xenial (2) recvfrom.2.gz

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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.

       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.

              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 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.

   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 {
               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 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 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

       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 4.04 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
       http://www.kernel.org/doc/man-pages/.