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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  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 non-blocking (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 OR'ing one or more of the  follow-
       ing 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 non-blocking 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 depen-
              dent 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 pay-
              load 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.

       MSG_OOB
              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 proto-
              cols.

       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 subse-
              quent 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),  and
              netlink  (since  Linux 2.6.22) sockets: return the real length of the packet
              or datagram, even when it was longer than the  passed  buffer.   Not  imple-
              mented for Unix domain 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 satis-
              fied.  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  sup-
       plied 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 */
               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 uncon-
       nected; 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 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.  Additional errors
       may be generated and returned from the underlying protocol modules; see their  man-
       ual pages.

       EAGAIN or EWOULDBLOCK
              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.   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.

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  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 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), sock-
       atmark(3), socket(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-03-10                           RECV(2)

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