msgrcv, msgsnd - System V message queue operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
The
msgsnd() and
msgrcv() system calls are used to send messages
to, and receive messages from, a System V message queue. The calling
process must have write permission on the message queue in order to send a
message, and read permission to receive a message.
The
msgp argument is a pointer to a caller-defined structure of the
following general form:
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
The
mtext field is an array (or other structure) whose size is specified
by
msgsz, a nonnegative integer value. Messages of zero length (i.e.,
no
mtext field) are permitted. The
mtype field must have a
strictly positive integer value. This value can be used by the receiving
process for message selection (see the description of
msgrcv() below).
The
msgsnd() system call appends a copy of the message pointed to by
msgp to the message queue whose identifier is specified by
msqid.
If sufficient space is available in the queue,
msgsnd() succeeds
immediately. The queue capacity is governed by the
msg_qbytes field in
the associated data structure for the message queue. During queue creation
this field is initialized to
MSGMNB bytes, but this limit can be
modified using
msgctl(2). A message queue is considered to be full if
either of the following conditions is true:
- •
- Adding a new message to the queue would cause the total number of bytes in
the queue to exceed the queue's maximum size (the msg_qbytes
field).
- •
- Adding another message to the queue would cause the total number of
messages in the queue to exceed the queue's maximum size (the
msg_qbytes field). This check is necessary to prevent an unlimited
number of zero-length messages being placed on the queue. Although such
messages contain no data, they nevertheless consume (locked) kernel
memory.
If insufficient space is available in the queue, then the default behavior of
msgsnd() is to block until space becomes available. If
IPC_NOWAIT is specified in
msgflg, then the call instead fails
with the error
EAGAIN.
A blocked
msgsnd() call may also fail if:
- •
- the queue is removed, in which case the system call fails with
errno set to EIDRM; or
- •
- a signal is caught, in which case the system call fails with errno
set to EINTR;see signal(7). (msgsnd() is never
automatically restarted after being interrupted by a signal handler,
regardless of the setting of the SA_RESTART flag when establishing
a signal handler.)
Upon successful completion the message queue data structure is updated as
follows:
- •
- msg_lspid is set to the process ID of the calling process.
- •
- msg_qnum is incremented by 1.
- •
- msg_stime is set to the current time.
The
msgrcv() system call removes a message from the queue specified by
msqid and places it in the buffer pointed to by
msgp.
The argument
msgsz specifies the maximum size in bytes for the member
mtext of the structure pointed to by the
msgp argument. If the
message text has length greater than
msgsz, then the behavior depends
on whether
MSG_NOERROR is specified in
msgflg. If
MSG_NOERROR is specified, then the message text will be truncated (and
the truncated part will be lost); if
MSG_NOERROR is not specified, then
the message isn't removed from the queue and the system call fails returning
-1 with
errno set to
E2BIG.
Unless
MSG_COPY is specified in
msgflg (see below), the
msgtyp argument specifies the type of message requested, as follows:
- •
- If msgtyp is 0, then the first message in the queue is read.
- •
- If msgtyp is greater than 0, then the first message in the queue of
type msgtyp is read, unless MSG_EXCEPT was specified in
msgflg, in which case the first message in the queue of type not
equal to msgtyp will be read.
- •
- If msgtyp is less than 0, then the first message in the queue with
the lowest type less than or equal to the absolute value of msgtyp
will be read.
The
msgflg argument is a bit mask constructed by ORing together zero or
more of the following flags:
- IPC_NOWAIT
- Return immediately if no message of the requested type is in the queue.
The system call fails with errno set to ENOMSG.
- MSG_COPY (since Linux 3.8)
- Nondestructively fetch a copy of the message at the ordinal position in
the queue specified by msgtyp (messages are considered to be
numbered starting at 0).
- This flag must be specified in conjunction with IPC_NOWAIT, with
the result that, if there is no message available at the given position,
the call fails immediately with the error ENOMSG. Because they
alter the meaning of msgtyp in orthogonal ways, MSG_COPY and
MSG_EXCEPT may not both be specified in msgflg.
- The MSG_COPY flag was added for the implementation of the kernel
checkpoint-restore facility and is available only if the kernel was built
with the CONFIG_CHECKPOINT_RESTORE option.
- MSG_EXCEPT
- Used with msgtyp greater than 0 to read the first message in the
queue with message type that differs from msgtyp.
- MSG_NOERROR
- To truncate the message text if longer than msgsz bytes.
If no message of the requested type is available and
IPC_NOWAIT isn't
specified in
msgflg, the calling process is blocked until one of the
following conditions occurs:
- •
- A message of the desired type is placed in the queue.
- •
- The message queue is removed from the system. In this case, the system
call fails with errno set to EIDRM.
- •
- The calling process catches a signal. In this case, the system call fails
with errno set to EINTR. (msgrcv() is never
automatically restarted after being interrupted by a signal handler,
regardless of the setting of the SA_RESTART flag when establishing
a signal handler.)
Upon successful completion the message queue data structure is updated as
follows:
- msg_lrpid is set to the process ID of the calling process.
- msg_qnum is decremented by 1.
- msg_rtime is set to the current time.
On failure both functions return -1 with
errno indicating the error,
otherwise
msgsnd() returns 0 and
msgrcv() returns the number of
bytes actually copied into the
mtext array.
When
msgsnd() fails,
errno will be set to one among the following
values:
- EACCES
- The calling process does not have write permission on the message queue,
and does not have the CAP_IPC_OWNER capability in the user
namespace that governs its IPC namespace.
- EAGAIN
- The message can't be sent due to the msg_qbytes limit for the queue
and IPC_NOWAIT was specified in msgflg.
- EFAULT
- The address pointed to by msgp isn't accessible.
- EIDRM
- The message queue was removed.
- EINTR
- Sleeping on a full message queue condition, the process caught a
signal.
- EINVAL
- Invalid msqid value, or nonpositive mtype value, or invalid
msgsz value (less than 0 or greater than the system value
MSGMAX).
- ENOMEM
- The system does not have enough memory to make a copy of the message
pointed to by msgp.
When
msgrcv() fails,
errno will be set to one among the following
values:
- E2BIG
- The message text length is greater than msgsz and
MSG_NOERROR isn't specified in msgflg.
- EACCES
- The calling process does not have read permission on the message queue,
and does not have the CAP_IPC_OWNER capability in the user
namespace that governs its IPC namespace.
- EFAULT
- The address pointed to by msgp isn't accessible.
- EIDRM
- While the process was sleeping to receive a message, the message queue was
removed.
- EINTR
- While the process was sleeping to receive a message, the process caught a
signal; see signal(7).
- EINVAL
- msqid was invalid, or msgsz was less than 0.
- EINVAL (since Linux 3.14)
- msgflg specified MSG_COPY, but not IPC_NOWAIT.
- EINVAL (since Linux 3.14)
- msgflg specified both MSG_COPY and MSG_EXCEPT.
- ENOMSG
- IPC_NOWAIT was specified in msgflg and no message of the
requested type existed on the message queue.
- ENOMSG
- IPC_NOWAIT and MSG_COPY were specified in msgflg and
the queue contains less than msgtyp messages.
- ENOSYS (since Linux 3.8)
- MSG_COPY was specified in msgflg, and this kernel was
configured without CONFIG_CHECKPOINT_RESTORE.
POSIX.1-2001, POSIX.1-2008, SVr4.
The
MSG_EXCEPT and
MSG_COPY flags are Linux-specific; their
definitions can be obtained by defining the
_GNU_SOURCE feature test
macro.
The inclusion of
<sys/types.h> and
<sys/ipc.h> isn't
required on Linux or by any version of POSIX. However, some old
implementations required the inclusion of these header files, and the SVID
also documented their inclusion. Applications intended to be portable to such
old systems may need to include these header files.
The
msgp argument is declared as
struct msgbuf * in glibc
2.0 and 2.1. It is declared as
void * in glibc 2.2 and later, as
required by SUSv2 and SUSv3.
The following limits on message queue resources affect the
msgsnd() call:
- MSGMAX
- Maximum size of a message text, in bytes (default value: 8192 bytes). On
Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax.
- MSGMNB
- Maximum number of bytes that can be held in a message queue (default
value: 16384 bytes). On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmnb. A privileged process (Linux: a process
with the CAP_SYS_RESOURCE capability) can increase the size of a
message queue beyond MSGMNB using the msgctl(2)
IPC_SET operation.
The implementation has no intrinsic system-wide limits on the number of message
headers (
MSGTQL) and the number of bytes in the message pool
(
MSGPOOL).
In Linux 3.13 and earlier, if
msgrcv() was called with the
MSG_COPY flag, but without
IPC_NOWAIT, and the message queue
contained less than
msgtyp messages, then the call would block until
the next message is written to the queue. At that point, the call would return
a copy of the message,
regardless of whether that message was at the
ordinal position
msgtyp. This bug is fixed in Linux 3.14.
Specifying both
MSG_COPY and
MSC_EXCEPT in
msgflg is a
logical error (since these flags impose different interpretations on
msgtyp). In Linux 3.13 and earlier, this error was not diagnosed by
msgrcv(). This bug is fixed in Linux 3.14.
The program below demonstrates the use of
msgsnd() and
msgrcv().
The example program is first run with the
-s option to send a message and
then run again with the
-r option to receive a message.
The following shell session shows a sample run of the program:
$ ./a.out -s
sent: a message at Wed Mar 4 16:25:45 2015
$ ./a.out -r
message received: a message at Wed Mar 4 16:25:45 2015
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
struct msgbuf {
long mtype;
char mtext[80];
};
static void
usage(char *prog_name, char *msg)
{
if (msg != NULL)
fputs(msg, stderr);
fprintf(stderr, "Usage: %s [options]\n", prog_name);
fprintf(stderr, "Options are:\n");
fprintf(stderr, "-s send message using msgsnd()\n");
fprintf(stderr, "-r read message using msgrcv()\n");
fprintf(stderr, "-t message type (default is 1)\n");
fprintf(stderr, "-k message queue key (default is 1234)\n");
exit(EXIT_FAILURE);
}
static void
send_msg(int qid, int msgtype)
{
struct msgbuf msg;
time_t t;
msg.mtype = msgtype;
time(&t);
snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
ctime(&t));
if (msgsnd(qid, (void *) &msg, sizeof(msg.mtext),
IPC_NOWAIT) == -1) {
perror("msgsnd error");
exit(EXIT_FAILURE);
}
printf("sent: %s\n", msg.mtext);
}
static void
get_msg(int qid, int msgtype)
{
struct msgbuf msg;
if (msgrcv(qid, (void *) &msg, sizeof(msg.mtext), msgtype,
MSG_NOERROR | IPC_NOWAIT) == -1) {
if (errno != ENOMSG) {
perror("msgrcv");
exit(EXIT_FAILURE);
}
printf("No message available for msgrcv()\n");
} else
printf("message received: %s\n", msg.mtext);
}
int
main(int argc, char *argv[])
{
int qid, opt;
int mode = 0; /* 1 = send, 2 = receive */
int msgtype = 1;
int msgkey = 1234;
while ((opt = getopt(argc, argv, "srt:k:")) != -1) {
switch (opt) {
case 's':
mode = 1;
break;
case 'r':
mode = 2;
break;
case 't':
msgtype = atoi(optarg);
if (msgtype <= 0)
usage(argv[0], "-t option must be greater than 0\n");
break;
case 'k':
msgkey = atoi(optarg);
break;
default:
usage(argv[0], "Unrecognized option\n");
}
}
if (mode == 0)
usage(argv[0], "must use either -s or -r option\n");
qid = msgget(msgkey, IPC_CREAT | 0666);
if (qid == -1) {
perror("msgget");
exit(EXIT_FAILURE);
}
if (mode == 2)
get_msg(qid, msgtype);
else
send_msg(qid, msgtype);
exit(EXIT_SUCCESS);
}
msgctl(2),
msgget(2),
capabilities(7),
mq_overview(7),
sysvipc(7)