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       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
       connect(2)) and  is  identical  to  recvfrom()  with  a  NULL  src_addr

       All  three  routines  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.

       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
              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

              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

              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

       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
              (unix(7)) sockets.

              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

           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
       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

       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.
       Additional errors may be generated and  returned  from  the  underlying
       protocol modules; see their manual pages.

              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.

              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
       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 (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)


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