msgop() - Unix, Linux System Call
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NAME
msgop - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
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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, 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;
/* 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 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 initialised to
MSGMNB bytes, but this limit can be modified using
msgctl().) If insufficient space is available in the queue, then the default
behaviour 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:
Tag | Description |
|
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 behaviour 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 isnt 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:
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If
msgtyp is 0,
then the first message in the queue is read.
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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.
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|
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_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.
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If no message of the requested type is available and
IPC_NOWAIT isnt 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.
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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:
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msg_lrpid is set to the process ID of the calling process.
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msg_qnum is decremented by 1.
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msg_rtime is set to the current time.
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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:
Tag | Description |
EACCES |
The calling process does not have write permission on the message queue,
and does not have the
CAP_IPC_OWNER capability.
|
EAGAIN |
The message cant 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 isnt 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 non-positive
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. |
When
msgrcv() fails,
errno will be set to one among the following values:
|
E2BIG |
The message text length is greater than
msgsz and
MSG_NOERROR isnt 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.
|
EAGAIN |
No message was available in the queue and
IPC_NOWAIT was specified in
msgflg. |
EFAULT |
The address pointed to by
msgp isnt 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.
|
EINVAL |
msgqid was invalid, or
msgsz was less than 0.
|
ENOMSG |
IPC_NOWAIT was specified in
msgflg and no message of the requested type existed on the message queue.
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CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
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:
Tag | Description |
MSGMAX |
Maximum size for a message text: 8192 bytes
(on Linux, this limit can be read and modified via
/proc/sys/kernel/msgmax). |
MSGMNB |
Default 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() 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). SEE ALSO
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