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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  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 use 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 provides similar  behavior  to  setting  the
              O_NONBLOCK  flag  (via  the  fcntl(2)  F_SETFL  operation),  but  differs  in  that
              MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a setting on the open file
              description (see open(2)), which will affect all threads in the calling process and
              as well as other processes that hold file descriptors referring to  the  same  open
              file description.

       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.

              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.

       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()  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  and  addrlen  should  be
       specified as NULL.

       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));

       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 */

       The  msg_name  field points to a caller-allocated buffer that is used to return the source
       address if the socket is unconnected.  The caller should set msg_namelen to  the  size  of
       this buffer before this call; upon return from a successful call, msg_namelen will contain
       the length of the returned address.  If the application does not need to know  the  source
       address, msg_name can be specified as NULL.

       The  fields  msg_iov  and  msg_iovlen  describe  scatter-gather locations, as discussed in

       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 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

              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 received.

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


       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.


       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  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 file descriptor sockfd does not refer to a socket.


       POSIX.1-2001, POSIX.1-2008, 4.4BSD (these interfaces first appeared in 4.2BSD).

       POSIX.1 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.


       The socklen_t type was invented by POSIX.  See also accept(2).

       According to POSIX.1, 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.


       An example of the use of recvfrom() is shown in getaddrinfo(3).


       fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2), shutdown(2), socket(2), cmsg(3),
       sockatmark(3), socket(7)


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