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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
MSGOP(2) Linux Programmer's Manual MSGOP(2)
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, respectively, 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).
msgsnd()
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.
msgrcv()
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.
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
Program source
#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), svipc(7)
This page is part of release 4.12 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/.
Linux 2016-10-08 MSGOP(2)
Pages that refer to this page: ipcs(1), lsipc(1), pcp-ipcs(1), ipc(2), msgctl(2), msgget(2), syscalls(2), capabilities(7), mq_overview(7), signal(7), svipc(7)