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man:recvfrom

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.
            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 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 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), 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/.

Linux 2017-09-15 RECV(2)

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