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Linux 2.6 - man page for recv (linux section 2)

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  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 connect(2)) and is  iden-
       tical to recvfrom() with a NULL src_addr argument.

       All  three  routines return the length of the message on successful completion.	If a mes-
       sage 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 receipt of the full amount requested.

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

       The  flags  argument to a recv() call 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 reasons 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 mem-
	      ber 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
		  {
		      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_origin  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  mem-
	      ber  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.

       MSG_OOB
	      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
	      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 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 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 */
	   };

       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 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
	       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 extended errors, IP
       options, or file descriptors over UNIX domain sockets.

       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  data-
	      gram 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 occurred.  In the event
       of an error, errno is set to indicate the error.  The return value will be 0 when the peer
       has performed an orderly shutdown.

ERRORS
       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.

       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 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().

       ENOTCONN
	      The socket is associated with a connection-oriented protocol and has not been  con-
	      nected (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  prototypes	given  above follow glibc2.  The Single UNIX Specification 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 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 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-11-04					  RECV(2)


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