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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 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 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  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.

              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 flag 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 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  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()
       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  connect(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 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 cmsg(3).

       As  an  example,  Linux  uses  this  ancillary  data mechanism to pass extended errors, IP
       options, or file descriptors over UNIX domain 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.  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.  Additional errors may be
       generated and returned from the underlying protocol 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() 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.

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), ip(7), ipv6(7), socket(7), tcp(7), udp(7), unix(7)

COLOPHON

       This page is part of release 4.16 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/.