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

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

SYNOPSIS
       #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);

DESCRIPTION
       The  recv(),  recvfrom(),  and recvmsg()	calls are used to receive mes-
       sages from a socket.  They may be used to receive data on both  connec-
       tionless	 and  connection-oriented  sockets.  This page first describes
       common features of all three system calls, and then describes the  dif-
       ferences	between	the calls.

       All  three calls	return the length of the message on successful comple-
       tion.  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.

       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
       is set to EAGAIN	or EWOULDBLOCK.	 The receive calls normally return any
       data available, up to the requested amount, rather than waiting for re-
       ceipt of	the full amount	requested.

       An  application	can  use  select(2), poll(2), or epoll(7) to determine
       when more data arrives on a socket.

   The flags argument
       The flags argument is formed by ORing one or more of the	following val-
       ues:

       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 nonblocking 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 fcntl(2)).

       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-
	      tion.

	      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  re-
	      turns  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-
	      tion.

       MSG_OOB
	      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.

       MSG_PEEK
	      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
	      data.

       MSG_TRUNC (since	Linux 2.2)
	      For    raw   (AF_PACKET),	  Internet   datagram	(since	 Linux
	      2.4.27/2.6.8), netlink (since Linux 2.6.22), and	UNIX  datagram
	      (since  Linux 3.4) sockets: return the real length of the	packet
	      or datagram, even	when it	was longer than	the passed buffer.

	      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  re-
	      quest  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.

   recvfrom()
       recvfrom() places the received message into the buffer buf.  The	caller
       must specify the	size of	the buffer in len.

       If  src_addr  is	 not  NULL,  and  the underlying protocol provides the
       source address of the message, that source address  is  placed  in  the
       buffer pointed to by src_addr.  In this case, addrlen is	a value-result
       argument.  Before the call, it should be	initialized to the size	of the
       buffer  associated  with	 src_addr.  Upon return, addrlen is updated to
       contain 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.

       If the caller is	not interested in the source address, src_addr and ad-
       drlen should be specified as NULL.

   recv()
       The  recv()  call is normally used only on a connected socket (see con-
       nect(2)).  It is	equivalent to the call:

	   recvfrom(fd,	buf, len, flags, NULL, 0));

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

	   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 */
	       size_t	     msg_controllen; /*	ancillary data buffer len */
	       int	     msg_flags;	     /*	flags on received message */
	   };

       The msg_name field points to a caller-allocated buffer that is used  to
       return  the  source  address  if	the socket is unconnected.  The	caller
       should set msg_namelen to the size of this  buffer  before  this	 call;
       upon return from	a successful call, msg_name will contain the length of
       the returned address.  If the application does not  need	 to  know  the
       source address, msg_name	can be specified as NULL.

       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 an-
       cillary 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  se-
       quence.

       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	be  accessed  only  by	the  macros defined in
       cmsg(3).

       As an example, Linux uses this ancillary	data  mechanism	 to  pass  ex-
       tended  errors,	IP options, or file descriptors	over UNIX domain sock-
       ets.

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

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

       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	data.

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

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

RETURN VALUE
       These calls return the number of	bytes received,	or -1 if an error  oc-
       curred.	In the event of	an error, errno	is set to indicate the error.

       When a stream socket peer has performed an orderly shutdown, the	return
       value will be 0 (the traditional	"end-of-file" return).

       Datagram	sockets	in various domains (e.g., the UNIX  and	 Internet  do-
       mains) permit zero-length datagrams.  When such a datagram is received,
       the return value	is 0.

       The value 0 may also be returned	if the requested number	 of  bytes  to
       receive from a stream socket was	0.

ERRORS
       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.

       EAGAIN or EWOULDBLOCK
	      The socket is marked nonblocking 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.

       ECONNREFUSED
	      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 ad-
	      dress 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().

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

       ENOTSOCK
	      The argument sockfd does not refer to a socket.

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

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

NOTES
       The socklen_t type 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 types it as
       size_t.

       See recvmmsg(2) for information about a Linux-specific system call that
       can be used to receive multiple datagrams in a single call.

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

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

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			       RECV(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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