NAME
CMSG_DATA,
CMSG_FIRSTHDR,
CMSG_LEN,
CMSG_NXTHDR,
CMSG_SPACE —
socket control message
routines
SYNOPSIS
#include <sys/socket.h>
unsigned char *
CMSG_DATA(
struct
cmsghdr *);
const unsigned char *
CCMSG_DATA(
struct
cmsghdr *);
struct cmsghdr *
CMSG_FIRSTHDR(
struct
msghdr *);
size_t
CMSG_LEN(
size_t);
struct cmsghdr *
CMSG_NXTHDR(
struct
msghdr *,
struct cmsghdr
*);
size_t
CMSG_SPACE(
size_t);
DESCRIPTION
The control message API is used to construct ancillary data objects for use in
control messages sent and received across sockets.
Control messages are passed around by the
recvmsg(2) and
sendmsg(2) system calls. The
cmsghdr structure, described in
recvmsg(2), is used to specify
a chain of control messages.
These routines should be used instead of directly accessing the control message
header members and data buffers as they ensure that necessary alignment
constraints are met.
The following routines are provided:
-
-
- CMSG_DATA(cmsg)
- This routine accesses the data portion of the control
message header cmsg. It ensures proper alignment
constraints on the beginning of ancillary data are met.
-
-
- CMSG_FIRSTHDR(mhdr)
- This routine accesses the first control message attached to
the message msg. If no control messages are attached
to the message, this routine returns
NULL
.
-
-
- CMSG_LEN(len)
- This routine determines the size in bytes of a control
message, which includes the control message header.
len specifies the length of the data held by the
control message. This value is what is normally stored in the
cmsg_len of each control message. This routine
accounts for any alignment constraints on the beginning of ancillary data.
This macro might not evaluate to a compile-time constant.
-
-
- CMSG_NXTHDR(mhdr,
cmsg)
- This routine returns the location of the control message
following cmsg in the message
mhdr. If cmsg is the last
control message in the chain, this routine returns
NULL
.
-
-
- CMSG_SPACE(len)
- This routine determines the size in bytes needed to hold a
control message and its contents of length len,
which includes the control message header. This value is what is normally
stored in msg_msgcontrollen. This routine accounts
for any alignment constraints on the beginning of ancillary data as well
as any needed to pad the next control message. This macro might not
evaluate to a compile-time constant.
EXAMPLES
The following example constructs a control message containing a file descriptor
and passes it over a socket:
struct msghdr msg;
struct cmsghdr *cmsg;
/* We use a union to make sure hdr is aligned */
union {
struct cmsghdr hdr;
unsigned char buf[CMSG_SPACE(sizeof(int))];
} *cmsgbuf;
/*
* We allocate in the heap instead of the stack to avoid C99
* variable stack allocation, which breaks gcc -fstack-protector.
*/
if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
err(1, "malloc");
(void)memset(&msg, 0, sizeof(msg));
msg.msg_control = cmsgbuf->buf;
msg.msg_controllen = sizeof(cmsgbuf->buf);
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
*(int *)CMSG_DATA(cmsg) = fd;
if (sendmsg(s, &msg, 0) == -1)
err(1, "sendmsg");
free(cmsgbuf);
And an example that receives and decomposes the control message:
struct msghdr msg;
struct cmsghdr *cmsg;
union {
struct cmsghdr hdr;
unsigned char buf[CMSG_SPACE(sizeof(int))];
} *cmsgbuf;
if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
err(1, "malloc");
(void)memset(&msg, 0, sizeof(msg));
msg.msg_control = cmsgbuf->buf;
msg.msg_controllen = sizeof(cmsgbuf->buf);
if (recvmsg(s, &msg, 0) == -1)
err(1, "recvmsg");
if ((msg.msg_flags & (MSG_TRUNC|MSG_CTRUNC))
errx(1, "control message truncated");
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
cmsg = CMSG_NXTHDR(&msg, cmsg)) {
if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
cmsg->cmsg_level == SOL_SOCKET &&
cmsg->cmsg_type == SCM_RIGHTS) {
fd = *(int *)CMSG_DATA(cmsg);
/* Do something with the descriptor. */
}
}
free(cmsgbuf);
SEE ALSO
recvmsg(2),
sendmsg(2),
socket(2)
HISTORY
The control message API first appeared in
4.2BSD.