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RedHat 9 (Linux i386) - man page for recvmsg (redhat section 2)

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>

       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

       int recvmsg(int s, 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  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 connect(2)) and is  identi-
       cal to recvfrom with a NULL from parameter.

       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 (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 arrives.

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

	      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.

	      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.

	      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.

	      This flag turns off raising of SIGPIPE on stream sockets when the other end  disap-

	      Return  the real length of the packet, even when it was longer than the passed buf-
	      fer. Only valid for packet sockets.

	      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
		  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_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.

       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 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_con-
       trollen 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 cmsg(3).

       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

	      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 data-
	      gram 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 received.

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

	      Enables non-blocking operation; if the operation would block,  EAGAIN  is  returned
	      (this can also be enabled using the O_NONBLOCK with the F_SETFL fcntl(2)).

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

       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.

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

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

	      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.

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

       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  argu-
       ment  is  `int *' in BSD 4.*, libc4 and libc5.  The present  `socklen_t *' was invented by
       POSIX.  See also accept(2).

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

Linux Man Page				    2001-06-19					  RECV(2)

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