recv(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLES | 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/socket.h>

       ssize_t recv(int sockfd, void *buf, size_t len, int flags);
       ssize_t recvfrom(int sockfd, void *restrict buf, size_t len, int flags,
                        struct sockaddr *restrict src_addr,
                        socklen_t *restrict 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 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.  For further information on the use of ancillary data in
       various socket domains, see unix(7) and ip(7).

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                        RECV(2)

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