recv

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 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.

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

       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 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 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.
              Not implemented for UNIX domain (unix(7)) 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 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 */
           };

       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 {
               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
       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.  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 nonblocking 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 describes only 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 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
       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 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 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2013-04-19                           RECV(2)