.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\" and Copyright 2015 Bill Pemberton <wfp5p@worldbroken.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Mon Jul 10 21:09:59 2000 by aeb
.\" Modified 1 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language clean-ups.
.\"	Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
.\"	Added note on restart behavior of msgsnd() and msgrcv()
.\"	Formatting clean-ups (argument and field names marked as .I
.\"		instead of .B)
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Added notes on /proc files
.\"
.TH MSGOP 2 2024-06-15 "Linux man-pages (unreleased)"
.SH NAME
msgrcv, msgsnd \- System V message queue operations
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/msg.h>
.P
.BI "int msgsnd(int " msqid ", const void " msgp [. msgsz "], size_t " msgsz ,
.BI "               int " msgflg );
.P
.BI "ssize_t msgrcv(int " msqid ", void " msgp [. msgsz "], size_t " msgsz \
", long " msgtyp ,
.BI "               int " msgflg );
.fi
.SH DESCRIPTION
The
.BR msgsnd ()
and
.BR 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.
.P
The
.I msgp
argument is a pointer to a caller-defined structure
of the following general form:
.P
.in +4n
.EX
struct msgbuf {
    long mtype;       /* message type, must be > 0 */
    char mtext[1];    /* message data */
};
.EE
.in
.P
The
.I mtext
field is an array (or other structure) whose size is specified by
.IR msgsz ,
a nonnegative integer value.
Messages of zero length (i.e., no
.I mtext
field) are permitted.
The
.I 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
.BR msgrcv ()
below).
.SS msgsnd()
The
.BR msgsnd ()
system call appends a copy of the message pointed to by
.I msgp
to the message queue whose identifier is specified
by
.IR msqid .
.P
If sufficient space is available in the queue,
.BR msgsnd ()
succeeds immediately.
The queue capacity is governed by the
.I msg_qbytes
field in the associated data structure for the message queue.
During queue creation this field is initialized to
.B MSGMNB
bytes, but this limit can be modified using
.BR msgctl (2).
A message queue is considered to be full if either of the following
conditions is true:
.IP \[bu] 3
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
.I msg_qbytes
field).
.IP \[bu]
Adding another message to the queue would cause the total number of messages
in the queue to exceed the queue's maximum size (the
.I 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.
.P
If insufficient space is available in the queue, then the default
behavior of
.BR msgsnd ()
is to block until space becomes available.
If
.B IPC_NOWAIT
is specified in
.IR msgflg ,
then the call instead fails with the error
.BR EAGAIN .
.P
A blocked
.BR msgsnd ()
call may also fail if:
.IP \[bu] 3
the queue is removed,
in which case the system call fails with
.I errno
set to
.BR EIDRM ;
or
.IP \[bu]
a signal is caught, in which case the system call fails
with
.I errno
set to
.BR EINTR ; see
.BR signal (7).
.RB ( msgsnd ()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
.B SA_RESTART
flag when establishing a signal handler.)
.P
Upon successful completion the message queue data structure is updated
as follows:
.IP \[bu] 3
.I msg_lspid
is set to the process ID of the calling process.
.IP \[bu]
.I msg_qnum
is incremented by 1.
.IP \[bu]
.I msg_stime
is set to the current time.
.SS msgrcv()
The
.BR msgrcv ()
system call removes a message from the queue specified by
.I msqid
and places it in the buffer
pointed to by
.IR msgp .
.P
The argument
.I msgsz
specifies the maximum size in bytes for the member
.I mtext
of the structure pointed to by the
.I msgp
argument.
If the message text has length greater than
.IR msgsz ,
then the behavior depends on whether
.B MSG_NOERROR
is specified in
.IR msgflg .
If
.B MSG_NOERROR
is specified, then
the message text will be truncated (and the truncated part will be
lost); if
.B MSG_NOERROR
is not specified, then
the message isn't removed from the queue and
the system call fails returning \-1 with
.I errno
set to
.BR E2BIG .
.P
Unless
.B MSG_COPY
is specified in
.I msgflg
(see below),
the
.I msgtyp
argument specifies the type of message requested, as follows:
.IP \[bu] 3
If
.I msgtyp
is 0,
then the first message in the queue is read.
.IP \[bu]
If
.I msgtyp
is greater than 0,
then the first message in the queue of type
.I msgtyp
is read, unless
.B MSG_EXCEPT
was specified in
.IR msgflg ,
in which case
the first message in the queue of type not equal to
.I msgtyp
will be read.
.IP \[bu]
If
.I 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
.I msgtyp
will be read.
.P
The
.I msgflg
argument is a bit mask constructed by ORing together zero or more
of the following flags:
.TP
.B IPC_NOWAIT
Return immediately if no message of the requested type is in the queue.
The system call fails with
.I errno
set to
.BR ENOMSG .
.TP
.BR MSG_COPY " (since Linux 3.8)"
.\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8
Nondestructively fetch a copy of the message at the ordinal position
in the queue specified by
.I msgtyp
(messages are considered to be numbered starting at 0).
.IP
This flag must be specified in conjunction with
.BR IPC_NOWAIT ,
with the result that, if there is no message available at the given position,
the call fails immediately with the error
.BR ENOMSG .
Because they alter the meaning of
.I msgtyp
in orthogonal ways,
.B MSG_COPY
and
.B MSG_EXCEPT
may not both be specified in
.IR msgflg .
.IP
The
.B 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
.B CONFIG_CHECKPOINT_RESTORE
option.
.TP
.B MSG_EXCEPT
Used with
.I msgtyp
greater than 0
to read the first message in the queue with message type that differs
from
.IR msgtyp .
.TP
.B MSG_NOERROR
To truncate the message text if longer than
.I msgsz
bytes.
.P
If no message of the requested type is available and
.B IPC_NOWAIT
isn't specified in
.IR msgflg ,
the calling process is blocked until one of the following conditions occurs:
.IP \[bu] 3
A message of the desired type is placed in the queue.
.IP \[bu]
The message queue is removed from the system.
In this case, the system call fails with
.I errno
set to
.BR EIDRM .
.IP \[bu]
The calling process catches a signal.
In this case, the system call fails with
.I errno
set to
.BR EINTR .
.RB ( msgrcv ()
is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
.B SA_RESTART
flag when establishing a signal handler.)
.P
Upon successful completion the message queue data structure is updated
as follows:
.IP
.I msg_lrpid
is set to the process ID of the calling process.
.IP
.I msg_qnum
is decremented by 1.
.IP
.I msg_rtime
is set to the current time.
.SH RETURN VALUE
On success,
.BR msgsnd ()
returns 0
and
.BR msgrcv ()
returns the number of bytes actually copied into the
.I mtext
array.
On failure, both functions return \-1, and set
.I errno
to indicate the error.
.SH ERRORS
.BR msgsnd ()
can fail with the following errors:
.TP
.B EACCES
The calling process does not have write permission on the message queue,
and does not have the
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EAGAIN
The message can't be sent due to the
.I msg_qbytes
limit for the queue and
.B IPC_NOWAIT
was specified in
.IR msgflg .
.TP
.B EFAULT
The address pointed to by
.I msgp
isn't accessible.
.TP
.B EIDRM
The message queue was removed.
.TP
.B EINTR
Sleeping on a full message queue condition, the process caught a signal.
.TP
.B EINVAL
Invalid
.I msqid
value, or nonpositive
.I mtype
value, or
invalid
.I msgsz
value (less than 0 or greater than the system value
.BR MSGMAX ).
.TP
.B ENOMEM
The system does not have enough memory to make a copy of the
message pointed to by
.IR msgp .
.P
.BR msgrcv ()
can fail with the following errors:
.TP
.B E2BIG
The message text length is greater than
.I msgsz
and
.B MSG_NOERROR
isn't specified in
.IR msgflg .
.TP
.B EACCES
The calling process does not have read permission on the message queue,
and does not have the
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EFAULT
The address pointed to by
.I msgp
isn't accessible.
.TP
.B EIDRM
While the process was sleeping to receive a message,
the message queue was removed.
.TP
.B EINTR
While the process was sleeping to receive a message,
the process caught a signal; see
.BR signal (7).
.TP
.B EINVAL
.I msqid
was invalid, or
.I msgsz
was less than 0.
.TP
.BR EINVAL " (since Linux 3.14)"
.I msgflg
specified
.BR MSG_COPY ,
but not
.BR IPC_NOWAIT .
.TP
.BR EINVAL " (since Linux 3.14)"
.I msgflg
specified both
.B MSG_COPY
and
.BR MSG_EXCEPT .
.TP
.B ENOMSG
.B IPC_NOWAIT
was specified in
.I msgflg
and no message of the requested type existed on the message queue.
.TP
.B ENOMSG
.B IPC_NOWAIT
and
.B MSG_COPY
were specified in
.I msgflg
and the queue contains less than
.I msgtyp
messages.
.TP
.BR ENOSYS " (since Linux 3.8)"
Both
.B MSG_COPY
and
.B IPC_NOWAIT
were specified in
.IR msgflg ,
and this kernel was configured without
.BR CONFIG_CHECKPOINT_RESTORE .
.SH STANDARDS
POSIX.1-2008.
.P
The
.B MSG_EXCEPT
and
.B MSG_COPY
flags are Linux-specific;
their definitions can be obtained by defining the
.B _GNU_SOURCE
.\" MSG_COPY since glibc 2.18
feature test macro.
.SH HISTORY
POSIX.1-2001, SVr4.
.P
The
.I msgp
argument is declared as \fIstruct msgbuf\ *\fP in
glibc 2.0 and 2.1.
It is declared as \fIvoid\ *\fP
in glibc 2.2 and later, as required by SUSv2 and SUSv3.
.SH NOTES
The following limits on message queue resources affect the
.BR msgsnd ()
call:
.TP
.B MSGMAX
Maximum size of a message text, in bytes (default value: 8192 bytes).
On Linux, this limit can be read and modified via
.IR /proc/sys/kernel/msgmax .
.TP
.B 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
.IR /proc/sys/kernel/msgmnb .
A privileged process
(Linux: a process with the
.B CAP_SYS_RESOURCE
capability)
can increase the size of a message queue beyond
.B MSGMNB
using the
.BR msgctl (2)
.B IPC_SET
operation.
.P
The implementation has no intrinsic system-wide limits on the
number of message headers
.RB ( MSGTQL )
and the number of bytes in the message pool
.RB ( MSGPOOL ).
.SH BUGS
In Linux 3.13 and earlier,
if
.BR msgrcv ()
was called with the
.B MSG_COPY
flag, but without
.BR IPC_NOWAIT ,
and the message queue contained less than
.I msgtyp
messages, then the call would block until the next message is written
to the queue.
.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
At that point, the call would return a copy of the message,
.I regardless
of whether that message was at the ordinal position
.IR msgtyp .
This bug is fixed
.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
in Linux 3.14.
.P
Specifying both
.B MSG_COPY
and
.B MSC_EXCEPT
in
.I msgflg
is a logical error (since these flags impose different interpretations on
.IR msgtyp ).
In Linux 3.13 and earlier,
.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
this error was not diagnosed by
.BR msgrcv ().
This bug is fixed
.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
in Linux 3.14.
.SH EXAMPLES
The program below demonstrates the use of
.BR msgsnd ()
and
.BR msgrcv ().
.P
The example program is first run with the \fB\-s\fP option to send a
message and then run again with the \fB\-r\fP option to receive a
message.
.P
The following shell session shows a sample run of the program:
.P
.in +4n
.EX
.RB "$" " ./a.out \-s"
sent: a message at Wed Mar  4 16:25:45 2015
.P
.RB "$" " ./a.out \-r"
message received: a message at Wed Mar  4 16:25:45 2015
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (msgop.c)
.EX
#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]\[rs]n", prog_name);
    fprintf(stderr, "Options are:\[rs]n");
    fprintf(stderr, "\-s        send message using msgsnd()\[rs]n");
    fprintf(stderr, "\-r        read message using msgrcv()\[rs]n");
    fprintf(stderr, "\-t        message type (default is 1)\[rs]n");
    fprintf(stderr, "\-k        message queue key (default is 1234)\[rs]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\[rs]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()\[rs]n");
    } else {
        printf("message received: %s\[rs]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 \[aq]s\[aq]:
            mode = 1;
            break;
        case \[aq]r\[aq]:
            mode = 2;
            break;
        case \[aq]t\[aq]:
            msgtype = atoi(optarg);
            if (msgtype <= 0)
                usage(argv[0], "\-t option must be greater than 0\[rs]n");
            break;
        case \[aq]k\[aq]:
            msgkey = atoi(optarg);
            break;
        default:
            usage(argv[0], "Unrecognized option\[rs]n");
        }
    }
\&
    if (mode == 0)
        usage(argv[0], "must use either \-s or \-r option\[rs]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);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR msgctl (2),
.BR msgget (2),
.BR capabilities (7),
.BR mq_overview (7),
.BR sysvipc (7)