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


   #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);


   The  recv(),  recvfrom(),  and  recvmsg()  calls  are  used  to receive
   messages from a socket.  They may be  used  to  receive  data  on  both
   connectionless   and  connection-oriented  sockets.   This  page  first
   describes common features of all three system calls, and then describes
   the differences 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
   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 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

   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 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

          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

          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 protocols.

          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

   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
          disconnect 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() 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.

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

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

   The recvmsg() call uses a msghdr structure to minimize  the  number  of
   directly  supplied  arguments.  This structure is defined as follows in

       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
   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 {
           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

   As an example,  Linux  uses  this  ancillary  data  mechanism  to  pass
   extended  errors,  IP  options,  or  file  descriptors over UNIX domain

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

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

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

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

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

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


   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

   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.


   These  are  some  standard  errors  generated  by  the  socket   layer.
   Additional  errors  may  be  generated and returned from the underlying
   protocol modules; see their manual pages.

          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.

          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().

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

          The file descriptor sockfd does not refer to a socket.


   POSIX.1-2001,  POSIX.1-2008, 4.4BSD (these interfaces first appeared in

   POSIX.1 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.


   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()
   consumes 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.


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


   fcntl(2),  getsockopt(2), read(2), recvmmsg(2), select(2), shutdown(2),
   socket(2), cmsg(3), sockatmark(3), socket(7)


   This page is part of release 4.09 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

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