recvfrom


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  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 con-
       nect(2)) and is identical to recvfrom() with a NULL src_addr 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 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 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).

              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  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.


              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.

       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  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  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-

       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.   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 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.1-2001 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.

       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 sockfd does not refer to a socket.

       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 accept(2).

       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.

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), socket(7)

COLOPHON
       This page is part of release 3.54 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                             2013-04-19                           RECV(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2017 Hurricane Electric. All Rights Reserved.