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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  recvfrom() and recvmsg() calls are used to receive messages from a socket, and may be used to receive data
       on a socket whether or not it is connection-oriented.

       If src_addr is not NULL, and the underlying protocol provides the source address, this source address is filled
       in.   When  src_addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL.
       The argument addrlen is a value-result argument, which the caller should initialize before the call to the size
       of  the  buffer  associated  with  src_addr,  and  modified on return to indicate 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.

       The  recv()  call  is  normally used only on a connected socket (see connect(2)) and is identical to recvfrom()
       with a NULL src_addr argument.

       All three routines 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  non-blocking  (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.

       The select(2) or poll(2) call may be used to determine when more data arrives.

       The flags argument to a recv() call is formed by OR'ing 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  non-blocking  operation;  if the operation would block, the call fails with the error EAGAIN or
              EWOULDBLOCK (this can also be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(2)).

       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 destina-
              tion 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_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 mes-
              sage.  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  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 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),  and  netlink  (since  Linux  2.6.22)
              sockets:  return  the  real  length  of  the packet or datagram, even when it was longer than the passed
              buffer.  Not implemented for Unix domain sockets.

              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.

       The recvmsg() call uses a msghdr structure to minimize the number of directly supplied arguments.  This  struc-
       ture 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 */
               socklen_t     msg_controllen; /* ancillary data buffer len */
               int           msg_flags;      /* flags on received message */
           };

       Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as
       a null pointer if no names are desired or required.  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 only be accessed by the macros defined in cmsg(3).

       As an example, Linux uses this auxiliary data mechanism to pass extended errors, IP options or file descriptors
       over Unix 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.  The return value will be 0 when
       the peer has performed an orderly shutdown.

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 non-blocking and the receive operation would block, or a receive timeout had been
              set and the timeout expired before data was received.  POSIX.1-2001 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 argument sockfd does not refer to a socket.

CONFORMING TO
       4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

       POSIX.1-2001 only describes the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.

NOTES
       The prototypes given above follow glibc2.  The Single Unix Specification agrees, except that it has return val-
       ues  of  type  ssize_t (while 4.x BSD and libc4 and libc5 all have int).  The flags argument is int in 4.x BSD,
       but unsigned int in libc4 and libc5.  The len argument is int in 4.x BSD, but size_t in libc4 and  libc5.   The
       addrlen  argument  is  int * in 4.x BSD, libc4 and libc5.  The present  socklen_t * was invented by POSIX.  See
       also accept(2).

       According to POSIX.1-2001, the msg_controllen field of the msghdr structure should be typed as  socklen_t,  but
       glibc currently (2.4) types it as size_t.

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

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-03-10                           RECV(2)