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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 (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

       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

              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.

              Return the real length of the packet, even when  it  was  longer
              than the passed buffer. 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  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
                  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 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 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
                  u_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.

              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. 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

       POSIX only describes the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.


       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).


       fcntl(2),  getsockopt(2),  read(2),  select(2), shutdown(2), socket(2),