Section (2) msgop
Name
msgrcv, msgsnd — System V message queue operations
Synopsis
#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) ; |
DESCRIPTION
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_zsingle_quotesz_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_zsingle_quotesz_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 theSA_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_zsingle_quotesz_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 typemsgtyp
is read, unlessMSG_EXCEPT
was specified inmsgflg
, in which case the first message in the queue of type not equal tomsgtyp
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 ofmsgtyp
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 ofmsgtyp
in orthogonal ways,MSG_COPY
andMSG_EXCEPT
may not both be specified inmsgflg
.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 theCONFIG_CHECKPOINT_RESTORE
option. MSG_EXCEPT
-
Used with
msgtyp
greater than 0 to read the first message in the queue with message type that differs frommsgtyp
. 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_zsingle_quotesz_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 theSA_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.
RETURN VALUE
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.
ERRORS
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_zsingle_quotesz_t be sent due to the
msg_qbytes
limit for the queue andIPC_NOWAIT
was specified inmsgflg
. - EFAULT
-
The address pointed to by
msgp
isn_zsingle_quotesz_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 nonpositivemtype
value, or invalidmsgsz
value (less than 0 or greater than the system valueMSGMAX
). - 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
andMSG_NOERROR
isn_zsingle_quotesz_t specified inmsgflg
. - 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_zsingle_quotesz_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, ormsgsz
was less than 0. - EINVAL (since Linux 3.14)
-
msgflg
specifiedMSG_COPY
, but notIPC_NOWAIT
. - EINVAL (since Linux 3.14)
-
msgflg
specified bothMSG_COPY
andMSG_EXCEPT
. - ENOMSG
-
IPC_NOWAIT
was specified inmsgflg
and no message of the requested type existed on the message queue. - ENOMSG
-
IPC_NOWAIT
andMSG_COPY
were specified inmsgflg
and the queue contains less thanmsgtyp
messages. - ENOSYS (since Linux 3.8)
-
MSG_COPY
was specified inmsgflg
, and this kernel was configured withoutCONFIG_CHECKPOINT_RESTORE
.
CONFORMING TO
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.
NOTES
The inclusion of <
sys/types.h
>
and <
sys/ipc.h
>
isn_zsingle_quotesz_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 theCAP_SYS_RESOURCE
capability) can increase the size of a message queue beyondMSGMNB
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
).
BUGS
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.
EXAMPLE
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] , prog_name); fprintf(stderr, Options are: ); fprintf(stderr, −s send message using msgsnd() ); fprintf(stderr, −r read message using msgrcv() ); fprintf(stderr, −t message type (default is 1) ); fprintf(stderr, −k message queue key (default is 1234) ); 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 , 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() ); } else printf(message received: %s , 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 _zsingle_quotesz_s_zsingle_quotesz_: mode = 1; break; case _zsingle_quotesz_r_zsingle_quotesz_: mode = 2; break; case _zsingle_quotesz_t_zsingle_quotesz_: msgtype = atoi(optarg); if (msgtype <= 0) usage(argv[0], −t option must be greater than 0 ); break; case _zsingle_quotesz_k_zsingle_quotesz_: msgkey = atoi(optarg); break; default: usage(argv[0], Unrecognized option ); } } if (mode == 0) usage(argv[0], must use either −s or −r option ); 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); }
COLOPHON
This page is part of release 5.04 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/.
Copyright 1993 Giorgio Ciucci <giorgiocrcc.it> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esrthyrsus.com> Modified Mon Jul 10 21:09:59 2000 by aeb Modified 1 Jun 2002, Michael Kerrisk <mtk.manpagesgmail.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.manpagesgmail.com> Added notes on capability requirements Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Added notes on /proc files |