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



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

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/socket.h>&gt;

       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);

DESCRIPTION
       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 argument, 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 con-
       nect(2)) and is identical to recvfrom() with a NULL from argument.

       All three routines return the length of the message on successful  com-
       pletion.   If  a  message  is  too  long to fit in the supplied buffer,
       excess bytes may be discarded depending on the type of socket the  mes-
       sage 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
       arrives.

       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  rea-
              sons as the O_CLOEXEC flag of open(2).

       MSG_DONTWAIT (since Linux 2.2)
              Enables  non-blocking  operation;  if the operation would block,
              the call fails with the error EAGAIN (this can also  be  enabled
              using the O_NONBLOCK with the F_SETFL fcntl(2)).

       MSG_ERRQUEUE
              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  opera-
              tion.

              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  (since  Linux 2.2) 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.

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

       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.

       The  recvmsg()  call  uses a msghdr structure to minimize the number of
       directly supplied arguments.  This structure has the following form, as
       defined in &lt;sys/socket.h&gt;:

           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 scat-
       ter-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  only  be  accessed  by  the  macros defined in
       cmsg(3).

       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:

       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.  The return value will be 0 when the peer  has  performed  an
       orderly shutdown.

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 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.

       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 argument s does not refer to a socket.

CONFORMING TO
       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
       flags.

NOTES
       The prototypes given above follow glibc2.  The Single  Unix  Specifica-
       tion  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.

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

SEE ALSO
       fcntl(2), getsockopt(2), read(2),  select(2),  shutdown(2),  socket(2),
       cmsg(3), sockatmark(3)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-14                           RECV(2)