recvmsg(2)



RECV(2)                    Linux Programmer's Manual                   RECV(2)

NAME
       recv, recvfrom, recvmsg - receive a message from a socket

SYNOPSIS
       #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);

DESCRIPTION
       The  recv(),  recvfrom(),  and recvmsg() calls are used to receive mes-
       sages from a socket.  They may be used to receive data on both  connec-
       tionless  and  connection-oriented  sockets.  This page first describes
       common features of all three system calls, and then describes the  dif-
       ferences between the calls.

       The  only  difference  between  recv()  and  read(2) is the presence of
       flags.  With a zero flags argument, recv() is generally  equivalent  to
       read(2) (but see NOTES).  Also, the following call

           recv(sockfd, buf, len, flags);

       is equivalent to

           recvfrom(sockfd, buf, len, flags, NULL, NULL);

       All  three calls return the length of the message on successful comple-
       tion.  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 val-
       ues:

       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 rea-
              sons 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.

              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_ori-
              gin 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 sock-
              addr 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 flag 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  opera-
              tion.

       MSG_OOB
              This flag requests receipt of out-of-band data that would not be
              received in the normal data stream.  Some protocols place  expe-
              dited  data  at the head of the normal data queue, and thus this
              flag cannot be used with such protocols.

       MSG_PEEK
              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  discon-
              nect  occurs,  or the next data to be received is of a different
              type than that returned.  This flag has no effect  for  datagram
              sockets.

   recvfrom()
       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.

   recv()
       The recv() call is normally used only on a connected socket  (see  con-
       nect(2)).  It is equivalent to the call:

           recvfrom(fd, buf, len, flags, NULL, 0);

   recvmsg()
       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 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 con-
       tain the length of the available buffer  in  msg_control;  upon  return
       from  a  successful call it will contain the length of the control mes-
       sage sequence.

       The messages are of the form:

           struct cmsghdr {
               size_t cmsg_len;    /* Data byte count, including header
                                      (type is socklen_t in POSIX) */
               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 sock-
       ets.

       The  msg_flags  field  in the msghdr is set on return of recvmsg().  It
       can contain several flags:

       MSG_EOR
              indicates end-of-record; the data returned  completed  a  record
              (generally used with sockets of type SOCK_SEQPACKET).

       MSG_TRUNC
              indicates  that the trailing portion of a datagram was discarded
              because the datagram was larger than the buffer supplied.

       MSG_CTRUNC
              indicates that some control data were discarded due to  lack  of
              space in the buffer for ancillary data.

       MSG_OOB
              is  returned to indicate that expedited or out-of-band data were
              received.

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

RETURN VALUE
       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.

ERRORS
       These  are  some  standard errors generated by the socket layer.  Addi-
       tional errors may be generated and returned from the underlying  proto-
       col modules; see their manual pages.

       EAGAIN or EWOULDBLOCK
              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 file descriptor.

       ECONNREFUSED
              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().

       ENOTCONN
              The socket is associated with a connection-oriented protocol and
              has not been connected (see connect(2) and accept(2)).

       ENOTSOCK
              The file descriptor sockfd does not refer to a socket.

CONFORMING TO
       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.

NOTES
       If  a  zero-length datagram is pending, read(2) and recv() with a flags
       argument of zero provide different  behavior.   In  this  circumstance,
       read(2) has no effect (the datagram remains pending), while recv() con-
       sumes the pending datagram.

       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.

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

SEE ALSO
       fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2),  shutdown(2),
       socket(2),  cmsg(3),  sockatmark(3), ip(7), ipv6(7), socket(7), tcp(7),
       udp(7), unix(7)

COLOPHON
       This page is part of release 4.11 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-03-13                           RECV(2)

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