mirror of https://github.com/mkerrisk/man-pages
253 lines
6.5 KiB
Groff
253 lines
6.5 KiB
Groff
.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
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.\"
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.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
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.\" Permission is granted to distribute possibly modified copies
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.\" of this page provided the header is included verbatim,
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.\" and in case of nontrivial modification author and date
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.\" of the modification is added to the header.
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.\" %%%LICENSE_END
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.\"
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.\" $Id: cmsg.3,v 1.8 2000/12/20 18:10:31 ak Exp $
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.TH CMSG 3 2020-06-09 "Linux" "Linux Programmer's Manual"
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.SH NAME
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CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR \- access ancillary data
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.SH SYNOPSIS
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.nf
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.B #include <sys/socket.h>
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.BI "struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *" msgh );
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.BI "struct cmsghdr *CMSG_NXTHDR(struct msghdr *" msgh ,
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.BR " struct cmsghdr *" cmsg );
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.BI "size_t CMSG_ALIGN(size_t " length );
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.BI "size_t CMSG_SPACE(size_t " length );
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.BI "size_t CMSG_LEN(size_t " length );
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.BI "unsigned char *CMSG_DATA(struct cmsghdr *" cmsg );
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.fi
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.SH DESCRIPTION
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These macros are used to create and access control messages (also called
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ancillary data) that are not a part of the socket payload.
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This control information may
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include the interface the packet was received on, various rarely used header
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fields, an extended error description, a set of file descriptors, or UNIX
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credentials.
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For instance, control messages can be used to send
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additional header fields such as IP options.
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Ancillary data is sent by calling
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.BR sendmsg (2)
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and received by calling
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.BR recvmsg (2).
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See their manual pages for more information.
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.PP
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Ancillary data is a sequence of
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.I cmsghdr
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structures with appended data.
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See the specific protocol man pages for the available control message types.
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The maximum ancillary buffer size allowed per socket can be set using
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.IR /proc/sys/net/core/optmem_max ;
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see
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.BR socket (7).
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.PP
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The
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.I cmsghdr
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structure is defined as follows:
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.PP
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.in +4n
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.EX
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struct cmsghdr {
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size_t cmsg_len; /* Data byte count, including header
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(type is socklen_t in POSIX) */
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int cmsg_level; /* Originating protocol */
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int cmsg_type; /* Protocol-specific type */
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/* followed by
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unsigned char cmsg_data[]; */
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};
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.EE
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.in
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.PP
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The sequence of
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.I cmsghdr
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structures should never be accessed directly.
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Instead, use only the following macros:
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.IP * 3
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.BR CMSG_FIRSTHDR ()
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returns a pointer to the first
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.I cmsghdr
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in the ancillary
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data buffer associated with the passed
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.IR msghdr .
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It returns NULL if there isn't enough space for a
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.I cmsghdr
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in the buffer.
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.IP *
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.BR CMSG_NXTHDR ()
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returns the next valid
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.I cmsghdr
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after the passed
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.IR cmsghdr .
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It returns NULL when there isn't enough space left in the buffer.
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.IP
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When initializing a buffer that will contain a series of
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.I cmsghdr
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structures (e.g., to be sent with
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.BR sendmsg (2)),
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that buffer should first be zero-initialized
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to ensure the correct operation of
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.BR CMSG_NXTHDR ().
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.IP *
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.BR CMSG_ALIGN (),
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given a length, returns it including the required alignment.
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This is a
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constant expression.
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.IP *
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.BR CMSG_SPACE ()
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returns the number of bytes an ancillary element with payload of the
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passed data length occupies.
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This is a constant expression.
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.IP *
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.BR CMSG_DATA ()
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returns a pointer to the data portion of a
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.IR cmsghdr .
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The pointer returned cannot be assumed to be suitably aligned for
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accessing arbitrary payload data types.
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Applications should not cast it to a pointer type matching the payload,
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but should instead use
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.BR memcpy (3)
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to copy data to or from a suitably declared object.
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.IP *
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.BR CMSG_LEN ()
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returns the value to store in the
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.I cmsg_len
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member of the
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.I cmsghdr
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structure, taking into account any necessary
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alignment.
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It takes the data length as an argument.
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This is a constant
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expression.
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.PP
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To create ancillary data, first initialize the
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.I msg_controllen
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member of the
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.I msghdr
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with the length of the control message buffer.
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Use
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.BR CMSG_FIRSTHDR ()
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on the
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.I msghdr
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to get the first control message and
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.BR CMSG_NXTHDR ()
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to get all subsequent ones.
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In each control message, initialize
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.I cmsg_len
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(with
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.BR CMSG_LEN ()),
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the other
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.I cmsghdr
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header fields, and the data portion using
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.BR CMSG_DATA ().
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Finally, the
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.I msg_controllen
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field of the
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.I msghdr
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should be set to the sum of the
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.BR CMSG_SPACE ()
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of the length of
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all control messages in the buffer.
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For more information on the
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.IR msghdr ,
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see
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.BR recvmsg (2).
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.SH CONFORMING TO
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This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
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the IPv6 advanced API described in RFC\ 2292 and SUSv2.
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.BR CMSG_FIRSTHDR (),
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.BR CMSG_NXTHDR (),
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and
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.BR CMSG_DATA ()
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are specified in POSIX.1-2008.
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.BR CMSG_SPACE ()
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and
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.BR CMSG_LEN ()
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.\" https://www.austingroupbugs.net/view.php?id=978#c3242
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will be included in the next POSIX release (Issue 8).
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.PP
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.BR CMSG_ALIGN ()
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is a Linux extension.
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.SH NOTES
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For portability, ancillary data should be accessed using only the macros
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described here.
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.BR CMSG_ALIGN ()
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is a Linux extension and should not be used in portable programs.
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.PP
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In Linux,
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.BR CMSG_LEN (),
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.BR CMSG_DATA (),
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and
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.BR CMSG_ALIGN ()
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are constant expressions (assuming their argument is constant),
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meaning that these values can be used to declare the size of global variables.
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This may not be portable, however.
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.SH EXAMPLES
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This code looks for the
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.B IP_TTL
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option in a received ancillary buffer:
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.PP
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.in +4n
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.EX
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struct msghdr msgh;
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struct cmsghdr *cmsg;
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int received_ttl;
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/* Receive auxiliary data in msgh */
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for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
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cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
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if (cmsg\->cmsg_level == IPPROTO_IP
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&& cmsg\->cmsg_type == IP_TTL) {
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memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(int));
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break;
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}
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}
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if (cmsg == NULL) {
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/* Error: IP_TTL not enabled or small buffer or I/O error */
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}
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.EE
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.in
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.PP
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The code below passes an array of file descriptors over a
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UNIX domain socket using
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.BR SCM_RIGHTS :
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.PP
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.in +4n
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.EX
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struct msghdr msg = { 0 };
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struct cmsghdr *cmsg;
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int myfds[NUM_FD]; /* Contains the file descriptors to pass */
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char iobuf[1];
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struct iovec io = {
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.iov_base = iobuf,
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.iov_len = sizeof(iobuf)
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};
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union { /* Ancillary data buffer, wrapped in a union
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in order to ensure it is suitably aligned */
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char buf[CMSG_SPACE(sizeof(myfds))];
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struct cmsghdr align;
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} u;
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msg.msg_iov = &io;
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msg.msg_iovlen = 1;
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msg.msg_control = u.buf;
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msg.msg_controllen = sizeof(u.buf);
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cmsg = CMSG_FIRSTHDR(&msg);
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cmsg\->cmsg_level = SOL_SOCKET;
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cmsg\->cmsg_type = SCM_RIGHTS;
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cmsg\->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
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memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));
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.EE
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.in
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.SH SEE ALSO
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.BR recvmsg (2),
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.BR sendmsg (2)
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.PP
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RFC\ 2292
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