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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 argument.

       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 arrives.

       The  flags  argument  to  a  recv()  call  is formed by ORing one or more of the following

       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.

              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.  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 <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

              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.  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 describes only 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 addrlen argument is int *
       in 4.x BSD, libc4 and libc5.  The present  socklen_t * was invented by  POSIX.   See  also

       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.


       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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