777 lines
20 KiB
HTML
777 lines
20 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of RECV</TITLE>
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</HEAD><BODY>
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<H1>RECV</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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recv, recvfrom, recvmsg - receive a message from a socket
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/sys/socket.h">sys/socket.h</A>></B>
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<B>ssize_t recv(int </B><I>sockfd</I><B>, void *</B><I>buf</I><B>, size_t </B><I>len</I><B>, int </B><I>flags</I><B>);</B>
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<B>ssize_t recvfrom(int </B><I>sockfd</I><B>, void *</B><I>buf</I><B>, size_t </B><I>len</I><B>, int </B><I>flags</I><B>,</B>
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<B> struct sockaddr *</B><I>src_addr</I><B>, socklen_t *</B><I>addrlen</I><B>);</B>
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<B>ssize_t recvmsg(int </B><I>sockfd</I><B>, struct msghdr *</B><I>msg</I><B>, int </B><I>flags</I><B>);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>recv</B>(),
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<B>recvfrom</B>(),
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and
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<B>recvmsg</B>()
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calls are used to receive messages from a socket.
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They may be used
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to receive data on both connectionless and connection-oriented sockets.
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This page first describes common features of all three system calls,
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and then describes the differences between the calls.
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<P>
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The only difference between
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<B>recv</B>()
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and
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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is the presence of
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<I>flags</I>.
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With a zero
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<I>flags</I>
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argument,
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<B>recv</B>()
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is generally equivalent to
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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(but see NOTES).
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Also, the following call
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<P>
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<BR> recv(sockfd, buf, len, flags);
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<P>
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is equivalent to
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<P>
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<BR> recvfrom(sockfd, buf, len, flags, NULL, NULL);
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<P>
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All three calls return the length of the message on successful
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completion.
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If a message is too long to fit in the supplied buffer, excess
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bytes may be discarded depending on the type of socket the message is
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received from.
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<P>
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If no messages are available at the socket, the receive calls wait for a
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message to arrive, unless the socket is nonblocking (see
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<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)),
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in which case the value -1 is returned and the external variable
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<I>errno</I>
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is set to
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<B>EAGAIN</B> or <B>EWOULDBLOCK</B>.
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The receive calls normally return any data available, up to the requested
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amount, rather than waiting for receipt of the full amount requested.
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<P>
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An application can use
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<B><A HREF="/cgi-bin/man/man2html?2+select">select</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+poll">poll</A></B>(2),
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or
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<B><A HREF="/cgi-bin/man/man2html?7+epoll">epoll</A></B>(7)
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to determine when more data arrives on a socket.
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<A NAME="lbAE"> </A>
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<H3>The flags argument</H3>
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The
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<I>flags</I>
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argument is formed by ORing one or more of the following values:
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<DL COMPACT>
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<DT id="1"><B>MSG_CMSG_CLOEXEC</B> (<B>recvmsg</B>() only; since Linux 2.6.23)
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<DD>
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Set the close-on-exec flag for the file descriptor received
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via a UNIX domain file descriptor using the
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<B>SCM_RIGHTS</B>
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operation (described in
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<B><A HREF="/cgi-bin/man/man2html?7+unix">unix</A></B>(7)).
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This flag is useful for the same reasons as the
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<B>O_CLOEXEC</B>
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flag of
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).
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<DT id="2"><B>MSG_DONTWAIT</B> (since Linux 2.2)
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<DD>
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Enables nonblocking operation; if the operation would block,
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the call fails with the error
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<B>EAGAIN</B> or <B>EWOULDBLOCK</B>.
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This provides similar behavior to setting the
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<B>O_NONBLOCK</B>
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flag (via the
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<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2)
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<B>F_SETFL</B>
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operation), but differs in that
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<B>MSG_DONTWAIT</B>
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is a per-call option, whereas
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<B>O_NONBLOCK</B>
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is a setting on the open file description (see
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)),
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which will affect all threads in the calling process
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and as well as other processes that hold file descriptors
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referring to the same open file description.
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<DT id="3"><B>MSG_ERRQUEUE</B> (since Linux 2.2)
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<DD>
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This flag
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specifies that queued errors should be received from the socket error queue.
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The error is passed in
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an ancillary message with a type dependent on the protocol (for IPv4
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<B>IP_RECVERR</B>).
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The user should supply a buffer of sufficient size.
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See
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<B><A HREF="/cgi-bin/man/man2html?3+cmsg">cmsg</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?7+ip">ip</A></B>(7)
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for more information.
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The payload of the original packet that caused the error
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is passed as normal data via
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<I>msg_iovec</I>.
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The original destination address of the datagram that caused the error
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is supplied via
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<I>msg_name</I>.
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<DT id="4"><DD>
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The error is supplied in a
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<I>sock_extended_err</I>
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structure:
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<DT id="5"><DD>
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#define SO_EE_ORIGIN_NONE 0
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#define SO_EE_ORIGIN_LOCAL 1
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#define SO_EE_ORIGIN_ICMP 2
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#define SO_EE_ORIGIN_ICMP6 3
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<P>
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struct sock_extended_err
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{
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<BR> uint32_t ee_errno; /* Error number */
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<BR> uint8_t ee_origin; /* Where the error originated */
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<BR> uint8_t ee_type; /* Type */
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<BR> uint8_t ee_code; /* Code */
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<BR> uint8_t ee_pad; /* Padding */
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<BR> uint32_t ee_info; /* Additional information */
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<BR> uint32_t ee_data; /* Other data */
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<BR> /* More data may follow */
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};
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<P>
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struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
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<DT id="6"><DD>
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<I>ee_errno</I>
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contains the
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<I>errno</I>
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number of the queued error.
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<I>ee_origin</I>
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is the origin code of where the error originated.
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The other fields are protocol-specific.
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The macro
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<B>SOCK_EE_OFFENDER</B>
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returns a pointer to the address of the network object
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where the error originated from given a pointer to the ancillary message.
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If this address is not known, the
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<I>sa_family</I>
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member of the
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<I>sockaddr</I>
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contains
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<B>AF_UNSPEC</B>
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and the other fields of the
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<I>sockaddr</I>
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are undefined.
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The payload of the packet that caused the error is passed as normal data.
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<DT id="7"><DD>
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For local errors, no address is passed (this
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can be checked with the
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<I>cmsg_len</I>
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member of the
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<I>cmsghdr</I>).
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For error receives,
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the
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<B>MSG_ERRQUEUE</B>
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flag is set in the
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<I>msghdr</I>.
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After an error has been passed, the pending socket error
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is regenerated based on the next queued error and will be passed
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on the next socket operation.
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<DT id="8"><B>MSG_OOB</B>
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<DD>
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This flag requests receipt of out-of-band data that would not be received
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in the normal data stream.
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Some protocols place expedited data
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at the head of the normal data queue, and thus this flag cannot
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be used with such protocols.
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<DT id="9"><B>MSG_PEEK</B>
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<DD>
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This flag causes the receive operation to
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return data from the beginning of the
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receive queue without removing that data from the queue.
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Thus, a
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subsequent receive call will return the same data.
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<DT id="10"><B>MSG_TRUNC</B> (since Linux 2.2)
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<DD>
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For raw
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(<B>AF_PACKET</B>),
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Internet datagram (since Linux 2.4.27/2.6.8),
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netlink (since Linux 2.6.22), and UNIX datagram
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(since Linux 3.4) sockets:
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return the real length of the packet or datagram,
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even when it was longer than the passed buffer.
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<DT id="11"><DD>
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For use with Internet stream sockets, see
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<B><A HREF="/cgi-bin/man/man2html?7+tcp">tcp</A></B>(7).
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<DT id="12"><B>MSG_WAITALL</B> (since Linux 2.2)
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<DD>
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This flag requests that the operation block until the full request is
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satisfied.
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However, the call may still return less data than requested if
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a signal is caught, an error or disconnect occurs, or the next data to be
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received is of a different type than that returned.
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This flag has no effect for datagram sockets.
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</DL>
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<A NAME="lbAF"> </A>
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<H3>recvfrom()</H3>
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<B>recvfrom</B>()
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places the received message into the buffer
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<I>buf</I>.
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The caller must specify the size of the buffer in
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<I>len</I>.
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<P>
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If
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<I>src_addr</I>
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is not NULL,
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and the underlying protocol provides the source address of the message,
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that source address is placed in the buffer pointed to by
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<I>src_addr</I>.
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In this case,
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<I>addrlen</I>
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is a value-result argument.
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Before the call,
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it should be initialized to the size of the buffer associated with
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<I>src_addr</I>.
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Upon return,
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<I>addrlen</I>
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is updated to contain the actual size of the source address.
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The returned address is truncated if the buffer provided is too small;
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in this case,
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<I>addrlen</I>
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will return a value greater than was supplied to the call.
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<P>
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If the caller is not interested in the source address,
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<I>src_addr</I>
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and
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<I>addrlen</I>
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should be specified as NULL.
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<A NAME="lbAG"> </A>
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<H3>recv()</H3>
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The
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<B>recv</B>()
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call is normally used only on a
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<I>connected</I>
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socket (see
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<B><A HREF="/cgi-bin/man/man2html?2+connect">connect</A></B>(2)).
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It is equivalent to the call:
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<P>
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<BR> recvfrom(fd, buf, len, flags, NULL, 0);
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<A NAME="lbAH"> </A>
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<H3>recvmsg()</H3>
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The
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<B>recvmsg</B>()
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call uses a
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<I>msghdr</I>
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structure to minimize the number of directly supplied arguments.
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This structure is defined as follows in
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<I><<A HREF="file:///usr/include/sys/socket.h">sys/socket.h</A>></I>:
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<P>
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struct iovec { /* Scatter/gather array items */
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<BR> void *iov_base; /* Starting address */
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<BR> size_t iov_len; /* Number of bytes to transfer */
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};
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<P>
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struct msghdr {
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<BR> void *msg_name; /* Optional address */
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<BR> socklen_t msg_namelen; /* Size of address */
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<BR> struct iovec *msg_iov; /* Scatter/gather array */
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<BR> size_t msg_iovlen; /* # elements in msg_iov */
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<BR> void *msg_control; /* Ancillary data, see below */
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<BR> size_t msg_controllen; /* Ancillary data buffer len */
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<BR> int msg_flags; /* Flags on received message */
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};
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<P>
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The
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<I>msg_name</I>
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field points to a caller-allocated buffer that is used to
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return the source address if the socket is unconnected.
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The caller should set
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<I>msg_namelen</I>
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to the size of this buffer before this call;
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upon return from a successful call,
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<I>msg_namelen</I>
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will contain the length of the returned address.
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If the application does not need to know the source address,
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<I>msg_name</I>
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can be specified as NULL.
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<P>
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The fields
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<I>msg_iov</I>
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and
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<I>msg_iovlen</I>
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describe scatter-gather locations, as discussed in
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<B><A HREF="/cgi-bin/man/man2html?2+readv">readv</A></B>(2).
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<P>
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The field
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<I>msg_control</I>,
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which has length
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<I>msg_controllen</I>,
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points to a buffer for other protocol control-related messages or
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miscellaneous ancillary data.
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When
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<B>recvmsg</B>()
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is called,
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<I>msg_controllen</I>
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should contain the length of the available buffer in
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<I>msg_control</I>;
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upon return from a successful call it will contain the length
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of the control message sequence.
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<P>
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The messages are of the form:
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<P>
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struct cmsghdr {
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<BR> size_t cmsg_len; /* Data byte count, including header
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<BR> (type is socklen_t in POSIX) */
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<BR> int cmsg_level; /* Originating protocol */
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<BR> int cmsg_type; /* Protocol-specific type */
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/* followed by
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<BR> unsigned char cmsg_data[]; */
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};
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<P>
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Ancillary data should be accessed only by the macros defined in
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<B><A HREF="/cgi-bin/man/man2html?3+cmsg">cmsg</A></B>(3).
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<P>
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As an example, Linux uses this ancillary data mechanism to pass extended
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errors, IP options, or file descriptors over UNIX domain sockets.
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<P>
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The
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<I>msg_flags</I>
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field in the
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<I>msghdr</I>
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is set on return of
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<B>recvmsg</B>().
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It can contain several flags:
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<DL COMPACT>
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<DT id="13"><B>MSG_EOR</B>
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<DD>
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indicates end-of-record; the data returned completed a record (generally
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used with sockets of type
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<B>SOCK_SEQPACKET</B>).
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<DT id="14"><B>MSG_TRUNC</B>
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<DD>
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indicates that the trailing portion of a datagram was discarded because the
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datagram was larger than the buffer supplied.
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<DT id="15"><B>MSG_CTRUNC</B>
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<DD>
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indicates that some control data was discarded due to lack of space in the
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buffer for ancillary data.
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<DT id="16"><B>MSG_OOB</B>
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<DD>
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is returned to indicate that expedited or out-of-band data was received.
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<DT id="17"><B>MSG_ERRQUEUE</B>
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<DD>
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indicates that no data was received but an extended error from the socket
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error queue.
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</DL>
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<A NAME="lbAI"> </A>
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<H2>RETURN VALUE</H2>
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These calls return the number of bytes received, or -1
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if an error occurred.
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In the event of an error,
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<I>errno</I>
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is set to indicate the error.
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<P>
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When a stream socket peer has performed an orderly shutdown,
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the return value will be 0 (the traditional "end-of-file" return).
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<P>
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Datagram sockets in various domains (e.g., the UNIX and Internet domains)
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permit zero-length datagrams.
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When such a datagram is received, the return value is 0.
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<P>
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The value 0 may also be returned if the requested number of bytes
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to receive from a stream socket was 0.
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<A NAME="lbAJ"> </A>
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<H2>ERRORS</H2>
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These are some standard errors generated by the socket layer.
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Additional errors
|
|
may be generated and returned from the underlying protocol modules;
|
|
see their manual pages.
|
|
<DL COMPACT>
|
|
<DT id="18"><B>EAGAIN</B> or <B>EWOULDBLOCK</B>
|
|
|
|
<DD>
|
|
|
|
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.
|
|
<DT id="19"><B>EBADF</B>
|
|
|
|
<DD>
|
|
The argument
|
|
<I>sockfd</I>
|
|
|
|
is an invalid file descriptor.
|
|
<DT id="20"><B>ECONNREFUSED</B>
|
|
|
|
<DD>
|
|
A remote host refused to allow the network connection (typically
|
|
because it is not running the requested service).
|
|
<DT id="21"><B>EFAULT</B>
|
|
|
|
<DD>
|
|
The receive buffer pointer(s) point outside the process's
|
|
address space.
|
|
<DT id="22"><B>EINTR</B>
|
|
|
|
<DD>
|
|
The receive was interrupted by delivery of a signal before
|
|
any data was available; see
|
|
<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7).
|
|
|
|
<DT id="23"><B>EINVAL</B>
|
|
|
|
<DD>
|
|
Invalid argument passed.
|
|
|
|
<DT id="24"><B>ENOMEM</B>
|
|
|
|
<DD>
|
|
Could not allocate memory for
|
|
<B>recvmsg</B>().
|
|
|
|
<DT id="25"><B>ENOTCONN</B>
|
|
|
|
<DD>
|
|
The socket is associated with a connection-oriented protocol
|
|
and has not been connected (see
|
|
<B><A HREF="/cgi-bin/man/man2html?2+connect">connect</A></B>(2)
|
|
|
|
and
|
|
<B><A HREF="/cgi-bin/man/man2html?2+accept">accept</A></B>(2)).
|
|
|
|
<DT id="26"><B>ENOTSOCK</B>
|
|
|
|
<DD>
|
|
The file descriptor
|
|
<I>sockfd</I>
|
|
|
|
does not refer to a socket.
|
|
</DL>
|
|
<A NAME="lbAK"> </A>
|
|
<H2>CONFORMING TO</H2>
|
|
|
|
POSIX.1-2001, POSIX.1-2008,
|
|
4.4BSD (these interfaces first appeared in 4.2BSD).
|
|
<P>
|
|
|
|
POSIX.1 describes only the
|
|
<B>MSG_OOB</B>,
|
|
|
|
<B>MSG_PEEK</B>,
|
|
|
|
and
|
|
<B>MSG_WAITALL</B>
|
|
|
|
flags.
|
|
<A NAME="lbAL"> </A>
|
|
<H2>NOTES</H2>
|
|
|
|
If a zero-length datagram is pending,
|
|
<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
|
|
|
|
and
|
|
<B>recv</B>()
|
|
|
|
with a
|
|
<I>flags</I>
|
|
|
|
argument of zero provide different behavior.
|
|
In this circumstance,
|
|
<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
|
|
|
|
has no effect (the datagram remains pending), while
|
|
<B>recv</B>()
|
|
|
|
consumes the pending datagram.
|
|
<P>
|
|
|
|
The
|
|
<I>socklen_t</I>
|
|
|
|
type was invented by POSIX.
|
|
See also
|
|
<B><A HREF="/cgi-bin/man/man2html?2+accept">accept</A></B>(2).
|
|
|
|
<P>
|
|
|
|
According to POSIX.1,
|
|
|
|
the
|
|
<I>msg_controllen</I>
|
|
|
|
field of the
|
|
<I>msghdr</I>
|
|
|
|
structure should be typed as
|
|
<I>socklen_t</I>,
|
|
|
|
but glibc currently types it as
|
|
<I>size_t</I>.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<P>
|
|
|
|
See
|
|
<B><A HREF="/cgi-bin/man/man2html?2+recvmmsg">recvmmsg</A></B>(2)
|
|
|
|
for information about a Linux-specific system call
|
|
that can be used to receive multiple datagrams in a single call.
|
|
<A NAME="lbAM"> </A>
|
|
<H2>EXAMPLE</H2>
|
|
|
|
An example of the use of
|
|
<B>recvfrom</B>()
|
|
|
|
is shown in
|
|
<B><A HREF="/cgi-bin/man/man2html?3+getaddrinfo">getaddrinfo</A></B>(3).
|
|
|
|
<A NAME="lbAN"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+getsockopt">getsockopt</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+recvmmsg">recvmmsg</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+select">select</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+shutdown">shutdown</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+socket">socket</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?3+cmsg">cmsg</A></B>(3),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?3+sockatmark">sockatmark</A></B>(3),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+ip">ip</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+ipv6">ipv6</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+socket">socket</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+tcp">tcp</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+udp">udp</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+unix">unix</A></B>(7)
|
|
|
|
<A NAME="lbAO"> </A>
|
|
<H2>COLOPHON</H2>
|
|
|
|
This page is part of release 5.05 of the Linux
|
|
<I>man-pages</I>
|
|
|
|
project.
|
|
A description of the project,
|
|
information about reporting bugs,
|
|
and the latest version of this page,
|
|
can be found at
|
|
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="27"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="28"><A HREF="#lbAC">SYNOPSIS</A><DD>
|
|
<DT id="29"><A HREF="#lbAD">DESCRIPTION</A><DD>
|
|
<DL>
|
|
<DT id="30"><A HREF="#lbAE">The flags argument</A><DD>
|
|
<DT id="31"><A HREF="#lbAF">recvfrom()</A><DD>
|
|
<DT id="32"><A HREF="#lbAG">recv()</A><DD>
|
|
<DT id="33"><A HREF="#lbAH">recvmsg()</A><DD>
|
|
</DL>
|
|
<DT id="34"><A HREF="#lbAI">RETURN VALUE</A><DD>
|
|
<DT id="35"><A HREF="#lbAJ">ERRORS</A><DD>
|
|
<DT id="36"><A HREF="#lbAK">CONFORMING TO</A><DD>
|
|
<DT id="37"><A HREF="#lbAL">NOTES</A><DD>
|
|
<DT id="38"><A HREF="#lbAM">EXAMPLE</A><DD>
|
|
<DT id="39"><A HREF="#lbAN">SEE ALSO</A><DD>
|
|
<DT id="40"><A HREF="#lbAO">COLOPHON</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:05:34 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|