NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

RECV(2)                   Linux Programmer's Manual                  RECV(2)

NAME         top

       recv, recvfrom, recvmsg - receive a message from a socket

SYNOPSIS         top

       #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         top

       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.

       The only difference between recv() and read(2) is the presence of
       flags.  With a zero flags argument, recv() is generally equivalent to
       read(2) (but see NOTES).  Also, the following call

           recv(sockfd, buf, len, flags);

       is equivalent to

           recvfrom(sockfd, buf, len, flags, NULL, NULL);

       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.  This flag has no effect
              for datagram sockets.

   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 {
               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         top

       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         top

       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 file 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         top

       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         top

       If a zero-length datagram is pending, read(2) and recv() with a flags
       argument of zero provide different behavior.  In this circumstance,
       read(2) has no effect (the datagram remains pending), while recv()
       consumes the pending datagram.

       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         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.08 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-07-17                          RECV(2)