with a NULL src_addr argument.

       All three routines return the length of the message on successful  com‐
       pletion.	  If  a	 message  is  too  long to fit in the supplied buffer,
       excess bytes may be discarded depending on the type of socket the  mes‐
       sage is received from.

       If  no messages are available at the socket, the receive calls wait for
       a message to arrive, unless the socket is non-blocking (see  fcntl(2)),
       in  which case the value -1 is returned and the external variable errno
       is set to EAGAIN or EWOULDBLOCK.	 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_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
	      Set the close-on-exec flag for the file descriptor received  via
	      a	 Unix  domain  file  descriptor using the SCM_RIGHTS operation
	      (described in unix(7)).  This flag is useful for the  same  rea‐
	      sons as the O_CLOEXEC flag of open(2).

       MSG_DONTWAIT (since Linux 2.2)
	      Enables  non-blocking  operation;	 if the operation would block,
	      the call fails with the error EAGAIN or  EWOULDBLOCK  (this  can
	      also  be	enabled	 using	the  O_NONBLOCK	 flag with the F_SETFL

       MSG_ERRQUEUE (since Linux 2.2)
	      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 opera‐

	      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
		      uint32_t ee_errno;   /* error number */
		      uint8_t  ee_origin;  /* where the error originated */
		      uint8_t  ee_type;	   /* type */
		      uint8_t  ee_code;	   /* code */
		      uint8_t  ee_pad;	   /* padding */
		      uint32_t ee_info;	   /* additional information */
		      uint32_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 sock‐
	      addr 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	opera‐

	      This flag requests receipt of out-of-band data that would not be
	      received in the normal data stream.  Some protocols place	 expe‐
	      dited  data  at the head of the normal data queue, and thus this
	      flag cannot be used with such protocols.

	      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_TRUNC (since Linux 2.2)
	      For    raw   (AF_PACKET),	  Internet   datagram	(since	 Linux
	      2.4.27/2.6.8), and netlink (since Linux 2.6.22) sockets:	return
	      the  real	 length	 of  the  packet or datagram, even when it was
	      longer than the passed buffer.  Not implemented for Unix	domain

	      For use with Internet stream sockets, see tcp(7).

       MSG_WAITALL (since Linux 2.2)
	      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 discon‐
	      nect occurs, or the next data to be received is of  a  different
	      type than that returned.

       The  recvmsg()  call  uses a msghdr structure to minimize the number of
       directly supplied arguments.  This structure is defined as  follows  in

	   struct iovec {		     /* Scatter/gather array items */
	       void  *iov_base;		     /* Starting address */
	       size_t iov_len;		     /* Number of bytes to transfer */

	   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 source 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 scat‐
       ter-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
	       unsigned 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:

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

	      indicates	 that the trailing portion of a datagram was discarded
	      because the datagram was larger than the buffer supplied.

	      indicates that some control data were discarded due to  lack  of
	      space in the buffer for ancillary data.

	      is  returned to indicate that expedited or out-of-band data were

	      indicates that no data was received but an extended  error  from
	      the socket error queue.

       These  calls  return  the  number  of bytes received, or -1 if an error
       occurred.  The return value will be 0 when the peer  has	 performed  an
       orderly shutdown.

       These  are  some	 standard errors generated by the socket layer.	 Addi‐
       tional errors may be generated and returned from the underlying	proto‐
       col modules; see their manual pages.

	      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.  POSIX.1-2001 allows either
	      error to be returned for this case, and does not	require	 these
	      constants	 to  have  the	same  value, so a portable application
	      should check for both possibilities.

       EBADF  The argument sockfd is an invalid descriptor.

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

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

       EINTR  The receive was interrupted by delivery of a signal  before  any
	      data were available; see signal(7).

       EINVAL Invalid argument passed.

       ENOMEM Could not allocate memory for recvmsg().

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

	      The argument sockfd does not refer to a socket.

       4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

       POSIX.1-2001 only describes  the	 MSG_OOB,  MSG_PEEK,  and  MSG_WAITALL

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

       According  to  POSIX.1-2001,  the  msg_controllen  field	 of the msghdr
       structure should be typed as socklen_t, but glibc currently (2.4) types
       it as size_t.

       An example of the use of recvfrom() is shown in getaddrinfo(3).

       fcntl(2),  getsockopt(2),  read(2),  select(2), shutdown(2), socket(2),
       cmsg(3), sockatmark(3), socket(7)

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2009-03-10			       RECV(2)

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