man-pages/man3/cmsg.3

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" 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.
.\" $Id: cmsg.3,v 1.8 2000/12/20 18:10:31 ak Exp $
.TH CMSG 3 1998-10-02 "Linux Man Page" "Linux Programmer's Manual" 
.SH NAME
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR \- Access ancillary data
.SH SYNOPSIS
.B #include <sys/socket.h>
.sp
.BI "struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *" msgh );
.br
.BI "struct cmsghdr *CMSG_NXTHDR(struct msghdr *" msgh ", struct cmsghdr *" cmsg );
.br
.BI "size_t CMSG_ALIGN(size_t " length );
.br
.BI "size_t CMSG_SPACE(size_t " length );
.br
.BI "size_t CMSG_LEN(size_t " length );
.br
.BI "unsigned char *CMSG_DATA(struct cmsghdr *" cmsg );       
.sp
.nf
struct cmsghdr {
    socklen_t cmsg_len;    /* data byte count, including header */
    int       cmsg_level;  /* originating protocol */
    int       cmsg_type;   /* protocol-specific type */
    /* followed by unsigned char cmsg_data[]; */
};
.fi
.SH DESCRIPTION
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 
.BR sendmsg (2)
and received by calling
.BR recvmsg (2).
See their manual pages for more information. 
.PP
Ancillary data is a sequence of 
.I struct cmsghdr 
structures with appended data. This sequence should only be accessed
using the macros described in this manual page and never directly. 
See the specific protocol man pages for the available control message types. 
The maximum ancillary buffer size allowed per socket can be set using the 
.B net.core.optmem_max
sysctl; see
.BR socket (7).  
.PP
.BR CMSG_FIRSTHDR ()
returns a pointer to the first 
.B cmsghdr 
in the ancillary
data buffer associated with the passed 
.BR msghdr .
.PP
.BR CMSG_NXTHDR ()
returns the next valid 
.B cmsghdr 
after the passed 
.BR cmsghdr . 
It returns NULL when there isn't enough space left in the buffer.
.PP
.BR CMSG_ALIGN (),
given a length, returns it including the required alignment. This is a 
constant expression.
.PP
.BR CMSG_SPACE ()
returns the number of bytes an ancillary element with payload of the 
passed data length occupies. 
This is a constant expression.
.PP
.B CMSG_DATA
returns a pointer to the data portion of a 
.BR cmsghdr . 
.PP
.B CMSG_LEN
returns the value to store in the 
.I cmsg_len
member of the 
.B cmsghdr 
structure, taking into account any necessary
alignment. It takes the data length as an argument. This is a constant
expression.  
.PP
To create ancillary data, first initialize the 
.I msg_controllen
member of the 
.B msghdr 
with the length of the control message buffer.  Use 
.BR CMSG_FIRSTHDR ()
on the 
.B msghdr 
to get the first control message and
.B CMSG_NEXTHDR
to get all subsequent ones.
In each control message, initialize
.I cmsg_len
(with 
.BR CMSG_LEN ), 
the other 
.B cmsghdr 
header fields, and the data portion using 
.BR CMSG_DATA .
Finally, the 
.I msg_controllen 
field of the 
.B msghdr 
should be set to the sum of the 
.BR CMSG_SPACE ()
of the length of 
all control messages in the buffer. 
For more information on the 
.BR msghdr ,
see
.BR recvmsg (2). 
.PP
When the control message buffer is too short to store all messages, the
.B MSG_CTRUNC 
flag is set in the 
.I msg_flags 
member of the 
.BR msghdr .
.SH EXAMPLE
This code looks for the 
.B IP_TTL 
option in a received ancillary buffer:
.PP
.RS
.nf
.ta 8n 16n 32n
struct msghdr msgh; 
struct cmsghdr *cmsg;
int *ttlptr;
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) { 
		ttlptr = (int *) CMSG_DATA(cmsg);
		received_ttl = *ttlptr; 
		break;
	}
}
if (cmsg == NULL) {
	/*
	 * Error: IP_TTL not enabled or small buffer
	 * or I/O error.
	 */ 
} 
.ta
.fi
.RE
.PP
The code below passes an array of file descriptors over a Unix socket using 
.BR SCM_RIGHTS :
.PP
.RS
.nf
.ta 8n 16n 32n
struct msghdr msg = {0};
struct cmsghdr *cmsg;
int myfds[NUM_FD]; /* Contains the file descriptors to pass. */ 
char buf[CMSG_SPACE(sizeof myfds)];  /* ancillary data buffer */
int *fdptr;

msg.msg_control = buf; 
msg.msg_controllen = sizeof buf;
cmsg = CMSG_FIRSTHDR(&msg); 
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD); 
/* Initialize the payload: */ 
fdptr = (int *)CMSG_DATA(cmsg); 
memcpy(fdptr, myfds, NUM_FD * sizeof(int)); 
/* Sum of the length of all control messages in the buffer: */ 
msg.msg_controllen = cmsg->cmsg_len;
.ta
.fi
.RE
.SH NOTES
For portability, ancillary data should be accessed only using the macros
described here. 
.BR CMSG_ALIGN ()
is a Linux extension and should be not used in portable programs.
.PP
In Linux,
.BR CMSG_LEN , 
.BR CMSG_DATA ,
and
.BR CMSG_ALIGN ()
are constant expressions (assuming their argument is constant);
this could be used to declare the size of global
variables. This may be not portable, however.
.SH "CONFORMING TO"
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
the IPv6 advanced API described in RFC\ 2292 and the SUSv2.
.B
CMSG_ALIGN 
is a Linux extension. 
.SH "SEE ALSO"
.BR recvmsg (2),
.BR sendmsg (2)
.PP
RFC\ 2292