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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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MSGOP(2) System Calls Manual MSGOP(2)
msgrcv, msgsnd - System V message queue operations
Standard C library (libc, -lc)
#include <sys/msg.h>
ssize_t msgrcv(size_t msgsz;
int msqid, void msgp[msgsz], size_t msgsz, long msgtyp,
int msgflg);
int msgsnd(size_t msgsz;
int msqid, const void msgp[msgsz], size_t msgsz,
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).
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 success, msgsnd() returns 0 and msgrcv() returns the number of
bytes actually copied into the mtext array. On failure, both
functions return -1, and set errno to indicate the error.
msgsnd() can fail with the following errors:
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.
msgrcv() can fail with the following errors:
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)
Both MSG_COPY and IPC_NOWAIT were specified in msgflg, and
this kernel was configured without
CONFIG_CHECKPOINT_RESTORE.
POSIX.1-2008.
The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their
definitions can be obtained by defining the _GNU_SOURCE feature
test macro.
POSIX.1-2001, SVr4.
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 <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <time.h>
#include <unistd.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)
{
time_t t;
struct msgbuf msg;
msg.mtype = msgtype;
time(&t);
snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
ctime(&t));
if (msgsnd(qid, &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, &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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-08-11. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.15 2025-06-28 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), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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