RECV(2)                    Linux Programmer's Manual                   RECV(2)
       recv, recvfrom, recvmsg - receive a message from a socket

       #include <sys/types.h>
       #include <sys/socket.h>

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

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

       ssize_t recvmsg(int s, struct msghdr *msg, int flags);

       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 underlying protocol provides the source
       address, this source address 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.

       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

       The  flags argument to a recv() call is formed by OR'ing one or more of
       the following values:

              Enables non-blocking operation; if the  operation  would  block,
              EAGAIN  is  returned  (this can also be enabled using the O_NON-
              BLOCK with the F_SETFL fcntl(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 opera-

              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-

              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.

              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

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

              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.

       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 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
             u_char    cmsg_data[]; */

       Ancillary data should  only  be  accessed  by  the  macros  defined  in

       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:

              indicates  end-of-record;  the  data returned completed a record
              (generally used with sockets of type SOCK_SEQPACKET).

              indicates that the trailing portion of a datagram was  discarded
              because the datagram was larger than the buffer supplied.

              indicates  that  some control data were discarded due to lack of
              space in the buffer for ancillary data.

              is returned to indicate that expedited or out-of-band data  were

              indicates  that  no data was received but an extended error from
              the socket error queue.

       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.

       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 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.

       EBADF  The argument s is an invalid descriptor.

              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.

       EINVAL Invalid argument passed.

       ENOMEM Could not allocate memory for recvmsg().

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

              The argument s does not refer to a socket.

       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

       The  prototypes  given above follow glibc2.  The Single Unix Specifica-
       tion agrees, except that it has return values 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 argu-
       ment is 'int' in 4.x BSD, but 'size_t' in libc4 and libc5.  The fromlen
       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.

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

Linux Man Page                    2002-12-31                           RECV(2)