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.

              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.

       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), socket(7)

COLOPHON
       This page is part of release 4.08 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                             2016-07-17                           RECV(2)

Man(1) output converted with man2html
list of all man pages