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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 s, void *buf, size_t len, int flags);

       ssize_t recvfrom(int s, void *buf, size_t len, int flags,
                        struct sockaddr *from, socklen_t *fromlen);

       ssize_t 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 underlying protocol provides the source
       address, this source address 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 identical to recvfrom() with a NULL from  parameter.

       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 non-blocking (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

       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).

              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)).

              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;
                      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

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

              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  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_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.

       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.

       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).

       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.

       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 s 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 fromlen
       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)


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