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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
       messages from a socket.  They may be  used  to  receive  data  on  both
       connectionless   and  connection-oriented  sockets.   This  page  first
       describes common features of all three system calls, and then describes
       the differences between the calls.

       All  three  calls  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.

       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.

       An  application  can  used 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
       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
              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 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
                  {
                      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  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.

       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.

   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  should
       be specified as NULL and addrlen should be specified as 0.

   recv()
       The  recv()  call  is  normally  used  only  on a connected socket (see
       connect(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 */
           };

       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 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
       occurred.   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
       domains)  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.
       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 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  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 3.65 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/.



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