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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>

       int recv(int s, void *buf, size_t len, int flags);

       int  recvfrom(int  s, void *buf, size_t len, int flags, struct sockaddr
       *from, socklen_t *fromlen);

       int recvmsg(int s, 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 from is not NULL, and the socket  is  not  connection-oriented,  the
       source  address of the message is filled in.  The argument fromlen is a
       value-result parameter, initialized to the size of the  buffer  associ-
       ated  with  from, and modified on return to indicate the actual size of
       the address stored there.

       The recv call is normally used only on a  connected  socket  (see  con-
       nect(2)) and is identical to recvfrom with a NULL from parameter.

       All  three routines return the length of the message on successful com-
       pletion.  If a message is too long  to  fit  in  the  supplied  buffer,
       excess  bytes may be discarded depending on the type of socket the mes-
       sage is received from (see socket(2)).

       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 set
       to EAGAIN.  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_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_WAITALL
              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  dif-
              ferent type than that returned.

       MSG_NOSIGNAL
              This  flag  turns  off raising of SIGPIPE on stream sockets when
              the other end disappears.

       MSG_TRUNC
              Return the real length of the packet, even when  it  was  longer
              than the passed buffer. Only valid for packet sockets.

       MSG_ERRQUEUE
              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  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
              {
                  u_int32_t       ee_errno;   /* error number */
                  u_int8_t        ee_origin;  /* where the error originated */
                  u_int8_t        ee_type;    /* type */
                  u_int8_t        ee_code;    /* code */
                  u_int8_t        ee_pad;
                  u_int32_t       ee_info;    /* additional information */
                  u_int32_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  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.

       The recvmsg call uses a msghdr structure  to  minimize  the  number  of
       directly  supplied  parameters.  This structure has the following form,
       as defined in <sys/socket.h>:

              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  scat-
       ter-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 con-
       tain 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
                  u_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.

       MSG_DONTWAIT
              Enables non-blocking operation; if the  operation  would  block,
              EAGAIN   is  returned  (this  can  also  be  enabled  using  the
              O_NONBLOCK with the F_SETFL fcntl(2)).

RETURN VALUE
       These calls return the number of bytes received,  or  -1  if  an  error
       occurred.

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.

       EBADF  The argument s is an invalid descriptor.

       ECONNREFUSED
              A remote host refused to allow the network connection (typically
              because it is not running the requested service).

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

       ENOTSOCK
              The argument s does not refer to a socket.

       EAGAIN 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.

       EINTR  The  receive  was interrupted by delivery of a signal before any
              data were available.

       EFAULT The  receive  buffer  pointer(s)  point  outside  the  process's
              address space.

       EINVAL Invalid argument passed.

CONFORMING TO
       4.4BSD (these function calls first appeared in 4.2BSD).

NOTE
       The  prototypes  given above follow glibc2.  The Single Unix Specifica-
       tion agrees, except that it has return values of type `ssize_t'  (while
       BSD  4.*  and  libc4  and libc5 all have `int').  The flags argument is
       `int' in BSD 4.*, but `unsigned int' in libc4 and libc5.  The len argu-
       ment is `int' in BSD 4.*, but `size_t' in libc4 and libc5.  The fromlen
       argument is  `int  *'  in  BSD  4.*,  libc4  and  libc5.   The  present
       `socklen_t *' was invented by POSIX.  See also accept(2).

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



Linux Man Page                    2001-06-19                           RECV(2)