.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\"

.\" 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 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

msgrcv, msgsnd \- System V message queue operations

.SH SYNOPSIS

.nf

.B #include <sys/types.h>

.B #include <sys/ipc.h>

.B #include <sys/msg.h>

.PP

.BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \

", int " msgflg );

.PP

.BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \

", long " msgtyp ,

.BI "               int " msgflg );

.fi

.SH DESCRIPTION

The

.BR msgsnd ()

and

.BR 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.

.PP

The

.I msgp

argument is a pointer to a caller-defined structure

of the following general form:

.PP

.in +4n

.EX

struct msgbuf {

    long mtype;       /* message type, must be > 0 */

    char mtext[1];    /* message data */

};

.EE

.in

.PP

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 .

.PP

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 * 2

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 *

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.

.PP

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 .

.PP

A blocked

.BR msgsnd ()

call may also fail if:

.IP * 2

the queue is removed,

in which case the system call fails with

.I errno

set to

.BR EIDRM ;

or

.IP *

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.)

.PP

Upon successful completion the message queue data structure is updated

as follows:

.IP

.I msg_lspid

is set to the process ID of the calling process.

.IP

.I msg_qnum

is incremented by 1.

.IP

.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 .

.PP

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 .

.PP

Unless

.B MSG_COPY

is specified in

.IR msgflg

(see below),

the

.I msgtyp

argument specifies the type of message requested, as follows:

.IP * 2

If

.I msgtyp

is 0,

then the first message in the queue is read.

.IP *

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 *

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.

.PP

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,

.BR MSG_COPY

and

.BR MSG_EXCEPT

may not both be specified in

.IR msgflg .

.IP

The

.BR 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.

.PP

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 * 2

A message of the desired type is placed in the queue.

.IP *

The message queue is removed from the system.

In this case, the system call fails with

.I errno

set to

.BR EIDRM .

.IP *

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.)

.PP

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 failure both functions return \-1

with

.I errno

indicating the error,

otherwise

.BR msgsnd ()

returns 0

and

.BR msgrcv ()

returns the number of bytes actually copied into the

.I mtext

array.

.SH ERRORS

When

.BR msgsnd ()

fails,

.I errno

will be set to one among the following values:

.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.

.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 .

.PP

When

.BR msgrcv ()

fails,

.I errno

will be set to one among the following values:

.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

.BR 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)"

.I MSG_COPY

was specified in

.IR msgflg ,

and this kernel was configured without

.BR CONFIG_CHECKPOINT_RESTORE .

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

.PP

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 NOTES

The inclusion of

.I <sys/types.h>

and

.I <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.

.\" Like Linux, the FreeBSD man pages still document

.\" the inclusion of these header files.

.PP

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.

.PP

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.

.PP

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

.BR 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.

.PP

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 EXAMPLE

The program below demonstrates the use of

.BR msgsnd ()

and

.BR msgrcv ().

.PP

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.

.PP

The following shell session shows a sample run of the program:

.PP

.in +4n

.EX

.RB "$" " ./a.out \-s"

sent: a message at Wed Mar  4 16:25:45 2015

.RB "$" " ./a.out \-r"

message received: a message at Wed Mar  4 16:25:45 2015

.EE

.in

.SS Program source

\&

.EX

#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 \(aqs\(aq:

            mode = 1;

            break;

        case \(aqr\(aq:

            mode = 2;

            break;

        case \(aqt\(aq:

            msgtype = atoi(optarg);

            if (msgtype <= 0)

                usage(argv[0], "\-t option must be greater than 0\\n");

            break;

        case \(aqk\(aq:

            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);

}

.EE

.SH SEE ALSO

.BR msgctl (2),

.BR msgget (2),

.BR capabilities (7),

.BR mq_overview (7),

.BR svipc (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.