msgop, msgrcv, msgsnd — message 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 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 caller-defined structure of the following
general form:
struct msgbuf { long mtype;char mtext[1];};
The mtext field is
an array (or other structure) whose size is specified by
msgsz, a non-negative
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).
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 defined by the msg_bytes 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).) 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). (msgsnd and msgrcv are 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.
The system call msgrcv()
removes a message from the queue specified by msqid and places it in the
buffer pointed to 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.
The argument msgtyp 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_NOWAITReturn immediately if no message of the requested
type is in the queue. The system call fails with
errno set to ENOMSG.
MSG_EXCEPTUsed with msgtyp greater than 0 to
read the first message in the queue with message type
that differs from msgtyp.
MSG_NOERRORTo 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.
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:
The calling process does not have write permission
on the message queue, and does not have the
CAP_IPC_OWNER
capability.
The message can't be sent due to the msg_qbytes limit for
the queue and IPC_NOWAIT
was specified in msgflg.
The address pointed to by msgp isn't
accessible.
The message queue was removed.
Sleeping on a full message queue condition, the process caught a signal.
Invalid msqid value, or
non-positive mtype value, or invalid
msgsz value
(less than 0 or greater than the system value
MSGMAX).
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:
The message text length is greater than msgsz and MSG_NOERROR isn't specified in
msgflg.
The calling process does not have read permission on
the message queue, and does not have the CAP_IPC_OWNER capability.
No message was available in the queue and
IPC_NOWAIT was specified
in msgflg.
The address pointed to by msgp isn't
accessible.
While the process was sleeping to receive a message, the message queue was removed.
While the process was sleeping to receive a message, the process caught a signal.
msgqid was
invalid, or msgsz was less than
0.
IPC_NOWAIT was
specified in msgflg and no message of
the requested type existed on the message queue.
The msgp argument
is declared as struct msgbuf * with
libc4, libc5, glibc 2.0, glibc 2.1. It is declared as
void * with glibc 2.2
and later, as required by SUSv2 and SUSv3.
The following limits on message queue resources affect the
msgsnd() call:
MSGMAXMaximum size for a message text: 8192 bytes (on
Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax).
MSGMNBDefault maximum size in bytes of a message queue:
16384 bytes (on Linux, this limit can be read and
modified via /proc/sys/kernel/msgmnb). The
superuser can increase the size of a message queue
beyond MSGMNB by a
msgctl(2) system
call.
The implementation has no intrinsic limits for the system
wide maximum number of message headers (MSGTQL) and for the system wide maximum
size in bytes of the message pool (MSGPOOL).
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright 1993 Giorgio Ciucci <giorgiocrcc.it> 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. 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 |