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