recvmsg

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 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.

              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 flag 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 was 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 was
              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.

       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
       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 was 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
       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
       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, and the msg_iovlen field should be typed as
       int, but glibc currently types both 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.

EXAMPLES
       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), ip(7), ipv6(7), socket(7), tcp(7),
       udp(7), unix(7)

COLOPHON
       This page is part of release 5.08 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                              2020-06-09                            RECV(2)